Savitribai Phule Pune University

Semester VII (Electronics & Telecommunication Engineering)

Radiation and Microwave Theory (Code : 404181)
Chapter 4: Passive Microwave Components
The network representation which is based on
Explain S matnx representation along with the
resultant wave or propagating wave is known as
Q.1
properties for multiport network.
scattering parameters.
SPPUSDecs 15, May 16 Aug 16, May.18,7 Marks
How to find S-parameters for given Network?
Ans.
Consider the N-port network shown in Fig. 4.2.
S-matrix representation

Consider the following two port network which is

vVVN Incident voltages at corresponding
ports.
shown in Fig. 4.1.
,V2 VN Reflected voltages from
VP Two port O/P
port network oport corresponding ports.
Very basic assumption for S-parameters analysis is
Fig. 4.1: Two port network
that there is linear relationship between incident and
Consider that we want to find out Z-parameters of reflected voltages of propagating wave along the
above network. To find the Z-parameter open either transmitting media. To formulate and to define 'S'
Input or Output port parameters let's consider the 'N' port network shown in
Fig 4.2.
By open circuiting, we will calculate open circuit
voltages at the corresponding ports. Let VN Incident voltage wave
Using formulas for Z-parameters, and these open
VNReflectedvoltage wave.
Circuited voltages, We will calculate the
Port 1

Z-parameters.
Thus to find either Z, Y or ABCD parameter, very
essential step is to either open or short circuit the
2Port 2
corresponding port
Ifwant to analyze the same network at microwave
frequency, then we have to either short or open
Port 3
circuit the network. S-parameters are based on the
Na
concept of travelling waves and provide a complete Port N VN
characterization of N-port network.
over
Circuit terminations are difficult to implement
a broad range of frequencies. Therefore we cannot Fig.4.2: 'N' port Network
use the Z, Y or ABCD parameters
For N-port network
To overcome above difficulty, instead of considering
= S11 V +S12 V2 *+ SN V
net voltage or net current at particular port, we can V
consider the net voltage a net current due to V2 = S21 Vi +S22 V2t+SzN V .4.1)
ravelling wave [may be in form of incident,
retlected or transmitted wave
VW
SN1
V+
Sw2 V.+SNN V
 Radiation and Microwav: Throry
(PPD 1-2 (uik
V,V, Vu Reflected waves from a. 2 Explairn the properties o Scattering rnatri fo
Corresponding, ports muliport riotwork.
VV2 VI1 SPPURDOC15 May162Au16:6MAa
Trannitled waves
Let's represent Equation Ans.: Properties of S-Parameters:
(4.1) in matriz form
S11S12 1. 1S matrix is always a square rnatrix of order (N
Su for 'N' port network.
S212
1 VH
4.2)
2. Symmetry of [S) natrix.
The reciprocal networks are the nctworks wh
LSu Stuz
SnnL V satisfies the Reciprocity theorem. It can be shown that
OR for all passive Reciprocal network S-Matrix is
IVT= ISIIV'J SYMMETRICAL.

The linear relationship Consideri&j are the two different parts of a passive
expressed in Equation (4.2)
is the ratio of two phasors reciprocal network.
quantities. These are complex
in naturc having
the magnitude such that there Since the network is reciprocal,
less than or equal to unity. ratio is
The element in [SJ matriz is
defined as,
Where Zj and Zii are the equivalent impedances.
Obviously is
S1 OR SNN =
V1 VN Thus from the definition of S-parameters

Input reflection coefficient when rest of the

v Sj VforV =Transmitted voltage
all ports are terminated with matched
terminator except port 1 or port 'N.
V= S V for VTransmitted voltage
According to definition of reciprocal network,
OR though we interchange the excitation from V,
S21 OR SlN-1)] = 2 to V; OR Vi
vv-1
Forward transmission
=
to Vi, the response of network should
.We can say that
remain the same

coefficient when Output port is Siy S AND S=Sjj

terminated with matched load Thus we can conclude that scattering
matrix is a
i.c. When Input is applied to 1 port and port 2 is SYMMETRICAL MATRIX.
terminated with matched load. For symmetrical two port network.
OR When Input is applied to (N 1]th port and Nth port
-
ISl IS2zl AND IS21l = |S12l
is terminated with matched load. 3. [S] is a unitary matrix.

S12 OR Su-1) N = * OR
V IS]"> Complex conjugate of [S]
Reverse transmission coefficient when Input
= [] = Unit matrix of the same order as that of [S].
port is terminated with matched load. 4. Shifting at the reference plane
i.c. When Input is applied to 2 port and port 1 is For a given location of port along
terminated with matched load. the network at
given frequency, the scattering matrix elements Sij
OR When Input applied to w port and (N- 1)th port is have definite values.
terminated with match load,
With change in frequency the
scattering matrix
clements are also going to change. This
change in
S22 = Output reflection coefficient when values of the clements cannot judged analytically.
V2V2=0 For example we cannot conclude
that with increase
Input port is terminated with in frequency scattering
matrix elements will
matched load. increase and vice versa.

es Casy3so1uT.ons
Radiation and Microwave Theory (SPPU)_ 1-3 Quick Read
Howeve, at a fixed frequency the change in the Equation (4.6) shows that phase of Si is shifted by
scattering matrix elements arising from a shift in the twice the electrical length because the incident
terminal plane location is readily found. For sake of wave travels twice over this length upon reflection.
understanding, consider the two port network On the other hand, at port i (i = 1, 2), Sij{i * i)is
shown in Fig. 4.3. shifted by the sum of the electrical lengths because
the incident wave must pass through both lengths in
Two port order to travel from one shifted port to the other.
network 5. The zero property of [S] matrix: This property
Port 1' Port Port 2 Port 2 states that for a passive lossless N-port network, the
1

sum of the product of each term of any row or any

New reference column multiplied by the complex conjugate of the
New reterence
plane plane corresponding terms of any other row or column is
zero.
Fig. 4.3:Two Port Network N
Consider a two port network in which the reference 2SaS.
=
0for iti
plane at port 1 has moved a distance 11' to port 1. k=1
Similarly, the .reference plane at port 2 has moved a where, ,J
, 4, ., N
distance l2 to port 2.
Q.3 Explain with the help of neat diagram properties of
4.3) E-plane Tee.
SPRU Dec16, May18, Dec. 17, Dec: 18,4 Marks
Ans.: E-PLANE TEE
V Ve
V2 e
..(4.4)
The basic configuration for E-plane is as shown in
V2= Fig. 4.4(a).
Bl1, are the electrical length
02 = Bl2 Port 1
Port
corresponding to the reference plane shift at Primary
waveguide
each port E-ARM-
Also,
SecondaryV.
V = Vi*e waveguide
V2=V2*ea
V1=V1 ei 4.5)
Port 21
V2 V2 e
(a) Structure of F-plane Tee
From Equation (4.2), [S] matrix with change in Port 3
reference plane can be written as,
.[V] = [S][v'i
S11e S12e*7 .(4.6)
S21 ei) S22e2
Sz2
Port 2
S11'e2 S2'er1 "9 7
OR S= S21'e
0 S22'e22 Port 1

Conclude that
e
S'i S i=1,2,...n Etied Longitudinal
curront
S'y Sye) itj i=1, 2, ..,n j=1, 2, .n (b)
Fig. 4.4 E-Plane Tee (a) Structure of E-plane Tee
..(4.7) (b) Cross-sectional view of E-plane Tee

Es Dasy-soluions
Radiation andl MicrowaveTlheory (SPPU_ Quick Rtad
1-4
The arns represented by port1 and port 2 are primary waveguide by cutting slot along dimensin
kown as collinear arms. nd is
is show
The This arm is also known as H-ARM and shown in
-plane Tee can be described construction or of the primary waveguide are knoDwn
by very simple Fig. 4.5. The arms
following steps:
1.
as PORT-1 and PORT-2.is formed. When electromapo
Rectangular waveguide
of appropriate dimensions energy is fed to the H-ARM, electric field at port.1:is
ccording to frequency has
. According to design
been chosen.
specilications rectangular hole
same as that of the electric field at port-2. Wherea.
magnetic field at port-1 and port-z is not same. Thus this
of dimension b' is also is also called as SERIES TEE or CURRENT JUNCTiON
made on the top side of the
waveguide. Plane of
Coplanar symmetry
3. Anotlher waveguide am
of the same dimensions has
been plough togetlher in perpendicular Porl2
way. This is Port
shown in Fig. 44(a). 1
Power cyfrent flow
Functioning of E-plane Tee
1. When 1/P is given
to E-ARM i.e. PORT-3
When 1/P is given to the E-ARM field, a
Electric field
will propagate through the
secondary waveguide
and entered in the secondary waveguide. Harm(Side arm)
This is
shown in the Fig. 4.4(b).
This shows that half of the electric field wil
propagate towards the Port-1 and rest of the half of
T Port 3

the electric field will propagate to the Port-2. Fig. 4.5: Construction of H-plane Tee

Fig. 4.4(b) also shows that coupled electric field to Functioning of H-plane Tee
Port-1 and Port-2 are 180 degrees out of phase. . When 1/P is given to H-ARM i.e. PORT-3
Thus when signal is given to the E-ARM, E-Plane When 1/P is given to the H-ARM, Electric field will
TEE acts as HALF POWER DIVIDER. In this equal propagate through the secondary waveguide and
amount of the power gets divided into both ports entered in the secondary waveguide. This is shown
and they are out of phase to each other. in the Fig. 4.5.
2. When 1/P is given to either port of the collinear This shows that half of the electric field
will
arm (port1 or port 2) propagate towards the Port-1 and rest of the half of
When signal of same amplitude and 180° out of the electric field will propagate to the Port-2. Signal
phase is fed at the collinear arm i.e.at the PORT-1 propagating to both port is in phase. Thus when
and PORT-2, summation signal is obtained at signal is given to H-ARM, H-Plane TEE acts as HALF
PORT-3. POWER DIVIDER. In this equal amount
of the power
gets divided into both ports and they are in
Whereas signal of same amplitude and same phase phase to
each other.
is fed at the collinear arm i.e. at the PORT-1 and
PORT-2, no signal is obtained at the PORT-3. Thus E- 2. When I/P is given to either port of
the collinear
ARM is also known as DIFFERENCE ARM. arm (port1 or port 2)
Thus we can say that E-field at collinear arm is not When signal of same amplitude
and same phase is
same. Thus it is also called as.SERIES TEE. fed at the collinear arm i.e. at the
PORT-1 and PORT
2; summation signal is
obtained at PORT-3. Whereas
Q. 4 With the help of neat diagram, s- matrix & properties signal of same amplitude and 1800
out of phase is
explain H-plane Tee ? SPPU: Dec. 18, 8 Marks fed at the collinear arm i.e. at
the PORT-1 and PORT-
Ans.: H-plane or Shunt Tee 2, no signal is obtained at the PORT-3.
Thus E-ARM
is also known as SUM ARM.
To form H-Plane Tee, two rectangular waveguides
are used. Out of that one is called as Primary and another Thus we can say that H-field at
collinear arm is
waveguide is inserted in the same. Thus it is also called as SHUNT TEE.
is Secondary. Secondary

es casyESolutionE
 Padiation and MicrOwave The/ (SPPU)_ Quick Kea0
S1111S121 15131= |S1zl + |S2i + |S1214
s-Parameters for H-Plane Tee
H-TEE divides the magnetic field. Electric field
S111 IS22l

C within all the three arms is same and magnetic field From Equation (4.19),
is aliso called as current
S is different Therefore, it
s junction. Since it has three ports, [S] matriz will be Since S12 9
S of 33 and is given by Equation (4.2),
S11 S12 S13 S1=-S12
i.
= S21 S22 S23 Put it in Equation (4.16),
S] 4.8)
L Sa1 S32 S2z
Now why H-plane Tee is known as current tee. Due
1. .S11 1/2
to physical symmetry, we can say that [S] matrix for
:. Equation {4.14) hich is complete
IS131=S22 4.9)
H-plane Tee becomes
2. Let part 3 be pefectly matched 1/2 -1/2 1//2
1S331 0 4.10) 1/2 1//2
IS] =| -1/2
From symmetry property of parameter S
3.
4.11)
1/2 12
IS12i IS21l
S12 1Sal 412) a.5 V/th the help of diagramn explain the Magic Tee used
IS231 = ISa22l 4.13) to measure the impedance.

= 1S31l= |Sazzl
SPPU: Aug. 16, 4 Marks
IS131 IS23l
(4.13) Ans.: Magic Tee
From Equations (4.12) and
Hybrid Tee or Magic Tee is a combination of E and H
4. Equation (4.8) reduces to,
S13 Tee and is shown in the Fig. 4.6. E-Tee and H-Tee
S11 S12
are coupled in perpendicular manner to each other
4.14) to form Magic Tee. It has four port. Collinear arm
ISS12
L S13
Sz2
S13
S13
0 forms PORT-1 and PORT-2. Out of two
perpendicular arm, one is E-ARM and another is H
Now we have to find four unknovyns.
ARM. The arm along dimension 'b' of the collinear
5. From the unitary property arm is H-Arm. whereas arm along the dimension 'a'
of the collinear arm is H-ARM. This four port hybrid
S12 S131S11 S12" Tee or magic tee consists of all properties of E and H
fS1 S3] [1 0 01
plane Tee. Therefore, all port of magic tee can be
S12 S22 Sy3| S2 S22*
S1a
S13=01o
SoJ Lo 1 utilized for matching.
L S13 S13 0Jls12 o

R1 C1 S11S11+S12S12 + S13S13=1 -4.15) Port 3

R2 C2 S12S12 +Sz2S22 + S13S13 =1 4.16) E aim
Coplanar
RzCa S13Si3 +S135131 4.17) am Coplanar am

From Equation (4.17).

