UNIT 3A – MICROWAVE ANTENNAS 1

SET-1 [19331A0418]
[7 MARKS]:
1. Draw and explain how gain can be improved in Yagi antenna?
A:

• Yagi Uda Antenna is used for receiving single TV channel.

• It contains a reflector,a driven element or folded dipole and directors.
• The active element is folded dipole as the feed is given to it.
• Field Transmission lines is connected to folded dipole.Length is exactly equal to λ / 2.
• Reflector rod is placed behind folded dipole & length is greater than λ / 2. It is used to
reflect energy towards driven elements.
• Directors are placed in front of driven element. Its l< λ / 2. The minimum no. of
directors is 3.
• Reflector acts like a mirror, driven element acts like a source and directors acts like
lens.
• Reflector have inductive effect, driven elements have resistive effect, directors have
capacitive effect.
DESIGN EQUATIONS:
1.Reflectors = 152/f(MHz) mts.
2.Driven element = 143/f(MHz) mts.
3.Directors = 137/(MHz) mts.
4.Spacing between reflector & driven element = 0.2 to 0.25 λ
5.Spacing between director = 0.3 to 0.4 λ
6.Length of Directors = 0.3 to 0.4 λ
7.Length of Driven element = 0.4 to 0.55 λ
8.Length od Reflector 0.5 to 0.55 λ
3 MARKS:
1. Derive the input impedance of two element folded dipole used in Yagi antenna?
A:

𝑉
= Z11 I1 + Z12 I2
2
𝑉
= Z21 I1 + Z22 I2
2

If ‘t’ is very small,

I1 = I2 = I
If S is small,
Z11=Z12
Z22=Z21
𝑉
= [Z11 + Z12] I
2
𝑉
= [Z21+ Z22] I
2
𝑉
= 2 IZ11
2
𝑉
= 4 IZ11
2

𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑑𝑖𝑝𝑜𝑙𝑒 𝑖𝑠 73Ω

𝑉
Hence, 2 = 4 ∗ 73Ω

FORMULAE:
• If d1 is not equal to d2 and s is small, Zin = Z11[1+r2/r1]2 where r1=radius of 1st
dipole, r2 = radius of 2nd dipole.
• If s is very small and d1 is not equal to d2, Zin = Z11[1+log(s/r1)/log (s/r2)]2
2MARKS:
1.A 3 element Yagi antenna is designed for GSM1800 then finds it reflector length?
A: Given Frequency is 1800Hz,
152
Reflector length = 𝑓(𝐻𝑧)

= 152/1800
= 0.0844mts.
 19331A0435

UNIT-3A
ASSIGNMENT-3
2. Draw the structure of YAGI antenna and explain its operation.
YAGI UDA antenna:
This antenna contains 3 types of elements. They are:
1. Rod reflects
2. Folded dipole or driven element
3. Directors

YAGI UDA antenna is used for receiving single TV channel.

YAGI UDA antenna

Feed transmission line is connected to folded dipole or driven element and its length is exactly
equal to λ/2.
Reflector rod is placed behind the folded dipole and its length is greater than λ/2.
L > λ/2
It is used to reflect the energy towards folded dipole.