Port 1

S1a1+IS11 i
IS13 = 1//2 Po 2
4.18)
But four unknown and 3 equations Hamn
Also consider Pon 4
Rg C S13Si1+S13512 =0 .4.19)
(a) Construction of Magic Tee
Put values compare Equations (4.15) and (4.16) Fig. 4.6:The hybrid (magic) tee junction
Ces casyESOUTIonE
 1-6 symmetry property
Radiation and Microwave Theory (SPPU)_ 4.
From the IS34 S43l
Signal into IS12l 1S21
E arn IS24-S42l
S13 31
IS41 S141
Pons IS32
S23l S13 14
S12
Port 1 Por 2 S11
S22 S13 -S14
S12
output S13
0
Oulpul
S:gnal
'Porl 4 sigjnial S S13
0
-
S14 S14
Signal into
Ham . From unitary
property

S12 S13 S14

(b) Field within Magic Tee
Fig. 4.6:The hybrid (magic) teejunction S11 S12S13 S14 7 1 Sa2 S13 -314
S12 S22 S13 -S14 S12
S12
Functioning of the MAGIC TEE
S13 0 S13 S13 0 0
ARM : When two
S13
1. When 1/P is applied at collinear 0 0
are LS14 -S14 0 -S14
signal having same amplitude and same phase
S14
signal at
applied at PORT-1 and PORT-2, there is no i0 0
H-ARM.
E-ARM. Summation of signal is obtained at 010 0
1/P is applied
When 1/P is applied at E-ARM When
:
2.
PORT-1
at E-ARM, power is distributed equally at Lo o 0 1
Amplitude of the signal at both PORT is S13 +1S141 =1
and PORT-2.
Also there is no .R1 C 1S l+|S12l+| S24 12 =1
same but they are out of phase. S13 12+ |
power coupled at H-ARM.
R2C2 IS +|S221 +1
...(4.20)
When 1/P is applied at H-ARM: When
1/P to the H-
3.
PORT-2 will be
ARM, signal available at PORT-1 and RgCa 1S13 l+1S3 =1
no signal at
same in amplitude and phase. There is S14 1
R4 C4 S14 1+ =1
1

E-ARM.
can be obtained as
The S-matrix for Hybrid Tee IS1sl
below:
1. S] is 4 x 4 matrix since there are
4 ports i. IS14l
S11 S12 S13 S14
S21 S22 S23 S24 From Equation (4.20) becomes,
i.e. S S31 S32 S33 S34
Ra Ca 1Sa2 +1 S22 *2+1
S41 Sa2 S43 S44
S12 1+1 S22 1 = 0
from the Fig. 4.6[a), due to E-Tee junction symmetTy IS121 = IS11l= 0
IS13l=-IS23l =
IS22l 0
Also due to H-Tee junction symmetry 1/N2
0 1/N2
IS24l = S14

2. Looking at the geometry, port (3) and (4)

are
S| =
0 -1/2 1/N
perfectly perpendicular to each other so they are 1N -1/N2
isolated from each other. L 1N2 1/N2 0 0
IS34l |S43l= 0
a.6 Explain applications of Magic tee.
3. If ports (3) and (4) are perfectly matched to the
junction SPRU DeC19.21Marks
IS33l = IS4al = 0

es 0asySoiutionS
ad
Radiation and Microwave Theory (SPPU) -7 Quick Read

Ans. Where = 4o [1x Xml

Applications of Magic Tee A

Xm = Tensor magnetic susceptibility

Mainly used to measure unkrnown impedance.
1. [ Xm jk
2. Magic Tee-as a mixer.
Am
3. Magic Tee as Isolator. 0

Explain the Faraday's rotation principle. Diagonal susceptibility

a.7
K off diagonal susceptibility when a DC
SPRU AUd5.May17Deci
magnetic field is applied to ferrite,
Dec 18MayY19, Deo 19Marks
The propagation constant for a linearly polarized
Ans.: Ferrite Devices wave inside the ferrite can be given as,
Ferrites are non-metallic materials with resistivity jon/euo (+ )
(p) nearly 101* times greater than metals and with
Dielectric constants (e) around 10-15 and relative where = 1tXmn
permeability of the order of 1000. H +k
They have magnetic property similar to' those of
ferrous metals.
HT -k
The relative permeability H changes with applied
They are oxide based compounds which are mixture
DC magnetic field as given by,
of metallic oxide and ferric oxide.
Ye Me
They are obtained by firing powdered oxides of
materials at 1100°C or more and pressing them into
= 1++ 4.21)

different shapes. This processing gives them added Ye Gyromagentic ratio of electron
characteristics of ceramic insulators so that they can Me Saturation magnetization
be used at microwav frequencies.
Ferrites have atoms with large number of spinning
= Angular Frequency of microwave field
Hde DC magnetic field
electrons resulting in strong magntic properties.
These magnetic properties are due to magnetic A Relative permeability in clockwise direction
dipole moment associated with electron spin. Right circularly polarized].
Because of above properties, ferrites find HT Relative permeability in anticlockwise
application in a number of microwave devices to direction [Left circularly polarized]).
reduce reflected power, for modulation purposes It can be seen from Equation (4.21) that if,
and in switching circuits.
0 = lYel Hdc
Ferrites are non-reciprocal in nature i.e. when two
circularly polarized waves one rotating clockwise
Then f is infinite.

and other anticlockwise are made to propagate This phenomenon is called the gyromagnetic
through ferrite, the material reacts differently to the resonance of ferrite.
two rotating fields, thereby presenting different If 4 is much larger than, K (u
), the wave in
>>
effective permabilities to both the waves. ferrite is rotated in clockwise direction.
Microwave Propagation in Ferrites and Faraday's Consequently, the propagation phase constant p"
Rotation for the forward direction differs from propagation
When magnetic field is applied to
the slab of the phase constant p" for the backward direction.
ferrite material, Faraday's Rotation. is observed By choosing the lenigth of ferrite slab and DC
which is described below, magnetic field so that
B
H o = (B-B)l=n/2

es 0asy-solutios
 Radiation and Microwave Theory (SPPU)_
1-8 Quick Re
Q.8 With neat schematic
diagram explain the operation or Further this signal passes through the 90 dee.
Gyrator. Also state S matrix
for it. twister resulting in the phase shift of 900 Thus
SPPU: Aug,17, May 19, Dec. plane of wave polarization rotated by 90°.
Ans. Gyrator 19,6Marks
This wave now passes through the ferrite rod.
gyrator is two port
A
ferrite device which exhibits a Length of the rod is designed to give phase shift n
phase shift of 180 degrees 90. When wave come out irom port-2 total chane
when signal propagates from
port-1 to port-2 whereas in the phase is of 180°.
signal propagates from
there is no phase change when
port-2. Now when signal enters at port-z instead at port-1,
Construction of Gyrator the transition at port 2 shifts the signal phase by 90
Fig. 4.7 shows in clockwise direction.
basic construction of Gyrator. Various The ferrite rod does not change the phase shift. This
components used in Gyrator.
90° phase shifted signal passes through circular
Circular waveguide
operating in mode TE11. waveguide where phase shift of the 90° occurs in
Transition anticlockwise direction. This compensates phase
2. 900 Twister change of 90° in clockwise direction.
Ferrite Rod Thus signal obtained at port-2 do not have any
phase change. Thus gyrator gives phase shift of 180°
When signal enters at port-1
it passes through the when signal travels from port-1 to port-2. There is
circular waveguide and 90° phase change when signal travels from port-2 to
phase shift and is
entered in the transition. The role of port-1.
the transition
is to convert mode of
circular waveguide to the
equivalent rectangular waveguide Q. 9 With the help of schematic, explain
mode. the working
principle of an isolator.

WG
SPPU: Aug,15, May 17,Aug.17,Dec. 17, Dec.18
Transition
Transition
TE,, May 19, Dec. 19, 6 Marks
Ferrite Ans.: Isolator
dr rod
90 An Isolator is a device which
WG Wist is mainly used to
lE10 isolate particular component in
the microwave
system. It is N- port device,
in which signal
propagates only in one direction.
Foam
two ports are used.
Actual practice
This is also known as non-reciprocal
device. Ideal
Ferite isolator prevents propagation in one direction
completely. The construction
for it is shown in
TE10 Fig. 4.8.
TE11 2
Permanent Isolator contains following components,
magnet
TE10 1. Resistive Vane at each port
2. Ferrite Rod

ecton Output
V wavaguic
Reslstive of rotation
Fig. 4.7:Construction of Gyrator vane

Operation
H PORT-2

TE Magneuo
field
When electromagnetic signal enters at port 1, it Input Rellected Fernte rod
gets waveguldo wave
phase shifted, by 90° via circular waveguide vector
in PORT-1
clockwise direction.
Fig. 4.8:Construction details for Isolator

Ces Casy-soluuion
 Quick Read
Radiation and Microwave Theory (SPPU)|
wo port isolator is described as the sigual took place. No signal will reach to the
The working for
port-1.
follows
Thus signal gets isolated.
1. When wave enters at port-1, the clectromagnetic 7.

signal is parallel to the resistive vane. Thus resistive of circulator using two magic
a.10 Explain the operation
vane dose not absorb the signal. SPPU: Dec. 18,4 Marks
tees.
It passes to the Ferrite rod. The length of the rod is
2.
Ans.: Circulator
adjusted to give phase shift 45 degree.
A microwave circulator is a multiport waveguide
This 45 degree phase shifted signal is obtained at
3.
junction in which wave can flow from n" port to
port-2. power
(n +1)" port in one direction i.e. there no
is
4. When signal enters at port-2, it is parallel to the flow in reverse direction i.e. from (n + 1) "
port to
resistive vane at port-2. Thus no absorption of the nth port.
signal.
The four port circulator is shown in Fig. 4.9 whichis
5. This signal enters in ferrite rod. This rod does not famous. One type of four port microwave circulator
provide phase shift of 45 degree since wave is is a combination of two directional
coupler as
entering in opposite direction. shown in Fig. 4.9{b) and rectangular
waveguide
6. Thus this wave becomes perpendicular to the with two non-reciprocal phase shifter.
resistive vane at port-1.Thus complete absorption of |
Port 4

Port 1 Port 3

Port 2

(a) Symbol of Circulator

Coupler 1 Coupler 2
************
*********}
Primary gulde

Phase shifter

4 180 0=90 180

j...******************* Port 2
Port 1 ************************************"*: *** ******* ***************************
90 0: 180

******************.

Port 3 O4 = 90 270 Port 4

**************************** T

Phase shifter
. 90
***********************************************************************************

Secondary guide

(b) Construction of Circulator

Fig. 4.9:Schematic diagram of four-port circulator

Ces 0ASy-sO10tionS
 Quick
Radiation and Microwave Theory (SPPU) 1-10 i
Working principle of Circulator 1. Transmitter and receiver are connected to difs.
Fig. 4.10.
ports of circulator as shown in
90 phase shifted signal by directional coupler
enters in the phase shifter which further gives some 2. Three port circulators can be used in tunnel 10d
phase shift in particular
direction. Thus coupler 1 or parametric amplifier.
splits the incident wave on
it in two different 3. Circulators can be used as low power devices- as
components. This results in powers only.
arrival of the signal at they can handle low
port 2 with phase shift of the 180°.
Four port circulator can be used to construct hybrid
Thus two waves reaching to the
port 2 have same or magic tee.
phase as Input. Thus signal is
getting transmitted 5. Four port circulator can be used to construct phase
from port-1 to the port-2. But
since at port 4 arrived
signal is out of phase there is no shifter.
Output at this port.
In general, a. 11 Explain the operation of circulator using two magic
1-3 =
(2m+1) T rad/s tees. SPPU: Aug15, Au.17, Deo. 19,4Mark
.02-4 2 nmrad/s Ans.

m, n any integers = 0, 1, Implementation of Circulator Using Magic Tee

The sequence of power flow is, The circulator is a device which isolates the power
123411 given between its two consecutive ports. Basic
The perfectly matched, lossless and non-reciprocal block diagram for N Port circulator is as shown in
-

four port circulator has s matrix as, Fig. 4.11.

0 S12 S13 S14 The functioning for the circulator is explain as

S21 0 S23 S24
following:
s () When power is fed to P1, there is no power
S31 S32 S34 to P3,
P4 .PN and is available at P2
L S41 S42 S43 only.
0
This can be simplified as,
0 0 0 1
10 0 0

01 0 0o
L0 10
P2

Applications of circulator
Circulators can be used ás duplexer for. radar
antenna system as shown in Fig. 4.10. P
Antennaa Fig. 4.11: N-port Circulator
(ii) When it is given to P2,
power will propagate to
and not to P1. Also there is no Pa
power at Pa, Ps,
Radar
Radar
P6.PN
Rx ii) This indicates that power will
circulate from P1 to
P2 P2 to P3, Pa to P4.
T
Fig. 4.10: Circulatoras Duplexer
But there is no power flow
provided P2 to is not matched
from p1 to p3 directly
In many radar system, single terminated.
antenna is used to The four port circulator can
transmit and receive the power. Therefore, be constructed using
to have two magic Tee and 180°
ideally infinite and practically very high phase shifter and it is
isolation shown in Fig. 4.12.
between transmitter and receiver, circulator is
used.

es Dasy3O1UtIonsS
 Quick Read
Radiation and icrowave Theory (SPPU) 1-11
Magic Teo-2 The wave coupled through two holes will add in
Magke Teo-1
phase at port 4 of auxiliary waveguide. In port 3 of
the auxiliary waveguide, the coupled wave through
the two holes will add 180° out of phase and will
cancel each other.

180 The directional coupler is frequency sensitive. Only

at a certain frequency, separation between the holes
Shittor will be A/4 and the two waves in port 3 will cancel.
Fig.4.12: Circulator using magic Tee and 180° phase At nearby frequencies, the phase shift will not be of
shifter 180° exactly, and the two waves will not cancel each
other. Matched termination will dissipate the power
Working of circulator using two magic tee and 180°
reaching it.
phase shiter is explained with following steps.
1. When power is fed to port-1 i.e. Pi which is H - arm
For Fig. 4.13, the properties of directional coupler
are defined as:
of magic Tee 2, Half power with same phase is
Coupling = C= 10 log10 Pa/P4
available at l' and 2' port of magic Tee-2.
Directivity= D=10 log10 P4/P3
2. Power from 2" flows to second port of ma Te

Whereas power fornm 1' gets 180° phase shifted and Isolation = l= 10 log10 P1/P3 or
reaches to 1st port of magic Tee-1. = Coupling+ Directivity = C + D

port 2 ie. P2 of magic Tee-1 these two power gets Isolated port Coupled port
At (3) (4)
added and thus we can say that power flows from Pi
Output of In -auxillary waveguide
to P2 of circulator.
phase phase
3. Since 4' and 3' areH- arm and E arm of magic Tee
-
hole 2
2, they are completely isolated from each other. As
1
hole
shown in Fig. 4.12, 3' it magic Tee-2 is P3 of
circulator and 4' of magic Tee - 2 is H arm of
-maln waveguide

circulator. Thus there is no power flow P1 to P3 of

Input port
circulator Thorough or
direct port
4 Similarly when power is fed to P2, it flows to pa3

when fed to Pa it flows to Pa. Fig. 4.13: Basic principle of directional coupler

S. Thus we can construct circulator using magic Tee. S-matrix for ideal directional coupler is given by

Q.12 What is a directional coupler? Draw and explain the

S21 S22 S23 S2
operatión two hole directional coupler. IS]
S31 S2 S33 S34
SPRUADEC 16, May 7 May
Aug 7Dec 17 Doc 1
19, Dec. 19,8 Marks
L S1 Ss2 S43 S44 -

O Si2 0 S14
Ans.: Directional Coupler S21 S23
0 0
The basic directional coupler is constructed using (S S32 0 .4.22)
wo waveguides which are joined one on the top of L Sa
the other as shown in Fig. 4.13.
0 Ss3
0
In directional coupler, all four ports are perfectly
Two holes are drilled in the common surtace
at a matched,
distance Ag/4.
When RF power is fed in
S11 S22= Sa3=S44
port 1, the power is
coupled in main waveguide to auxiliary Power reflected from that port
waveguide
through two holes. The Power incident on that port
remaining power is
available at port 2. S11 = S22 = S33 S44= 0
 Qui
Radiation and Microwave Theory (SPPU)_ 1-12
Coupling = C =10 log1o Pi/P4
From symmetric property Sij = Sji
.S23 S32, S13 = S31 S24 = S42 S14 or S41 = 0.05 < 90

S34 = S43, S41= S14 Directivity = D =10 log10 P4/P3 = Ss3

Ideally back power is zero. S34 0.96 <0

=

No coupling between port (1) and (3) Isolation= I=10 log10 P1/P3
S13 S31 = 0, on the same lin S13= S31 = 0.1 < 90

S24 S42=0
. We can represent s-matrix for directional coupler Q. 14 A signal of power 20 mW is fed into the one of the
t

Determine tha
by Equation (4.22). collinear ports of the H-plane Tee.
powers at the remaining poris when other ports are
Q.13 Ascattering matrix of a directional coupler
is: teminated by means of matched loads.
0.05 230 0.96 20. 0.1 290 0.05 290 7 SPPU: Aug. 16, Aug. 17, May 19,8 Markg
0.96 20 0.05 230 0.05 90 0.1 290 Ans.
IS]=
0.1 290 0.05 290 0.04 230 S-parameters for H plane tee are given by
0.96 20
Equation (4.24).
0.05 290 0.1 290 0.96 20 0.05 230
Find: Directivity, 1/2 -1/2 1N2
() Coupling factor, -1/2 1/2 1/2 4.24)
(i) Isolation, and
(i) Return loss at the input pot when the other ports
L 1N2 1N 0

Power fed to Collinear Arm is 20 mW

are teminated in matched loads.
As H plane is symmetrical device
SPPU: May 18,7 MarksS
IS12l IS21l =20 mW.
Ans.: For Fig. 4.13, the properties of directional coupler
are defined as: Thus power at another collinear arm is 20 mW.
Coupling = C=10 logi10 P1/P4 To know power at H-arm.
Directivity D=10 log10 P4/P3
=
From Equation (4.24),
Isolation =I = 10 log10 Pi/P3 S13l S23l= |S31l= IS32l =
1/2
S-matrix for ideal directional coupler is given by Therefore,
S1 S12 S13 Si4
Power at H-ARM = y2 x Power at collinear arm
S21 S22 S23 S24
IS =
S31 Sz2 S33 S34
2 * 20 = 28.28 mw ...Ans.
Q.15 An isolator has an insertion
Sa1 S42 S43 S44 loss of 0.5 dB and an
isolation of 30 dB. Detemine the
S12 0 S14 scattering matrix of
S21
the isolator if the isolated ports the perfectly
0 S23
0 matched
IS 0 S32 0 S34 4.23) to the junction. SPPU: Aug,16,4 Marks
L S41 Sa3 Ans.:
0
For Equation (4.23) the properties of directional
S11 Si2
29.57
S21 S22 J o
coupler are defined as:
Power at port 2 is not 30.
 Radiation and Microwave Theory (SPPU) 1-13 Quick Read

Chapter5 : Active Microwave Components

(a) When transit time across the gap is negligible

Q.1 What are the limitations of conventional tubes at
microwave frequencies. It-is assumed that the gap spacing and frequency is
SPPUR Dec.15, Dec. 19,4 Marks such that the transit time through gap (Buncher
grids) is small fraction of a cycle and is negligible.
Ans.: Limitations of conventional tubes
The velocity of electrons before entering the
There are few certain reasons for why not to use
buncher grid is given by,
conventional tubes at microwave frequencies
Do V2n Vo m/s .(5.1)
1. Inter electrode capacitance effect.
Lead inductance effect. Charge on electron
2. m Mass of electron
3. Transit time effect.
The energy of an electron emerging from the gap at
4. Gain bandwidth limitations.
time t, is,
5. Effect due to radiation losses.
eV = e[V% + V1 sin wt, ].
6. Effect due to RF losses.
7. Skin effect. eVo 1+V sin ot 5.2)
8. Dielectric losses.
V
=
Vo+ Vj sin wt,Instantaneous voltage at
Q.2 Explain the concept of velocity modulation with the time t
help of apple-gate diagram. How this is used for the
This energy is equal to kinetic energy mu' where
construction of microwave sources?
SPPUR May 16, 9 Marks is the velocity of electron at time t.
Ans.: Velocity Modulation
mv V
Velocity Modulation is the key procedure for any
evo1sin ot Vo

microwave tube. This is the process in which basic V

velocity of the electron beam is changed according
.u 2n Vo1sin oti
to RF 1/P. When electron beam- is escape from the D = Vo V1+a sin ot 5.3)
cathode, it enters into the accelerating grid. This
= 2n V% is injected velocity
acceleration is sufficient for beam to reach next grid
in the tube. This grid further changes velocity of
beam as per the requirement
and a. Vis known as Depth of Modulation.
Here either acceleration or de-acceleration occurs As V Vo ie. a <« 1, Equation (S.3) can be written
which results in change in beam velocity. Thus when as,
beam exits this grid. It is velocity modulated. at,
The analysis for velocity modulation is carried out
V V(1asin (5.4)

with following assumption: It is noted from Equation (5.4) that successive

1. When electrons leave the cathode its velocity is electrons are not travelling with same velocity when
zero. Also we will assume that electron density is they leave the first gap.
uniform. The electrons which leave the gap at a time given
2. We will neglect the space charge. below, will experience no change in velocity,
3. The magnitude (V) of input microwave signal is ot= 2tn; n=0, 1, 2, 3, .
much smaler than de-accelerating voltage (Vo) i.e. | The electrons which leave at a time corresponding
VV to,
We will carry out the analysis for velocity
modulation for following two different cases,
es casy-SOUtions
 Quick R

Radiation and Microwave Theory (SPPU) 1-14 Buncher Catcher

Accelerating cavity
caviy cavity
The maximum accelerating phase of buncher anode
will be moving with maximum velocity. Cathode
Dnft space
Umax

The minimum accelerating phase of bunches voltage

will be moving with minimum voltage: Velou
min 1-) Ltds -L+2d
Thus the electrons which leave during the 'time input
t=0RF RF output

20 t 0 will slow down and those leave during

Klystron
0<t20
20
will move fast
Fig. 5.1: Schematic diagram for two cavity
amplifier
Equation (5.4) the equation
is of velocity
modulation took place inside
modulation of cavity frequency '0'. 3. The process of velocity
buncher cavity has two
the buncher cavity. The
One electric field is
Q.3 Explain constuction, operation and applications of electric field present within it.
is due to RF input to
two cavity klystron ? due to electron beam and other
be amplified. The force attraction
(during positive
SRRUEMay18,Dec. 19,6.Marks (during
half cycle of the RF input) and repulsion
negative half cycle of the RF input) between
Ans. two

field changes velocity of the electron beam. This

Two Cavity Klystron Amplifier
process is termed as 'Velocity Modulation'. The
The two cavity Klystron is used microwave depth of velocity modulation -is directly
amplifier. Its operation is based on the following proportional to the peak amplitude of the RF input
two principles to be amplify.
1. Velocity modulation of the electron beam. t. This velocity modulated beam passes through the
2. Current modulation of the velocitý modulated drift space of length 'L.
electron beam. 5. This drift space is drifted from external potential.
The basic principle of Klystron amplifier, let us This means that no external potential is present in
assume that the electron beam is having uniform this drift space.
density. Also DC accelerating voltage is much 6. Thus velocity modulated electron beam starts giving
greater than microwave signal which is to: be up its energy.
amplify. This energy is collected by catcher cavity which is
7
Operation of two cavity Klystron amplifier placed at the end of the drift space.
8. Thus kinetic energy of the velocity modulated
The basic principle of operation is described in
electron beam is converted to its equivalent
followingsteps
electrical energy across the load. This load is
1. The electron beam is emitted from the cathode connected across the catcher cavity. This is also
surface. passes through the accelerating beam.
It known as 'Current Modulation'.
The accelerating beam focuses beam into a sharp
electron beam. High velocity electron beam
9 The magnetic field across the electron beam helps
him to move axially without spreading.
produced by accelerating anode.
10. The process of velocity and current modulation is
2. This focused beam now enters into buncher cavity
explained with help of Applegate diagram. This is
and is shown in Fig. 5.1.
drawn between time and distance and is shown in
Fig. 5.2.
es 0asy-SOJutios
 1-15 Quick Read
Radiation and Microwave Theory (SPPU_
Output gap
How reflex klystron generates microwave oscillations?
oistanca voltago
1. Schematic diagram for Reflex Klystron is as shown
Catcher
gina in Fig. 5.3. Electron beam is generated from the
-Fastest veloaty cathode surface and is accelerated by accelerating
Unchanged velocity grids.
Slowast velocty
2. This beam passes through the re-entrant cavity.
Burcher Thus this cavity has two 1/P, one is electron beam
grd
and other is RF noise or RF Modulating lP which
Vo / +T/4 is
-T74
present in cavity.
Klystron
Fig. 5.2: Applegate diagram for two cavity 3. Thus there are two electric fields present within
amplifier cavity which interact with each other.
The following steps gives mathematical analysis
of
Either force of attraction or repulsion exhibits
velocity modulation and current modulation for two inside cavity.
cavity klystron amplifier. This changes velocity of the electron beam. During
5.
Applications of Two Cavity Klystron positive half cycle, force of attraction increases
amplifier of microwave signal for: beam velocity. Whereas force of repulsion during
1. As Thus
negative half cycle decrease beam velocity.
) In UHF TV transmitter.
velocity modulation occurs.
troposphere scatter transmitter.
(ii) In . The depth modulation is dependent on the cavity
6.
(ii) Satellite communication ground station. gap d'as well.
(iv) Radar transmitter. This velocity modulated electron enters in the drift
2. As power oscillator used as Klystron oscillator. .space.
diagram 8. The length of the drift space is L.
Q.4 With the help of construction and applegate
explain woking of Reflex Klystron ? 9. When velocity modulated beam enters in the drift
SPPU: Dec. 16; Dec. 17, Dec, 18, May 19,8 Marks space, it experiences force of repulsion from the
repeller. The repeller is nothing but metallic plate
Ans.
placed at distance 'L' from the re-entrant cavity in
:

Reflex Klystron the drift space. The repeller is always at the negative
In case of two cavity klystron amplifier if output potential.
voltage is fed back to input and if positive feedback 10. Since repeller is at the negative potential, it exerts
is taken then klystron will oscillate. But two
cavity force of repulsion on the velocity modulatedd
klystron oscillator is not used because the slight electron beam. Thus electron within beam pushes
change in frequency is necessary to tune both cavity back to the cavity. This acts as the feedback.
proper positive feedback
to get 11. By proper selection of the drift space L' and repeller
This disadvantage of multi-cavity klystron
is
voltage VR', this feedback can be positive one.
known
Overcome by single cavity klystron which
is
12. Thus here process of velocity modulation results in
as REFLEX KLYSTRON. amplification and repulsion of the electron beam
Characteristics of reflex Klystron from repeller results in positive feedback. Thus
amplifier.along with positive feedback results in
1. Low power generator of 10 to 500 MW at frequency
oscillations at the microwave frequency.
range of 1 to 25 GHz.
13. Thus reflex klystron generates oscillations.
2. Efficiency = 20 to 30%.
are
14. Thus these entire velocity modulated electron
Applications of reflex Kiystron
bunched together.
1. Mainly used in laboratory as microwave source loosing there
15. When these are bunched, they starts
kinetic energy in the re-entrant cavity.
2. Also used in Radars.

escasySOfUiionS
Radiation and Micnow.ave Theory SPTUL 1-10
l6. lf enery loose by electron uncth is greater tin
losses wvithin cavity, electroagnetic power
nerated. - Eunh
17. These microwave oscillations are given to the loaa
connected across the re-entrant cavity. Dislaixxy

18. The Applegate diagram

for the conplete process 1s
as shown in Fig. 5.3.

hodor
AA
RF Fig. 5.4:Applegate diagram for Reflex klystron
output

RF DP= Vs
Mode of oscillation for Reflex Klystron
t Cavity
Ekctron
bOum
anode(A)
Initially electron beam s
velocity modulated beam
repelled by repeller to the cavity back. When this
Acoslarating gd(G)
beam enters back to cavity it give up its energy. At
thoda what instant energy is bunched to the cavity decides
EBlectron Vs=V, sn(ut) various operating modes for the reflex klystron.
gun :RF input
Fig. 5.5 also shows that
amount of power collected
and mode of oscillations depends on distance
Fig. 5.3: Schematic for reflex klystron between cavity and repeller as well as repeller
voltage.

Frequence.

GHz AF Repeller voltage in volts

node
mode
3 mode
Power
output
in
mW

Repeller voltage in volts

Fig. 5.5: Mode curves for Reflex
klystron

Ces casy
auuiona
Radiation and Microwave Theory (SPPU) 1-17 Quick Read
If T' resonant frequency, to
is the time period at the Do

is the time taken by reference

electron to travel in
repeller space between entering the repeller space
at b' and returning the cavity at positive
peak Cathode

voltage on formation of bunch, then

n)T
t = NT
db
N
n+n=0,1,2,..
Anode
Thus by adjusting repeller voltage for a given
dimensions of Reflex klystron, bunching can be Output coupling
loop
made to occur at N =n+ positive half cycle. Magnetron
N =
Accordingly mode of oscillations are labeled as Fig. 5.6: Magnetron resonant modes in a magnetron

,2for modes n = 0, 1, 2, . respectively. DC voltage Vo in between cathode and anode. This

generates radial electric field.
It is obvious that lowest order mode occurs for
The permanent magnet is placed along the z
maximum value of repeller voltage when transit direction which generates magnetic field Bo.
time t of electrons in repeller spaçe is, minimum. Thus electric field is along the radial direction
Higher modes occur at lower repeller voltages. Since whereas magnetic field is along the Z-axis. Thus
at the highest repeller voltage acceleration of both are perpendicular to each other. Therefore
bunche electrons on return is maximum, the magnetron is Crossed Field Device.
power output of lowest mode is maximum. This is Due applied DC voltage electron is emitted from the
shown in Fig. 5.5. cathode surface. This electron experiences two
forces which are perpendicular to each other. Thus
a.5 Explain construction and principal of operation of
the resultant force acting on the electron enforces to
cylindrical magnetron
to move along the radial path.
SPPU: May 18, May 19, 6 Marks it
This mathematically represented as given below.
Ans.
F eE- e (U xB)
Magnetron
U Velocity of electron
A magnetron oscillator is used to generate high
This electron is also experiences electric feld due to
microwave power, This word magnetron can be
RF Noise present within cavity.
split as magnet-electron which implies that device
which uses both electric and magnetic field for its During positive half cycle of the RF noise electron
operation. It is cross field device in which electric experiences force of attraction. Whereas during
and magnetic field are perpendicular to each other. negative half cycle it experiences force of repulsion.
Schematic for it is as shown in Fig. 5.6. Thus velocity modulation occurs.
It consists of cylindricalcathode of finite length and The accelerated electron when retarded by RF field,
radius 'a' at the centre surrounded by a cylindrical transfer energy from electron to cavities to grow RF
anode of radius 'b'. oscillations till the system RF losses balances the RF
In magnetron anode is slow wave structure. It oscillations for stability.
consists of. re-entrant cavities around
circumference of the cylindrical cathode. The
thea.6 What are cross field devices? Explain the cavity
magnetron with hull cut off condition in detail.
physical separation between two consecutive
cavities is constant. SPPU: May 17, 8 Marks
es casy-soiutions
 Radiation and Microwave Theory (SPPU)_ Quick
1-18 Rea
Ans. Vo the electrons
Thus if Bo> B, for a given
Cut-off Magnetic Field and reach the anode. For a given magnetic field BOR
Voltage h
From Fig. 5.6, emitted cut-off voltage is given by,
electron from cathode
experiences following three
1. -
forces
eE> Force due to electric field.
m-b1
V. for a given B,, the electrons will not reas
If V<
-e (VxB)> Force due to magneticfield.
the anode.
3. Centrifugal force mv In above Equation V. and are called Hull cutof
Be
r
voltage and magnetic equations.
For equilibrium mv2 + eE = evB
The nature of field distribution in magnetror
The electric field is in radial cavities is such that alternating RF magnetic flu
direction.
lines pass through cavities parallel to cathode axis
Letdt - emBz rdr
d 22 da
and alternating RF fields are concentrated acros
the slot and fringe out to the interaction space
OR between the anode and cathode in the transverse
Or direction.
d k
For N resonant coupled cavities of the anode, there
=
k Integration constant exist N resonant frequencies or modes.
Atr = a Since the slow wave structure is closed on itself, the
total phase shift around the internal periphery must
dt 0 be an integral multiple of 21t for possible

k - oscillations.
The phase shift between two adjacent cavities is
Since the electrons move in direction perpendicular given by,
to magnetic field, the anguiar energy of electron is,

n 0,t1,t2., tN/2, indicates nth mode of

oscillation.
Since electrons move in directi perpendicular to
magnetic field, the kinetic energy is given by electric n# 0 because this would indicate
zero RF fringing
field only fields in the interaction region.

Q.7 Explain the phase focusing

eV effect in cavity magnetron.
SPPURDec. 15, May 16, Deci
Atr = b, 16,Dec,173.Marks
Ans. Phase Focusing and Control
V
=
Vo and dr/ dt =0 for the electrons to just grazeMagnetron of Back Heating in
anode so that,
The bunching of electrons in known as
"Phase
focusing effect". In case of magnetron, the
number of
cavities decides the phase difference between
two
Put
eB consecutive cavities. If the electron was
=
m at grazing not focused,
it won't contribute in process of
oscillation
generation.
This effect is important because without it,
B electrons will fall behind the phase
favored
change of
electric field across the gaps. Such
electrons are
retarded at each interaction with the R.F field in

Ces 013Vsolutions
Radiation and Microwave Theory (SPPUL 1-19 Quick Read
focused For proper interaction of electron beam with RF1/P,
magnetron. If the particular electron is not
which affect life of Slow wave structures are designed. These
it may produce heating
structures elongates path of propagation whiich
magnetron.
scattered in any results in delay i.e. reduction in effective velocity.
The unfocussed electron may get
generated The commonly used helical structurc is shown in
direction which affects the power of
as Fig. 5.8.
oscillations and produces heat. This is known P
between Pilch
"Back Heating"'. Proper electrical length
two cavities can control back heating.
structures.
Q.8 Explain the different types of slow wave
with the
Brief the operation of travelling wave tube (a) Helical coil
help of neat diagram. Pitch P
SPPU:Dec: 15, May 16; May 17
Dec.18, May 18,9 Marks
(TWT)
Ans.: Travelling Wave Tube nd
x=Vp+
Travelling wave tube uses slow wave structure.
Slow Wave Structure
(b) One form of helix
Slow wave structure is non-resonating circuits Fig. 5.8
which are purposely designed to obtain high output Vp along the pitch to
The ratio of the phase velocity
power microwave frequency range.
high phase velocity along coil is given by,
The limitation of LC resonating circuits at
frequency is with increase in frequency both tch Pitch sin y
inductance and capacitance of resonant circuit must
x p?+ T7d
be decrease. Also gain bandwidth product
is limited c 3x10" m/s is the velocity of light in free
LC resonator cannot
by the resonant circuit, the space
generate a large. output. To overcome above P Helix pitch
problem slow wave structures are used. They are
which d Diameter of the helix
also called as non-resonant periodic structure
is designed' for producing large gain
over à wide Pitch àngle
bandwidth. The phase velocity in the axial direction within
These are available in various forms such as
helical dielectric filled tube is given as,
line, Zigzag line or corrugate waveguide and many
UpE=
Pitch
more. These structures are shown in Fig. 5.7.

Q000 The above equation that if t is too high, slow wave

structure may produce loss in micro wave devices
which reduces the efficiency.
(a) Helical line
For small pitch angle y, phase velocity along the coil

We (b) Zigzag line

in free pace is give as,

p
TtdB
PC

Equation (5.5) concludes that a circuit to be a slow

(5.5)

eZZZIIIDIIINIZZIIA wave structure, is must have property of periodicity

of some of
in the axial direction. The phase velocity
obtained
the spatial harmonics in the axial direction
(C) Corrugated wave guide waveguide field may be
by Fourier analysis of the
Fig. 5.7
smaller than velocity of light.
C@s casy-SOfutionS
 Radiation and Microwave Theory (SPPU)_ Quick
1-20
Lets how input gets amplified by slow wave The helix is made positive with respect to catig
structure. As we know that very basic structure o and collector.
micro wave contains electron gun
at one end and Since helix is at positive potential it gets attract
electron collector arrangement at another end of the
tube. The slow wave structure has
towards the anode.
been placed in
between the electron gun and
electron collector
assembly.
Charge density
Generally electron beam moves along inelectron beam
the axis of the
slow wave. Whereas RF field is conducting
through
slow wave structure. Velocity

This is described by following

beam is greater than
velocity of RF signal. This helps to
transfer energy.
equations
AA
F = -
eF
E VV .(5.6)
Since the DC. velocity
of electrons is slightly greater
than the axial wave velocity, more electrons
are in
the retarding field than in the accelerating field and Input end
a great amount of energy is
transferred from beam
to electromagnetic field. Voltage
in TWT Output
The bunch continues to become more compact
and end
a large amplification of signal voltage occurs Distance along the
at the interaction space
end of the.slow wave structure. Thus amplification
Ocurs by slow wave structure. Fig. 5.10: Growth of signal and bunching along TWT
A structure of TWT using helix is as shown in The signal to be amplified is applied at 1/P
terminal
Fig. 5.9. of the slow wave structure.
aF input The interaction takes place between
electron beam
soLENOIO MACNET
and RF 1/P.

AF output At the first winding of the helical

structure for the
Cathodo first time beam interacts with RF
I/P.
Beam
This interaction changes beam
velocity. The processS
is known as Velocity Modulation'.
This velocity modulated beam
Heux again undergoes the
process of velocity modulation
Accelerating sOLENOID MAGNET
RF Attenuator when it enters in
olectrode second winding of the helical structure.
Fig. 5.9:Schematic Diagram of Travelling wave tube Thus speed of the electron beam is
given as,
There is an electron gun at one end of the tube. Speed of electron beam =
Velocity of light
Anode is placed at another end of the tube.
helix pitch
Slow wave helical structure is placed between helix circumference
cathode and anode. Thus each winding of the helical
structure acts as
Electron gun produces narrow beam of electron amplifier. Thus total helical structure
is nothing but
which passes through the axis of the helical slow amplifiers connected in cascade.
Travelling of wave
wave structure. is as shown in Fig. 5.10.

The magnetic field generated by solenoid prevents At the anode end bunching
occurs which transfers
spreading of the beam. energy from field to the load.
Thus this interaction results in high
gain.

Ces casy-sOIuTions
FRadiation and Microwave Theory (SPPU)_ 1-21 Quick Read

Along the forward path of the propagationAns.

attenuator is placed. There are two main roe| 1. Cyclotron angular frequency
performed by the attenuator. The first is to control eBo
1.76 x 10x0.34
amplifier gain so that it won't go to saturation.
=

It also prevents reflections

from the load side to the
0 59.84 Rad/S
cathode. If these reflections get added with the
2. The cutoff voltage
amplified signal it may acts as positive feedback.|
Thus it may results in unwanted oscillations.
VHC
The gain of TWT amplifier is given as, 8m
Output voltage 034x176 100on
Gain20 108 Inputvoltage .

Gain - 9.54 + 47.3 N.C. dB VHC 143.05 kV

-9.54dB Loss
3. Cut-off magnetic flux density = Bc

N Length of interaction
Bx 25 x 103
Length of slow wave structure Bc
y8xVoxm/e 1.76x101
10l-)
Bc 142.13 uwb/m

Q.11 A two cavity Klystron amplifier has the following

parameters:
C Gain parameter = 4V
Beam voltage: V,= 900 V
= DC beam current
DC beam voltage
Beam current: = 30 mA
.V. Frequency:f=8 GHz
2 Characteristic impedance of helix structure
Gap spacing in either cavity: d = 1
mm

Q.9 Distinguish between TWTA and Klystron tube. Spacing between centers of cavities: L= 4 cm
SPPU:Dec, 16, Dec. 17,8 Marks Effective shunt impedance: R =40 k2
Ans.: Comparison for TWTA and Klystron Amplifier Determine:

Sr. (a) The electron velocity.

Klystron amplifjer TWTA
No. (b) The d.c. electron transit time.
.
1 Linear beam or "0' | Linear beam or '0' type (c) The input voltage for maximum output voltage.
type device. device.
(d) The voltage gain in decibels.
2. Uses cavity for input | Uses non-resonant wave
circuit.
SPPU: Dec,15,9 Marks
and output circuit.
3. Narrow band device Wide band device due to Ans.
due to resonant non-resonant wave Given
cavity. circuit.
Beamvoltage = V,=900.Volts,
Q.10 A pulsed cylindrical magnetron is operated with the Beam current lo= 30 mA
following parameters :Anode voltage = 25 kV, Beam
current=25A, Frequency = 8 GHz

Magnetic Hlux density = 0.34 wb/m; a = 5 cm, d 1mm

b= 10 cm. L
4 cm
Calculate:
Rsh=40k2
() Angular frequency. (i) The cut off voltage.
i) Cut off magnetic flux density. 9Marks
Ces 0asy-Solutions
 Radiation and Microwave Theory (SPPU)_ 1-22 to give
input gap voltage
(a) Find the
1. Electron velocity = Uo = 0.593 x 106V voltage 2 negíecting the
= 0.593 x 105900 (b) Find the voltage gain,
output cavity.
10 m/S loading in the
= 17.79 x
eficiency of the amplifer, neg
.
nec
Transit Time = »To (c) Find the
x 10-2 beam loading.
. To 4a17.79x 105 (d) Celculate the
beam loading
conductance
justified
neglecting it was
= 2.24 nano Seconds show that
preceding calculations.
= 2.24 x 10-9 Seconds SPPU May17,10 Mar
Maximum I/P voltage = Yox 3.68 maximum. Thi
V2 J X) must be
B8o Ans. For maximum
:
=
X 1.841.
means ), (X) = 0.582 at
Oo = To= 2 nfx To
=0.593 x 10 xVV
27tx 8 x 10° x 2.24 x 10-9 Beam velocity = v,
0.593 x 106xV103
= 112.59 Radians
2 nt v 1.88 x 10 m/s
gap transit angle =
d
0.001
The 6 0
= 2nx8 x 10°* 17.79 x 10°
, 103
21tx3x10'*1.88 x 107
2.82 Radians
= 1 radian
6 in degree x2.82
T
The beam coupling coefficient =B;=B.
= 161° sin(O/2)sin (1/210.952

SinsinJ-0986 The DC transit angle

1/2
between cavities is
B Sin 98
10-2
2173x10)*4x
L

Bi =
=T o 1.88x107
0.493
Vox 3.68 900x 3.68 = 40 radians
Vi max = 0.493 112.59
Bix Go x
2V. X
Themaximum 1/P voltage= Vimax B
V max = 59.66 Volts

2x(10) (1.841) =
The voltage gain = B y*
(0.952) (40)

L07
96.5 Volts

0.493 x 112.59 x 0.582 x 50 x 102 (i1) The volitage gain = Ay=R

40 x 103 x 1.841
XJX*h
8.595
21.83 26.78 dB
iii) Eficiency
a. 12 A two-cavity klystron amplifier. has the following 21,V.
parameters: 21,J X)
Vo1000 V Ro 40 ks2
2x 25x 103x0.582 = 29 mA
MA
o25 f=3 GHz V Pols Ra
Gap spacing in eilther cavity: d=1 mm
0.952 x 29x 103 x 30x 103
Spacing between the two cavities: L= 1 cm
831 Volts
Etfective shunt impedance, excluding beam loading:
Rsh 30 kS2 Efficiency= Bol:V 46.2%
21,V.

es easy solutionS
 Quick Read
Radiation and Microwave Theory (SPPU 1-23
Klystron has the following
oporates under tne 1ollowing
Q. 14 A two cavity amplifier
o. 13 A rollex klystron characteristics
15 kS2, e/m = 1.759 x 10,

,conditions Vo= 600 V, Rsh=

9 GHz, L =
1 mm
cavity. Tho tube
Voltage gain: 15 dB
Input power; 5 mW
L spacing botwoen repollor and
is the
2 mode or cavity Rsh: 30 ks2
is oscillating at 1, at
the peak on n Total shunt impedance of input
transit time through the cavity Rsh: 40 k2
Total shunt impedance of output
mode. Assume that the
can be neglected. Load impedance at output
cavity R: 40 k2
gap and through beam loading
(i) Find the value of repeller
voltage Va. Determine:
give' microwave gap of (i) The input voltage (Vms)
(ii) Find the dc necessary to
voltage of 200 V. (Gi) The output voltage (Vms)
in watts.
efficiency. (ii) The power delivered to the load
(ii) Calculate the electronic 9IMarks
SPPUKMay18, 9 Marks

Ans.
Ans.
=
600 v, mm;
5x10"x 30 x 103
L 1
Given: V, = =
) Pin PinXx Rsh (Input)
Rh 15 k 1.759x 101.f, = 9 GHz . Pin 150
Find repeller voltage V,
V = 250
w8 mL-V
-V, V1 = 12.24 Volts
Ve2m-) Input RMS voltage= VRms (Input) =

VRMs
12.24 volts
(output)
2 XTX 9x 10)x 8x (10-3)x600 600 (ii) Gain Ay= z0108 VRMs (lnput)
dB
V,
1.759x 1011 2 mx2-5 VRMS (output)
15 = 20 log 12.24
1.534 x 101 600 VRMs (output) = 68.83 Volts
V 2.126 x 1013
(iii) Power delivered to load = Pout
= 849.43 -
600
V, 249.43 V VRMS (Output) (68.83)2

To find directcurrent or Ho
Rsh (output) 40x 103
2], Cx) Rsh
Pout 118.44 milli Watts.
V Cavity gap yoltage = 200 V

First order Bessel's function = 0.52 a. 15 A reflex klystron operates under following condition
X) =
15 k,
Vo = 600 V, L =
1 mm, Rsh
Rsh
15 k2 =
e/m = 1.759 x 10, fr 9 GHz. the tube is oscillating9
200 = 12.82 mA. 4 mode.
at fr at the peak of the n = 2 mode or
1
o2x 0.52 x 15x 10°
2X,(X) Assume that the transit time through the gap and
To find efficiency =
tn-n/2)
(2 beam loading can be neglected.
2x2.408 x 0.52 () Find the value of repeller voltage Vr

2 7px
2-2) i) Find the direct current necessary to givea
Microwave gap voltage of 200 V.
2.504
10.99 (ii) What is electric efficiency undr this condition.
9 Marks

es 0aSVESolutions
 1-24 direct current lo»
Theory (SPPU) To find
Radiatlon and Microwave
Ans.:
Vi
Rsh
600 V;
I = lmm; 2X) V
gap voltage = 200
Vo=
1.759x 1011
V= Cavity = 0.52
h15 k2 order Besse's function
J1 = First
(X)
9 GH
S =

Find repcller voltage V,

Rsh15k2 12.82 mA.
200 =

w8 ml4V o 2x 0.52 x 15 X
10
Vo
V,
2X (X)
e2 n-2) To find efticiency (2 tn- n/2)
(2xTx9x 10)x8x (10") x 600 600 2x2.408x 0.52
V 1.759x 102mx2-5) 2x2-5)
=
1.534 x 1019 600 849.43 600
2.504
1 10.99
2.126x 105
V
V 249.43
 (SPPU)_ 1-25 Quick Read
Radiation and Microwave Theory
Chapter 6Solid 'State Microwave Devices
The microwave devices can be broken down into
time devices? Explain
Q.1 What are avalanche transit two groups which are active and passive devices.
lMAPATT
construction, working and applications of The Table 6.1 shows classification of microwave
SPPU Deo May 18.6iMarks
diode. devices.

Ans. Very first category of devices is active devices which

IMPATT diode consist of various solid state devices. Various solid

been state devices along with their construction,

Devoted research in would not otherwise have
principle of working, applications and their
possible semiconductor microwave devices and electrical and mechanical limitations.
an
circuits, results in a tremendous proliferation of
The begins with treatment of microwave devices
improvement in semiconductor devices for beautiful
switching, such as PIN and varactor diode. Very
microwave amplification, oscillation,
application of varactor diode is in parametric
limiting, frequency multiplication and other
devices.
function.
PIN diode is the device which is exist property of
For the system designer, the result, of these
variable resistance of microwave frequency.
continuing endeavors has been greater of flexibility,
Therefore they are used as attenuator, modulator at
improved performance, greater reliability, reduced
microwave frequency.
sizes and reduced power requirement.
.The next microwave device which is Varactor diode
Due to which system designers are able to produce
mainly used as electronic tuner. The parametric
very efficient and sophisticated Lwave system that
amplifiers which is important application of
would not otherwise have been possible.
Varactor diode is the next topic of the chapter.
As we are aware that when device is made from
These are low noise amplifiers and can be build with
semiconductor material, main property of it is the
the help of variation of reactive parameters.
junction property. So we can conclude that to
Table 6.1 : Various Miicrowave devices
modify device property we have to play with Microwavo Dovices

junction property of semiconductor material only.

Passive
the frequency of operation increases,
ACUvG
o As
Diroctional couplor
semiconductor junction has to be made more and Solid stato Vaccum tubo Attonuators
more thinner. But to achieve high power from such
- Phaso shiftors

Cross iold
Antonna
thin semiconductor junction in microwave Undar beam
Rosonalors
, frequency band is quite difficult. Klystrons
LMagnotrons

Traveling wave tubo

Therefore other type of devices had been invented Hybrid tubes

which do not depend on junction behaviour and Solld State Dovicos

which are capable to produce enough power.

Translstors Transtorred Avalancho Quanlumn
Transit Timo Elocttonic
Above results mainly into two categories which are clcctrons

BJT Gunn diode BARITT diodo FRuby Masors

famous for such a type of devices and which are as -FET FLSA diodo IMPATT diodo Semiconductor laser
following: HEMT InPdiodo FTRAPATT diodo
-cOTe diodo Paramelric Devicos
1. Avalanche Transit Time Devices
Tunnelling phenomenon of the diode is responsible
2. Bulk devices.
for negative resistance in tunnel diode due to which
Avalanche transit time devices are designed to have microwave oscillations occurred.
minimum transit time of charge carriers whereas
The Gunn effect and Gunn diodes so called after
bulk devices design is based on bulk property of are bulk
inventor are next created. These
semiconductor material rather than junction semiconductor devices responsible for negative
property. power oscillators at
resistance. These are medium
DOsySOONIons es
 Radiation and Microwave Theory (SPPU)_ Ouick
1-26 Re2
microwave frequencies. Mechanism who is responsible of tunnelling
Another class of power devices depends on Electron from N-type of semiconductor can climb ou
controlled avalanche to the depletion layer and enter into P-type if and ony
produce microwave
oscillations or amplification. The IMPATT ifit possess sufficient energy.
and
TRAPATT are two
main devices of this family. But quantum mechanism shows that, there is
1
Q.2 Explain the working principle of small but finite probability that an electron which
tunnel diode.
has insuficient energy to climb up potential Tier
SBPU Dec.15, Dec.16, May.17 can go on other side of potential barrier, if poten
Dec. 17, Dec. 18 May 19.8 Marks barrier is very thin.
Ans.: Tunnel Diode Thin potential barrier is a result of heavy dopina
Tunnel diode is P-N junction semiconductor When this electron goes to other side of potenda
diode.
This device negative resistance barrier, it does not lose any energy. This i tunneling
when forward
biased and due to which it can be used as oscillator phenomenon which is responsible for the behaviouy
and amplifier. of the tunnel diode.
Thin depletion region of this diode To explain the behaviour of Tunnel Diode we wil
generates oscillations
at microwave frequencies. take the help Characteristics of Tunnel
of V-I Diode
[Fig. 6.2(a)] and its equivalent circuit diagram
Construction of tunnel diode
Fig. 6.2(b)
Basically it is a P-N junction device. The difference
between regular rectifier diode and tunnel diode is Curen Tunnel
that, tunnel diode is heavily doped. Heavy doping T
Giodo 4 Normal rectiter
results in a very thin depletion region. Due to which P
diode
transit time of charge carriers to cross the junction
reduces which allows the microwave operation.
Germanium, gallium antimonide [GaSb] and gallium
arsenide (GaAs) are preferred to fabricate Tunnel
diode. All these materials are having small Vollage
forbidden gap and high ion mobility which results in
good operational characteristics of diode which
leads to high frequency and speed of operation.
The construction of tunnel diode is as shown in the
Fig. 6.1(a).

(a)
ZIIA -KOVAR
Tin dot
GaSb, GaAs or Ge pellet W-
*"*******lt"d**is*aat*ase*

KOVAR
in
(a) Construction of tunnel diode -R

Anode Cathode
* *****i

(b) Symbol of tunnel diode Tunnel

Fig. 6.1: Tunnel Diode (a) Construction of Tunnel Diode diade
(b)Its Symbol
(b)
peration and V-I Characteristics of Tunnel Diode Fig. 6.2: (a) V-I characteristics of tunnel diode
(b) Equivalent circuit of tunnel diode
Consider one phenomenon which is Quantum

Cescasy-501u1ons
 (STPPU)_ 1-27 Quick Read
Radiation and Microwave Theory
potential is zero volts Thus when applied potential is zero, the levels to
1. When applied which energy states are occupied by electrons one
either side are at same height. Therefore electrons
Enoroy
Thin dopetion region can tunnel from one side of junction to other side
due to thin depletion layer. But tunnelling current in
both directions are same. Thus no effective overal
Conduction ban
Coducion
band Empty onorgy sta1o
current flows.
fobiklen Thin forbkldon ap zero volts
When applied potential is greater than
Thin
band 2.
Vakanco. Vaanco but less than peak voltage
bnnd Diode length
level of
When small forward bias is applied, energy
N-y P- typu

P-side decreases, as compared to N-side as shown

in
Fltod ona0y
slal Fig. 6.3(b). Decrease in energy at P-side of junction
can be justified as when potential is
zero due to
(a) Energy diagram when zero bias is applied
N-
tunnelling, electrons in Fermi level tunnel into
becomes
side of junction due to which Fermi-level
Thin doplotion region
Enerøy
empty.
are
When small forward bias is applied, electrons
from N to P side.
able to tunnel through junction
Tunnelling of electrons from N to P side is possible
find
Fill fomi Empty stata because electrons in conduction band at N-side
tovol
Valanco band opposite vacant state at P-side. Whereas
tunneiling
Valanco
from P to N side is not
band
1ypo
Diode length in opposite direction i.e.
N typo
possible.
(b) Energy diagram when forward potential
isapplied This is because valence band electron on P-side are
This is
Fig. 6.3: Energy band diagram for Explaining
Tunneling at lower energy level compared to N-side.
Phenomenon shown in Fig. 6.3(b).
Tunnelling of electrons results into tunnelling
When applied potential is zero yolts, Tunnelling
occurs which. can be explained as following.
current With increase in forward bias voltage
Normally for P-N junction diode, when zero
across tunnel diode, tunnelling current goes on
increasing. This is shown in V-I characteristic in
potentiai is applied, current through .junction is
Fig. 6.2(a).
zero.
3. When applied potential becomes equal to peak
Quantum mechanism says that for P-N diagram,
voltage
which is heavily doped and having very. thin
depletion region, electrons can tunnel through . With increase in forward bias voltage across tunnel
depletion layer instead of crossing it and enter into diode, tunnelling current goes on increasing. It
other side of junction. This state is shown by energy becomes maximum when applied voltage is made
diagram in Fig. 6.3(a). equal to peak voltage.
Fig. 6.3(a) can be explained as when semiconductor So we can define peak voltage as the voltage at
material such as GaAs is heavily doped, forbidden which maximum tunnel current is obtained. Peak
gap is negligible or very small. Therefore tunnellingg voltage and peak current are labelled s V, and p in
can be explained with the help of Fermi level V-I characteristic in Fig. 6.2(a)..
instead of forbidden gap. The point to be noted over 4. When applied potential becomes greater than
here is that, for P-type semiconductor material, peak voltage [Negative resistance region]
fermi level is very close to valence band whereas for
N-type it is very close to conduction band. When applied forward bias voltage increases above
peak voltage, gradual decrease. in tunnel
current
was observed.
Ces easySDIuTions
 Radiation and Microwave
Theory (SPPU) 1-28
Decause with increase in forward bias 1. When very high electric field had been n
voltage, free electrons on
free electrons
N-side increases. These across Gunn diode, valence electrons observe.
obser.
are not able to recombine with charge high energy and enter into conduction band.
carrier at P-side because
vacant state at P-side.
they do not find opposite
ba
energy of electrons is very high. Therefore tihey
transferred from conduction band to ome nighe
,
Thus with increase
in forward bias voltage, forward energy level which is empty but separated b.
Current decreases
which results in negatve small forbidden gap energy.
resistance. This negative
for microwave oscillations.resistance is responsible 2. When electron from conduction band enters
higher energy level, its mobility decreases.
Negative resistance
region is shown on V- 3. Thus with increase in applied electric field, velocin
characteristics, so we
can comment that tunnel of electron decreases, which in turn decreases th-
diode is useful in
microwave oscillations and current. This phenomenon is stated as negatiy
amplifier because it exhibits
a negative resistance resistance as shown in V-I characteristics in Fig. 64
characteristics in region
between I, and I
6. When applied voltage Negative
increases above valley
voltage resistance
When applied voltage Drift
increases above valley velocity
voltage, tunnelling stops completely
and tunnel
diode behaves as normal diode.
Q. 3 Explain V-I characteristics of
Gunn diode.
SPPUR Dec.15, May 16, Dec. 18;:2 Eth Ey Electric field
Marks
Ans.: Gunn Diode Fig.
6.4:V-I characteristic of Gunn diode
4. Eventually the voltage across
Though construction of Gunn the slice becomes
diode is very imple, sufficient to remove electrons
its operation is quite complicated from higher energy
and the simplified lower mobility band, so that
energy diagram. current will increase
with voltage again.
When very high electric field is applied
across the 6. This flow of current,
slice of N-type heavily doped GaAs, is called as "Gunn Domain' or
electron flow i.e. "High field Domain'.
current towards the positive end of slice.
Greater
the potential across the slide,
maximum will be the a.4 With the help of two
velocity -of electron and maximum valley theorem explain
will be the working of Gunn diode. the
current.
SPPU:Dec 15.MayM6,Dec:16
Thus Gunn diode is behaving as a
normal positive Dec. 17, Dec18 Marks
resistance. So we can conclude that with
increase in Ans.:Gunn Diode Two Valley 8
electric field across normal diode, Model
velocity of GaAs has multi-valley
electron increases which in turn increases structure. For such a type
number structure, electron densities of
of electrons flowing towards in lower and upper
positiv terminal in valleys remain same
turn current flow increases. GaAs is under an equilibrium
having multiple conditions.
energy levels in its energy. Let, E = Electric field of lower valley
Thus when very high electric field had E Electric field ofupper valley
been applied
across Gunn diode, we are expecting heavy
flow of E =
Applied electric field
Current.
Obviously, from the
But increase in. applied electric field, features of two valley model,
current when E<E, no electron will
reduces, which can be explained with transfer to upper valley.
the help of Whereas when E, < E<
followingsteps: E electron will begin to
transfer from lower to upper valley. All
electrons
transfers to upper valley. When E> E, all
electrons
transfers to upper valley.
CEs DasveSOions
 (SPPU)_ 1-29 Quick Read
Radiadon and Microwave Theor
lower vailey Explain applications of Gunn diode.
Le.n Electron densities of
Q.5
SPPU Dec15 May 16DechNa 2 MErKE
= Electron densities of upper valley
Ans.: Applications of Gunn Diode
Conductivity of n-type of GaAs is given as,
n, + n 6.1) 1. Gunn oscillator
elu,
Mobility Since Gunn diode consists of negative resistance and
is very thin, it can be used to generate
as oscillator
n Electron density= n+n
at microwave frequency.
[n] both
Elecoron mobility [u) and electron density [may be
With the help of certain tuning devices
are functions of applied electric. electrical or mechanical] stable oscillation at
desired
Derendate Equation (6.1) w.r.t applied electric frequency can be obtained. Noise performance
of
ield E Gunn oscillator is quite acceptable.
2. Gunn diode amplifier
dn dn can be used
e n dE
=
dE dEn dE6.2 Due to negative resistance, Gunn diode
as an amplifier.
Sayn= Total density n+ De Above 30 GHz of frequency also Gunn diode
to E',
it is assumed H 4: and u , are proportional amplifier provides high power output and low noise
P Some constant with good efficiency.
(n dn Gunn diode as pump source in parametric
dE-0
3.
dE amplifier
dn
dE .(6.3) The majority parametric amplifiers use Gunn diode
dE
as pump source.

EdEPE-PP-u-(64) They have advantage over IMPATT diodes of having

much lower noise.
From Equation (6.3) and (6.2) Equation (6.4)
becomes, a. 6 a Gunn diode with active length 20 um, the drift
In
velocity of electrons is 2* 10 cm/s. Calculate natural
dn 6.5) frequency of the diode. SPRUFDec54Mark
Ohm's LawJ = GE Ans.: For Gunn Diode
Differeniate it w.r.t 'E' L
do
dE dEE :
f = L/Vd = 20*10-6/2*107 = 1 GHz

nich can be written as,

Q.7 Explain construction, operation, equivalent circuit and
1+ /E applications of PIN diode in detail.

swe know that for negative resistance current SPPUFMayzt7 Decs17, Mayie
density decreases with increase in current May 19 Dec9eMarks
. do/dE Ans.: PIN Diode
o/E 1 PIN diode is a semiconductor device that operates
Put itin Equation (6.2) and say,
as a variable resistor at RF and microwave
frequencies. It is a current controlled device.
By controlling forward bias current continuously,
= 1.28 mho. -6.6) PIN diode can be used as attenuator, leveler and
Equauon (6.6) exactiy describes two valley model amplitude modulator. They are small in size, with
behaviour. high switching speed.

es asvSOutfonS
 Radiation and Microwave Theory (SPPU Quick
1-30
An important tance of "t7' regior
Thus we can say that resistance of
characteristic of PIN diode is its ability
to control large RF signals while using much
smaller inversely proportional to 'Q' and is expressedias,
levels of DC excitation.
W2
Construction and working of PIN Diode Rs(H+ Hp)

Construction of PIN diode is as W Width of region

l
shown in Fig. 6.5(a).
PIN is a semiconductor
diode in which intrinsic Ha. Hp mobility of electron and hole respectivel
material has been sandwich between P
and N type Thus,
of semiconductor material
and hence it is named as W2
PIN diode.
Rs(H+ Hp)T l
Above equation shows that
resistance offered
Anode
Cathode PIN diode depends on width of intrinsic region a
W carrier life time.
(a)
2. Reverse Biased PIN Diode
When diode is reverse biased, behaves as capacit
having value,

(b)
Rs
.o Rp C
e
A =
=
W
GA

Intrinsic material dielectric constant

Junction area
W Width of intrinsic region
The equivalent circuit of PIN when
reversed biase-
(c is as shown in Fig. 6.5(c). From
this Figure
Fig. 6.5: PIN Diode (a) Construction
of PIN diode C = Parallel phase capacitànce when is reverse
(b) Equivalent circuit when biased
PIN diode is forward biased (c) Equivalent Rp, Lp
circuit when Parasiticresistance and inductance
PIN diode is reverse biased
The lowest frequency at
which this effect begins tc
1. Forward Biased PIN Diode predominate is related
to dielectric relaxatior
When PIN diode is forward biased, above frequency of l-region,
its knee f.
voltage as well sudden flow of the
current is not f 2
observed. This is due to intrinsic
region which is TPe'
inserted in between P and N type P I-region resistivity
of the
semiconductor material. Slowly current
through the Q. 8 Explain how PIN
PIN diode will increase. diode acts as a modulator.
This gives variation in
resistance across the PIN
SPPU: May16,4 Marks
with change in the applied Ans.
forward biased voltage.
diode when forwardApplications of PIN Diode
The equivalent circuit of PIN
biased is as shown in Fig. 6.5.. 1. PIN diode as switch
The quantity of stored charge
in intrinsic region is
given as, At microwave frequency range,
PIN diode is used as
switch. When PIN diode is
forward biased, provides
very small resistance
T Carrierlife time; and when reverse biased
provides high resistance.
Ip = Forward
current These characteristics of
diode permits to used it as
switch at RF and microwave
frequencies.

es Dasy-SOIUtionsB
 1-31 Quick Read
Radiation and Microwave Theory (SPPU
RF Attenuators There I-V characteristics are similar to p-n junction
2. PIN diode as
diode, but have stepper 1-V slope, lower series
An attenuator is a network design to introduce resistance R, lower breakdown voltage and smaller
known amount of loss. turn-on voltage.
When diode is forward
bias, by changing applied - Metal contact (cathode)
changes.
bias voltage, diode resistance
Ceramic housing
know that resistance is which dissipate
the
As we Gold plated tungsten whisker
can use PIN
energy which is treated as loss. Thus we Silicon pellet
as attenuator.
Metal contact (anode)
3. PIN diode as RF modulator
switch or as attenuato,
We are using PIN diode as
the main difference between these
two applications Fig. 6.6: Construction of Schottky Barrier Diode
conditions are
are the manner in which the bias Application of Schottky Barrier Diode
defined for PIN diode used in Mixers,
SBD's are used as : Mainly it is
Now PIN diode is described as
modulator element
In case of PIN diode as modulator
two different Radars.
frequencies are present in PIN diode . Draw equivalent circuit of
Varactor diode. Explain in
simultaneously. These frequencies are RF carrier 0
detail its construction and operation.
and much lower signal frequency.
which SPPU 2Dec. 15 May.16 May17
Here signal frequency acts as 'biasing curren' Deck 173 May 18,3 MarksS
PIN diode
modulates impedance of I-region which
exhibits to RF carrier wave current causing the Ans.: Varactor Diode
Reactor"
amplitude of RF carrier to change. The term Varactor can be said as "Variable
which enlightens the property of it which results
in
Write a short note on Schottky Barrier diode.
Q.9
variable capacitance of reverse biased junction.
SPPUE May 16;Dec,16, May 17 Normally this diode is used in reverse bias mode
Dec: 17 May18, May 19:4 Marks when variable reactance is required. With change in
Ans.: Schottky Barrier Diode reverse biased voltage, capacitance offered by
Schottky diode is nothing but semiconductor Varactor diode changes. Thus can be used as
electronically controlled variable reactance device.
material connected to the metal.
When it is forward biased, heavy amount of current Construction and Behaviour of Varactor Diode
flows. This reduces "'Switch ON" time for the diode. Varactor diode is a PN junction device specially
Since metal is there at the another end of the P-Type designed to give variable capacitance at microwave
of the material, when the diode is reverse biased frequency when it is reverse biased.
immediately it 'Switch OFF.
The doping for Varactor diode is designed in such a
Thus this diode offers low SWITCH ON and Low way that with slight change in its applied reverse
swITCH OFF time. voltage measurable variation in its junction
Thus when diode is reverse biased it requires less capacitance is obtained.
time to take out the majority charge carriers and To have capacitance variations at microwave
vice versa. frequencies, basic rectifier diode is modified by
Silicon is the most preferred material for fabrication following two structures shown in Fig. 6.7(a) and
of this diode and is can be used till 100 GHz. Fig. 6.7(b) which are:
Construction of Schottky Barrier Diode 1 Abrupt Varactor
The construction is shown in Fig. 6.6. It consists of 2. Hyper Abrupt Varactor.
silicon wàfer connected to metal.

@s DasyESOIuions
 Radiation and Microwave Thcory (SPPUJ
1-32
Mainly to construct Varactor diode, is
GaAs
Forh2
preferred because it has advantage such as higher Curre
maximum operating frequency and better
functioning at lowest temperature. Both advantages Arca of interest of
are mainly due to higher mobility of charge carriers varactor applicaion

exhibited by GaAs. Co

When Varactor diode is forward biased, it behaves

-V
as normal rectifier diode.

(b) Variation of Varactor

capacitance
with applied bias voltage
Doping Fig.6.8:V-Icharacteristics of Varactor Diode
lcvol
When Varactor is reverse bíased, it offers variat
capacitance which is given by Equation (6.7).
X(Depth from anode
terminal) CO 6.7
(a

C (0) = The capacitance with no reverse bias applie

Doping n for abrupt and is 1/3 for hyper abrupt

lovel N
junction
VT = Maximum reverse bias voltage
X{0epih from ancde ->
terminal) VRApplied reverse bias voltage
(6 At this point we can conclude that depending on
frequency, by varying applied reverse bias only.
Fig. 6.7:Possible structures of Varactor Diode
capacitance of Varactor diode can be varied.
(a) Abrupt Varactor (b) Hyper abrupt Varactor
.The main electrical parameters of Varactor diode
The V-I characteristic and variation of capacitance
are
with applied reverse bias voltage is as shown in
1. Reverse breakdown
Fig. 6.8(a) and 6.8[b) respectively. Equivalent voltage and reverse
circuit saturation current.
of Varactor diode when reverse bias is as shown in
2. Capacitance value and
Fig 6.9. capacitance voltage
change behaviour.
3. Quality factor or figure of
Forwa merit.
d The quality factor, also known
current as figure of merit and
is an important parameter
for 'a Varactor diode
since it determines the frequency limit
applicability
for the diode.
+V

Saturated reverse curent

Avalanche
current

(a) V-I characteristic

Fig.6.8:V-I characteristics of Varactor Diode Fig. 6.9: Equivalent circuit of Varactor
when reverse
biased

es easysolutons
 (SPPU)
_
1-33 Quick Read
Radiation and Microwave Theory
R and R2 develop a dc voltage that reverse biases
Applications of Varactor Diode
varactor diode VD, and determines the rest
multiplier
Varactor diode as frequency frequency of the oscillator. The external modulating
DC bias,
Frequency multiplication is the phenomenon
which signal voltage adds to and subtract from
characteristics of either which changes capacitance of diode and thus
results from nonlinear
resistance or reactance. It is more
efficient when frequency of oscillation. Thus FM modulation.
impedance is pure reactance.
Coil
Capacitance of varactor diode varies with applied Cc
reverse bias, the diode acts. as
a nonlinear HHOH
Crystal
FM Output
capacitance. Coil

This characteristic of varactor diode is

used to get Coil
Amplitier
is very
frequency multiplication. The varactor diode Modulating
useful device, especially since it will
operate at input R Z VD
frequencies much higher.
2. Varactor diode as FM modulator
crystal. Fig. 6.10 : FMmodulator using Varactor Diode
Carrier frequency is generated by
changes
Capacitance offered by varactor diode, 3. Varactor diode as PUMP SOURCE in parametric
change in
according to applied reverse bias. This amplifier
capacitance contributes change in frequency which
nothing but Parametric amplifier uses a device whose reactance
is proportional to applied bias [which is
is varied in such a manner that amplification results.
frequency modulation].
FM Thus Varactor diodes are used as pump source in
Fig. 6.10 shows very basic circuit diagram for
parametric amplifiers.
modulator using varactor diode.
 Aiiiti MiTONaE tieor (SFPO 1-34
Techniques
Gnapter 7: Microwave Measurements
a short note on: Frequency Meter. (4 As.
(4 Mar
a.1 We eviaratery notes on Tunable detector. Q.2 Write
Wavemeter
Ans.: Frequency Meter /
SPRUEMay16 May 17, 4Narks
Frequency-meter is a resonant circuit provided
Ans.
calibration unit indicating frequency. Three types
Tunable Detector frequency-meter/ wave-meter are listed below
Tte funcion of tunable detector is to detect the 1. Absorption type frequency-meter
Square wave modulated microwave signal. For sake 2. Reaction type frequency-meter
of detection it consists of point contact or Schottky
3. Transmission type frequency-meter
Barrier diode as Demodulator.
Basic structure of Absorption type requency-meter
Thus with the help of Tunable detector microwave
is as shown in Fig. 7.2[a). The heart of this
signa geS convertad in low frequency sigmal. This
frequency meter is the resonant cavity mounted on
Low raquency signal can be observed on
the rectangle waveguide. Dimension change (length,
ascillascope or can be connected to the VSWR-meter
radius or height) of cavity changes its resonant
for the further measurements.
frequency.
Tunable detector with probe attachment in Fig. 7.1.
Bottom wall of cavity has small window allowing
The main part of the tunable detector is sensor. This energy propagation through cavity from waveguide.
sensar is inserted in the waveguide via slot which is Cavity rotation results in impedance mismatch
made on dhe top of the waveguide. between cavity and waveguide. This results in
The sensor detects the standing wave pattern which reflected power in guide and absorb/transmitted
exists within waveguide. This pattern is given to the power in avity. The energy is often said to be
diode detector. As length of the tunable detector can absorbed by the cavity and hence the name
be zdjusted for the maximum power, it is called as absorption type.
cunable probe. In given frequency band of operation, cavity is
Tuning mechanism designed to have maximum absorption of energy
when the cavity is tuned exactly to the frequency ot
energy to be measured. It indicates that the tuned
circuit is extremely sharp absorbing the maximum
of the energy at the exact frequency and much less
at slightty different frequencies. At exact dial setting,
where the maximum or peak absorption takes place
and accurate frequency measurement can be made.
Diode detector
The indicating meter in the circuit indicates the
energy level and shows a drop in such level the
Attachment for moment energy is absorbed by the frequency meter
probe cavity.
A
frequency measurement would therefore consist
of tuning the cavity of the frequency meter until
maximum. Dip occurs of the indicating meter and
then reading the micrometre position and the
corresponding frequency from the calibration chart
Sensor to sense provided with the frequency meter. The grapn
microwave signal
corresponding to output power is shown in
Fig. 7.1:Tunable detector Fig. 7.2(c).
CEs Casy-soluions
 Quick Read
-35

Rotating knob

Calibratod scalo
for froquency
moasuromont (GHz)
2.30 2.32
Markor-V
LL LLL
Markor-H-1
2.10
LLLLLLLILLEI
Maukor-tl-2
LLLLLLLLILLLLLLLL
1,04 1.86
LLLLLLLLILLLLLLLI

Wavaguldo
scale
(b) Enlarged view of calibrated
(a) Frequency meter/ Wave meter

O/P Power
(dB)

Maximum absorption at
resonant trequency

Frequency (GHZ)
power vs frequency
(c) O/P
Type Frequency meter for Microwave
Frequency Measurements
Fig. 7.2: Waveguide based Absorption
With increase in SWR, mismatch increases.
VSWR.
Q.3 Explain measurement techniques for Therefore ideal value of S is unity.
SPRU: DeC 15 May16, Dec. 16 Dec.1
Thus we can divide measurements in two parts:
May18 Dec 18.Máy:19,8 Marks
1. Measurement of Low VsWR (S « 10)
Wave Ratio
Ans.: Measurement of Voltage Standing
(S > 10)
(VSWA) 2. Measurement of High VSWR

Measurement of Low VSWR (S< 10)

When characteristics impedance of the device does 1.
not matehed with load impedance, microwave Microwava Slotted
Isolator Load
power gets retlected back. Incident signal in source line

direction of load and reflected signal away from the

load forms standing wave along the length of the Detector
device. VSWR is the measure of load mismatch. The mount
ratio of maximum to minimum voltage of standing
Wave gives the VSWR.
CRO

1+P
VSIWR =
1-P Fig.7.3: Set-up for VSWR measurement
reletal
P Reflection coetticient= p.
incident

@saasTas0ulons
Radiation and Microwave Theory (SPPU)
1-36 Quick
The following steps 2a for TE in
are to be follow to measure the A
VSWR:
1. AS waveguide inmpedance is not matching
with
unknown impedance, standing wave
pattern is
generated along the length of
This pattern is sensed by
the slotted waveguide. 1-o/AJ
the sensor.
2. Experimental set is shown Then the VSWR can be calculated using
in Fig. 7.3. h
3.
empirical relation
The tunable probe is
precisely moved along the
length of the waveguide.
VSWRT(d2-dd 7.3
4. Thus the detector detects
corresponding signal
along the length. Q. 4 Write short note on Microwave power measurement
5. Thus we can observe either maxima
or minima on sePU May18 Mak
the oscilloscope.
6 We can note down the magnitude Ans.: Power Measurement
of maxima and
minima from the oscilloscope. In the microwave region the power of the signal is

vSWR = Vmax/Vmin. as important as the frequency of operation.

Thus VSWR can be measured. Method of measurement for the microwave power
depends on the following factors
2. Measurement of High VSWR (S 10)
1. Frequency of operation.
For VSWR> 10, we use the Double
Minimum 2. Levels of power [low, medium or high power).
method. In this method, the probe is inserted
where 3.Whether the power is pulsed or continuous.
the minimum voltage can be read. This
minimum
voltage is noted down. For this let us consider following three
levels of
Now probe is removed and inserted again at microwave power:
the
location where the double of the minimum valuue 1 Low power [below 1 mv or 0
dBm]
can be noted down. 2. Medium power [between 1 to
10 W or 0 to 40
The procedure has to be repeat on the both dBm]
side of
location where the minimum value is obtained. Let 3. High power [greater than 10
W or 40 dBm]
this position be denoted by The probe is 'd. then Microwave power can be.
moved to twice the power point on the other side of continuous or pulsed.
Most power measuring devices
minimum (say d2). as shown in Fig. 74. actually measure the
average power of the signal
Twice minimum
over the number of
wave cycles.
power points
Voltage(voits)| The measurement of pulsed
power requires a
knowledge of thermal response of
sensing devices
V = 2 x Vin **** to. pulses of energy and greater
frequency
bandwidth required.
Vmin
Power is detected at microwave frequencies
by the
d1 2 followingdevices:
Distance (cm) 1. Bolometer
Fig.7.4: Graph distance versus V 2. Thermocouples
2 Pmin o V 3. Microwave crystals.
V4
a.5 Explain with neat block diagram
power measurement
of microwave generator
= using:
v, y2 Vmin 1. Bolometer.
7.1) 2. Calorimeter
SPPUDec: 15, Dec 18:10 Marks
Ces CasSOmiDns
Radiaion and licTowave Theory (SPU 1-37
Ans. Use of Boiometer for Low Microwave Power
Measurements
Boiometer is a simple temperature sensitive device
whose resistance varies with applied power and
they are capable of measuring low microwave ANNV
Scometen
poiwer.
N
Bolometer are of following two types:
1. Barrerters
Fig. 7.5: Power measurement using boiomeier balanced
2. Thermistors. bridge
BarTetters have positive temperature coefficient
When signai power is applied to the sensor element.
and their resistance increases with increase in
causing its temperature to change, the seusor
temperature. They are basicaliy short length wire of
resistance changes, causing the bridge to become
piatinum mounted in a caruridge like an ordinary
unbalanced. Reading of meter 'M' can be calibrated
fuse.
in terms of microwave power.
Thermistors have negative temperature coeficient
and their resistance increases with decrease in
Measurement of High Microwave Power
temperature. They are basically semiconductor Any power between 10 W to 50 k\¥ is considered
material. high power. These are normally meusured by
calorimetric watt meters. These meters can be
Power measurement using boiometer:
either dry or flow type.
Thermistor and Barretters are used with bridge
A dry-type calorimeter normally consists of a co
circuit to convert resistance to power.
axial cable which is filled by a dielectric with a high
Barretters are usually thin-pieces of wire such as
hysteresis loss. The flow type uses circulating water,
platinum. They are mounted as terminating devices oil or any liquid which is good absorber of
in a transmission line.
microwaves.
The section of transmission line with mounting
The fluid after flowing through he load, experiences
structure is called a Detector Mount. The increase in a temperature rise due to microwave energy. The
temperature of barretter due to power absorbed
difference between temperature (Ti) of a kuown
from the signal in line causes the temperature of the
quantity of liquid before entering the load and
device to increase.
temperature (T) after it emerges is a measure of
The temperature coefficient of the device causes the the power which has been absorbed.
resistance to change in value in proportion to
Knowing the rate of the luid flow the exact value of
change in temperature of the device.
power can be calculated by using the equation
Thermistors are usually beads of semiconductor
RK, (T-T)
material that are mounted across the line. P
4.18
Thermistors differ from barretters in that they have
negative temperature coefficient. Where P = Measured power in watts.
Variation in resistance due to thermal sensing R Rate of flow in cnm°/s
devices such as barretters or thermistors must be K Specific heat in cal/g
converted to reading on an indicating device such as P Specific gravity in g/cm
meter. This is done by balanced bridge arrangement
shown in Fig. 7.5. (T2-T,) = Temperature difference in °C.

with no power to detector mount.that contains Q.6 Explain any two methods of measuring impedance of
bolometer, resistance R, is adjusted to balanced the
a terminating load in a microwave system.
bridge shown in Fig. 7.5.
SPPUFDec. 15, Dec 16, May 17,Dec 19,3MarksS

s CaSyASOuon
 Radiation and Microwave Theory (SPPU) 1-38 Fig .7. Po
two ports as shown in
Ans.: Measurement of Impedance at Microwave travelling waves
Frequency forward and reverse
coupled to two of th
Impedance can be line are separately
of
microwave frequencies
auxiliary line as follows:
measured using following two methods:
1) The portion of
the wave travelling trom
1. Impedance Measurement Using Slotted Line (output) is coupled to
input) to port 4
Microwave
source Isolator Slotted
ine H Unknown
Load
2)
(forward coupled) but not port
A portion of reflected
to 2.

wave travelling from

no

1
port is coupled to port 2 {reverse coupled)
Detector
CRO
Moun not port3.
is us
Fig.7.6:Experimental set-up for impedance The performance of a directional coupler u
described in terms of its coupling and directi
measurement
which are defined in the following manner.
Incident and reflected waves will be present
proportional to the mismatch of the load under test 1) Coupling: Coupling factor or coupling (C) is deting
pow
whose inmpedance is to be measured) resulting in as the ratio, expressed in decibels of
standing waves. Usíng slotted line and that (incident) at input of main line to power coupled.
unknown load positions of Vmax and Vmin can be output in auxiliary arm.
exactly determined. 2) Directivity: Directivity of a directional coupler
Experimental set-up is as shown in Fig. 7.6. defined as the ratio, expressed in decibels, of t
Now replace this unknown load by short circuit. power output in the coupled auxiliary arm to ti
Now measure the shift in minima. Now if minima is power flowing in uncoupled auxiliary arm.
shifted to left, impedance is inductive whereas if Reverse Forward
coupled port coupled port
minima is shifted to right, impedance is capacitive.
This unknown impedance can be obtained by usual Auxiliary P2 P
methods using data recorded and smith chart. ine
2. Measurement of Impedance Using Reflectometer
A reflectometer arrangement is as shown in Fig. 7.7.
Port 1 - Port 4
input Main lino
output
Unknown impedance of a device can be measured
using this arrangement. The directional coupler Fig.7.8: Directional coupler
forms an important part of the arrangèment. as
Coupler reflectometer
One of the useful
CRO application of the directiona
coupler involves measuring
the reflection coefficier
Matched magnitude | P | and hence
Dotoctor SWR of an unknowa
tormination mount Device Under Test (DUT).
Once reflection coefficien
magnitude | P| of DUT is known,
impedance of DUT
wavelength and frequency can
Microwave 2
.4port 3 be calculated usin
SOurce directional Dovico various formulac.
couplor undortost
In actual experiment
set-up detectors at ports 3 anc
2 sample forward and reflected
Matchod waves respectively
tormination The VSWR meter reads
forward voltage in dB wher
Fig.7.7: Impedance measurement using connected to port 3 and
reflectometer port 2 is terminator. Thc
Directional coupler VSWR meter
reads reflected voltage in dB whem
connected to port 2 and
A directional coupler, consists port 3 is terminated. Using
of two transmission these values reflection coefficient
lines, the main and auxiliary magnitude, SWK
line, is basically a four impedance and wavelength
port network. The main and auxiliary are calculated.
line each have
 1-39 Quick Read
Radiation and Microwave Theory [SPPUJ
2. Practical Atténuation Measurements
ref
P Readings are in volts Attenuation measurements are made by comparing
known value of attenuation to unknown value. This
SwR can be accomplished by following three substitution
(1+P) methods:
Z= Unknown impedance = Zo-P)5 1. RFsubstitution
Z, = 377 Q 2. Audio or DC substitution
3. IF substitution.
RF substitution attenuation measurements
Z 3772 In order to use the RF substitution method,
generato, precision variable attenuator, detector
2.7 Explain attenuation measuremeni technique in detail.
and power meter or indicator are connected as
SPPU May 16, May 17, May 18. shown in Fig 7.9(a).
Dec 18, May 19,8 Mark Power
Signal Variable
Detector meter
generator attenuator
Ans. indicator
Attenuation Measurement
(a) RF substitution set up for calibrating output reading
Attenuation of microwave frequencies is the result
Signal Variable Unknown
of following two different effects generator attenuator attenuator
Detector
Attenuation due to energy dissipated within the
device as heat and is defined as reduction of power
Power
through a device. meter
2. insertion loss of the device due to mismatch which indicator
results as power loss which reflected back down the
(b) RF substitution set up with unknown added to the
line and not dissipated within the device as heat.
circuit
Attenuation measurement can be made when input
Fig. 7.9
and output parts of the device to be measured are
matched and no reflection occur. Insertion loss Measurements are made by first setting precision
measurements require taking into account that the attenuator at a higher value of attenuation than the
devices are not matched and power reflection expected value of unknown attenuator.
occurs back down the line. Initially a convenient scale reading is selected on
Matched Attenuation Measurements power meter or indicator. The unknown device is
placed in series between precision attenuator and
Attenuation is the power loss between source and
detector as shown in Fig. 7.9(b).
load when a device is inserted between two. t is
The precision attenuator value is reduced until the
assumed that both input and output ports of the
device are matched to the transmission line so that original scale reading on the meter is again
obtained.
thereare no reflected losses due to mismatch. With
this restriction, the power loss is therefore limited The unknown value of attenuation is the difference
to attenuation that is dissipated as heat within the between the value of attenuation read off the
device. precision attenuator before and after unknown
Attenuation for a device, when both input and attenuation is inserted in circuit.
output are matched to transmission line is defined Advantages
by equation
1 in this method, indicator is read at the same location
(
A (dB) = 10 log
P
on the meter scale for both steps. This eliminates
Scale-reading error due to non-linearity of the meter
outp
itself.

es 0as SO uRTOns
 Radiation and Microwave
Theory (SPPU)_ 1-40
2. The. ayof the resultant
value is directly dependent on the accuracy and the calibration of the
variable attenuator over its
attenuation range.
Limitation
The precision of available attenuators is greater at low
frequency
Q.8 Explain biock diagram
for specirum analyzer.
Ans.:
Spectrum Anaiyzer

OSCiloscope, a spectrum analyzer produces visible

e spectrum
a display on a screen. Unlike an oscil1oscope, howee
analyzer has only one function-to
Ana also like an produce a display of the frequency content or an input signa
oscilloscope, the spectrum
analyzer will always produce a picture on tne screen.
Most spectrum anaiyzers
are heterodyne spectrum anaiyzers (also cailed
neterodyne analyzer is scanning spectrum anaiyzers).
essentially a radio receiver (a very sensitive
diagram of it is as shown in and seiective receiver). The basic bio
Fig. 7.10.
Use of Spectrum Analyzer
for Measurement of Spectrum
of Microwave Signa
A spectrum is a
collection of sine waves that,
examination. Spectrum Analyzer when combined properly, produce the
has frequency Display. The time-domain signal und
frequency of each sine wave in frequency domain display plots the amplitude
the spectrum. The frequency versus t
spectrum analyzer measurements domain also has its measurement
include frequency, power, strengths. Commo
spectral content of a signal is modulation, distortion, and noise.
important, especially in systems Understanding th
key measurement. Too little with limited bandwidth. Transmitted
power may mean the.signal cannot power is anothe
drain batteries rapidly, create reach its intended destination.
distortion, and cause excessively Too much power ma
high operating temperatures.
i
Input signal

RF Input Pre-selector Mixer

attenuator OR
band-pass filter F GainFFiterLogFitor

Local
Envelope
detector
oscillator

Reference
oscillator Video
filter

Sweep
generator
Display
Fig. 7.10 : Block Diagram
for Spectrum Analyzer
Radiation and Microwave Theory (SPPU 1-41 Quick Rcad

Chapter8: Microwave Systems

The dissipated power within dielectric material is
Q.1 Write a short note on: Microwave heating technique.
given as,
SPPU Mäy.19,5 Marks
Ans.: Microwave Heating
average 2 EEJE-E* dv .8.2)

. Microwave heating uses electromagnetic energy in Where, Frequency

the microwave frequency range 300-3000 MHz. Free space dielectric constant
Eo
Energy in this frequency range is used to heat any
dielectric material. E" Imaginary part of dielectric constant of the
material to be heat
The way, in which a material will be heated by
E = Required electric field
microwaves depends on its shape, size, dielectric
constant and nature of the microwave equipment It can be shown that the depth of microwave power
used. penetration in the object which is to be heat is
The most important characteristic of microwave directly proportional to the wavelength of the
microwave signal. This shows that it is inversely
heating is volumetric heating. which is quite
differe from conventional heating In the proportional to the frequency. i.e. lower the
conventional heating the heat must diffuse in from frequency more will be depth of penetration for the
the surface of the material.
same amplitude of the microwave power.
If we use higher microwave frequencies for heating
Volumetric heating means that materials can absorb
microwave energy directly and internally and purpose, depth of penetration will be small as
convert it to heat. It is this characteristic that leads compared to object size. This results in non-uniform
object heating. Therefore frequencies 915 MHz to
to advantage such as rapid, controlled, selective, and
uniform heating. 2450 MHz is generally used for the heating

Now let's try to understand how microwaves Microwave heating is mainly caused by following
produce heat within material. The dielectric two mechanisms:
constant for any dielectric material which is to be 1 Dipole Rotation 2. lonic Migration
heat is given by Equation (8.1). Q.2 Explain different applications of Microwave Heating.
E e'tje" .(8.1) (8 Marks)

Equation (8.1) shows that dielectric constant is Ans.: Applications of Microwave Heating
complex term. Microwave Regeneration of Granular Activated
1.
The imaginary term e" is termed the effective loss Carbon for the Carbon In Pulp Process (CIP)
factor, and accounts for dipolar relaxation loss as
well as any conduction. The carbon-in-pulpP process is widely used for the
In general, e " is a function of temperature, moisture
recovery of gold. The carbon, which is used to
content, density and electric field direction The absorb the gold cyanide molecule, is periodically
effective loss factor controls the power that can be removed from the adsorption tanks to allow
dissipated in a material. If e" < 102 the material is removal of the gold by elution.
said to be of low loss type, and couples poorly with Advantages of the technology are primarily cited to
microwaves. be low maintenance costs. Microwaves can readily
In order for such a material to be heated with heat granular activated carbon directly, suggesting
microwaves, a very high electric field would be that microwave regeneration would offer possible
needed. For materials with e"> 5, the power advantages over conventional regeneration. These
penetration depth could be quite small. If the object include rapid and precise temperature control of the
to be heated is larger than this depth it is likely that
carbon inventory itself, a more compact furnace and
highly non-uniform heating will reult.
possible energy savings.
Bs easy-solutions
 Radiation and Microwave Theory (SPPU) 1-42
2. Microwave Heating for Mineral
Process A combination infrared/microwave heatin
p
Mineral processing applications, which achieves an effect that conventional heatine
has reached Ca
pilot plant scale, is the Infrared heat first cures, the surface to obta
regeneration of granular
activated carbon. This carbon needs to be smooth "skin." Microwave energy then cu
heated for
removal of impurities. interior to a uniform porosity.
Microwaves can readily
heat granular activatea 6. Chemical Processing
carbon directly, suggesting
that microwave A large potential for growth "lies in the hemi- cho
regeneration would offer possible advantages
over processing industry because the high value of
conventional regeneration. These
include rapid and final products justifies investment in microw-
precise temperature control
of the carbon inventory heating equipment. While most applications
itself, a more compact furnace
and possible energ proprietary, three general types have. emerge
savings.
Microwave energy is effective for drying.
3. Investment Casting
Heat-sensitive materials that must be 100 % free
Microwave heating can help convert residual water. Conventional methods require h
investment
casting for precision parts such as temperatures and long times to remove the
turbine blades
from a batch process to a traces of water, and many products degrade un
continuous one.
Refractory moulds for the parts are formed these conditions.
around
a wax model.
Microwave energy is effective for heating fluids
Microwave energy heats the mould; melting can ensure uniform heating in the manufacture
the
wax. Use of microwave not only saved chemicals that must avoid a thermal gradient
energy
required but also improves quality of the retain vital material properties.
product.
Frozen Food Tempering It can rapidly heat chemicals through critio
Food Tempering is the most famous application temperature zones. It can effectively heat nuclear
of toxic chemicals to decompose, detoxify, or compa
microwave heating in U.S. Food tempering takes few
minutes with microwave heating without diluting them.
food quality. Conventional heating takes days for Q.3 Explain the Biomedical application
food tempering. of Microwave.

Frozen food Tempering using microwave heating Ans. 5 Mar

also have following advantages:
Applications of Microwave to
(a) It can be performed in-carton, without
unpacking
Biomedical
the meat, which simplifies processing and Microwave plays important role in
clean-up. medicine.
(b) Tempering effectively replaces a batch process Medical applications
with of Microwave signal is main=
a continuous one and provides a for the treatment of
product of uniform patients with the use of eith-
temperature and texture, with excellent process thermal or non-thermal effects.
Microwaves a
control. used for treatment of patient
based on therm
(c) It eliminates bacterial growth, effect. The various treatments
spoilage, and drip are categorized
following:
loss; keeping a minimum inventory in process
at
one time. (a) Diathermia : Heating up to
41° C (applications i
5. Sponge Rubber
physiotherapy)
Diathermia is kind of treatment
Pipe insulation and gaskets for automobile method in whic
windows heat is induced electrically
and doors are made from sponge rubber. therapeutic purposes
Sponge Mainly it is
usd for muscle
rubber is formed by adding a blowing agent relaxation. The methoc
that of electro-cauterization is
used to control bleedinE
activates with heat, forming gas bubbles which
are or cut through tissues
entrapped in the rubber. with the help of microwave
heat.

es casy-301ui0n
 Quick Read
Radiation and Microwave Theory (SPPU)_ 1-43
when this 2. Penetration Capability of Microwaves are
Main feature of the diathermy is that
increases soft independent of day or night.
treatment is given the heat therapy
tissue elasticity and blood flow.
It also induces 3. Penetrates through vegetation and soil to a
muscle relaxation, stimulates tissues, bone and certain extent.
nerve healing. 4. Sensitive to moisture (in liquid or vapour
(b) Hyperthermia : Hyperthermia is a type of cancer forms).
out using
treatment in which body tissue is exposed to high Microwave remote sensing can be carried
temperatures (upto 113°F). This high temperature Thus we can have
passive and active sensors.
can damage and kill cancer cells. Hyperthermia is following two types for microwave sensing radars:
almost always used with other forms of cancer Passive Microwave Remote Sensors or
1.
therapy such as radiation therapy and Radiometers
chemotherapy It may make some cancer cells more
The passive microwave remote
sensors are known
sensitive to radiation or harm other cancer cells that radiations
radiation canot damage. as Radiometers. They sense natural
originating from the earth surface.
Write short note on the application of Microwave natural
Q.4
Marks
These sense emission from the
Microwave Remote Sensing. 8 environment. This sensing of the emissions from
the
soil is
Ans. natural environment such as land, ocean, and
termed as "Atmospheric Sounding"
Microwave Remote Sensing
profile of
The atmospheric sounding gives vertical
Remote sensing has diverse applications and it has
to temperature and molecular constituents with
been identified as a technique with good potential
some respect to the molecular resonance frequency.
help the nation's economic growth and solve on the
The performance of the radiometers depends
of its problems. path of
actual object size, different obstacles in the
These include better management of natural
the signal, atmosphere temperature.
resources through wasteland mapping, identifying
lood-prone areas, water in catchments
areas, There are following various types for the
assessment of situation of reservoirs, estimating radiometers:
forest area and prediction of crop yield and scarcity . Total power Radiometer.
in
of resources etc. Remote sensing can be applied 2. Dicke Radiometer.
areas like prediction of climatic conditions, rainfall, 3. Satellite Borne Radiometer.
cloud cover, etc, and to help identify the areas
4. Pushbroom and Synthetic Aperture
covered by clouds, and other physical parameters.
Radiometer.
During rainy season, another area of concern
is
flood. 2. Active Remote Sensing Radar Systemm
Movement of clouds could not be predicted as the Active Radar system uses active sensors such as
clouds restrict the observation by conventional camera to sense the natural emissions.
methods. Observation is not possible during night
The main advantage of the active remote sensin8
so one needs sensors which can work in night as
radar is that we can retrieve the information at any
well as in cloud covered areas. Remote Sensing is
time i.e. day, night or any season.
also applicable for Security Purpose.
Since they use active sensors, we can retrieve
For these applications Microwave from
all
information at any time. Mainly they are used to
Electromagnetic Spectrum are preferred due to
know wavelength of the signal which is not
following reasons:
sufticiently provided by the SUN.
L Microwaves have all weather penetration
capability through clouds. t.5 What is MST RADAR ? Discuss in brief with its
(5 Marks)
advantages.

es easy-solutions
 R.adiativn and MicrowaveTheory (SPPU).
1-44 Ou
Ans. MST Radars on the
The localizer airborne is way esters
at the opposite end to the
end the:
For MST Radars M centerline approach
Mesosphere, S-Stratosphere
-

at a distance which ensures

and T-Troposphere and It is positioned tha
is collectively termed as
MST-Radars. These types lies below the runway take-off obstructi
of tlhe Radars are able to
give the atmospheric
information from 10 Km to clearance plane.
100 Km. The field pattern of the localizer antenna show
These radars uses in Fig. 8.1.
following technique for
atmosplhericmeasurements:
1. Doppler Beam Swinging (DBS)
technique. 90Hz
2. Spaced Antena Drilts (SAD)
technique.
These techniques uses following
echoes for the
.Glide.Path.
measurements: 150 Hz

(a) Turbulent Scatter

(b) Thermal or Incoherent
Scatter. Glide Path Runway
()Fresnel Scatter
Principles of MST Radar
Fig. 8.1:The field pattern of thelocalizer antenna
The width of the localizer beam is taken by the
MST Radar is used to travel of the localizer needle on the aircraft cross
kn the atmospheric
conditions and various investigations in pointer indicator.
different
regions of the atmosphere which are Mesosphere,
Total width in terms of degrees will depend on
Stratosphere, Troposphere. It uses Doppler
radar position of airborne and length of runway.
operating typically around 50 MHz with an average
power. Q.7 Write a short note on :
Microwave moisture=
It receives echoes due to the scattering and measurement SPPU May 19, 5 MarkG
reflection from variation in radio refractive Ans.:
index of
neutral atmosphere. These reflection are depends
Microwave moisture measurement
on variability of humidity, temperature and
electron
density. Drying, storage, marketing,
and roasting are fou
It provides estimates of atmospheric important aspects of any product
winds on a handling in which
continuous basis at high temporal and spatial moisture plays an important role.
resolutions required to study of various dynamical Thus at each step desired
moisture contents within
processes of the atmosphere. product is of measure concern.
Microwave signal is
MST radar uses used to measure moisture
the echoes obtained over the height contents as RF energy 1s
range of 1-100 km to study winds, highly absorbed by
waves, moisture.
turbulence and atmospheric stability. The method does not
give very accurate moisture
Advantages of MST Radar Over Conventional contents as microwave energy
is also absorbed by
Radar other components. Therefore this
System technique needs
some basic calibration to
It can work for any atmospheric conditions.
improve accuracy.
There are following methods
Sophisticated antenna system gives which are mainly used
better results. to measure moisture contents.
Q.6 Write a short note on: Localizer. Direct Measurement
(5 Marks) of Moisture Contents
Ans.: Localizer Water content is determined
by removing moisture
and then by measuring
Function of the localizer weight loss. Different
transmitter is to give methods are used to remove
guidance in azimuth along the extended all the water but
runway chemically bound water:
centre line. heating in an oven, use o
microwaves or infrared radiation.

CEs casESOluiIDDS
Radiation and Microwave Theory (SPPU) 1-45 Quick Read

Moisture content is given with the relation % Ans.:

mCwb
Wy
ww dx 100, where mc is expressed on Microwave Satellite Communication System
The communication system in which
wet basis (Ww is wet weight and Wa id dry weight).
communication link is establish between earth
Moisture can be expressed on dry basis % satellite station which is
station and man-made
mCwb x 100, this value sometimes being revolving in orbit around earth is known as

used for particular studies. Satellite Communication.

is given by following Satellite commutation uses higher band of

.The.conversion mcwb mcab
formulae :
microwave frequencies. They offers large
bandwidth, high data transfer rate and large
mCdb
mCwb
1+ mcab number of communication channels. It is extremely
secured way of communication.
mCwb
and mcdb 1- mCwb As thereis now restriction of available space and
weight, earth station uses huge antenna array
Indirect measurement: An intermediate variable
is measured and then converted into moisture
system to achieve sharp beam and high gain.
Whereas as satellite station do have weight
content. Building up calibration charts before
restriction, they utilizes low gain antennas.
applying indirect measurements is a prerequisite;
Satellite stations are revolving around earth in low
All commonly used methods are based on electrical
earth orbit, medium earth orbit or geo-stationary
property of beans. An electrical current unit, satellite
orbit The distances involved
resistance or capacitance, is measured and then an interesting
communication result in
converted into moisture content. signals must travel
phenomenonBecause all
Resistance: The meter measures the electrical 22,300 miles to the satellite and 22,300 miles when
resistance of beans when a current is applied returning to a receiver, the time required to
between two electrodes. Beans are placed in a transmit a signal is independent of distance.
constant and known volume. It takes as long to transmit a signal to a receiver in
Capacitance : The meter measures an electrical the same state as it does to a receiver a third of the
current between two plates of a condenser which way around the world. The time required for a
constitute the walls of a recipient. A precise weight signal to arrive at its destinatión is called
of sample is required. propagation delay.
In both techniques,temperature corrections are Unfortunately, satellite communication is extremely
required for accurate meaurements. Most of expensive. Attenuation characteristics depend on
moisture meters are equipped with temperature frequency, power, and atmospheric conditions.
correction software. Transmitters and receiver operate on licensed
Limits of the method: Calibration charts must be frequencies and require an FCC license.
established for each grain type. This means that a a. 9 Write short note Radiation hazards and its
meter must be calibrated separately for robusta SPPU:: May 19, Dec. 19,5Marks
protection.
beans and arabica beans, but also for cherries and
Ans.:
parchment to obtain accurate measurements.
Accurate measurements are obtained within a range Radiation Hazardous and Protection
given by the manufacturer. Over this range, readings Most peopleare not aware of the risks
have no meaning.
surrounding them as a consequence of
Q.8 e lectromagnetic radiation. Appreciable efforts are
Explain microwave satellite communication system.
done y researchers to give reasoning behind
SPRU: Dec. 18, 4 Marks electromagnetic radiation hazardous, it's safe level
and protection.
Ces easyESdlutions
 Radiatíon and lMicrowave Theory (SPPU 1-46 Quick

Electromagnetic radiations are classified mainly As microwaves can penetrate the human bod
categorized as lonized (High level of radiations)and disturbs the melatonin secretion. If RF ansmis
non-ionized (Low Level
of radiations. Sources of is increase suficiently, disturbance inin melato
low level radiations are Bluetooth, secretion gives following effects
cell phones
microwave oven computers and power
lines. While (1) As the melatonin protects against cancer, Longterm
sources of high level radiations
are high voltage transmission of microwave signal on the hun
power lines, cell towers and
radioactive medical body disturbs the melatonin secretion. This ma
equipments.
results in breast cancer and prostate cancer.
Electromagnetic Radiation Affects
Human Begins and (2) Microwave transmission increases the nocturnal in
Animals 2?

melatonin level which disturbs the sleep

Magnetic field induce electric Disturbance in sleeping cycle may cause Insomnia.
currents in
electrical conductors including
human, animal or Microwave radiation does not have sufficient power
plant bio-systems.
to break covalent bond in DNA molecule as that of
Stress created by. these X-Rays and Gamma rays. But microwave can
currents affects
metabolism and hormone production. produce enough interaction between ions and
These effects
will vary on individual basis,
age and life-style. charged Macromolecules. This interaction produces
To avoid these effects specific some bio-chemical functions. This increases
radiation levels are
prescribed. Also one should use all production of free radicals and reactive oxidant
equipments in
proper manner to protect himself. species. This damages the DNA. Constant exposure
It is proved that visible light to microwave Radiation damages
does not penetrate the DNA which
through walls and Human body lead to risk of cancer. This has
been proved with
whereas Microwave
Radiations can penetrate through some survey taken.
walls and Human
body. The penetrations of The survey taken shows
microwave affect the that extensive use of cell
human body in various manners. phone which uses microwave
results in Brain
Tumour.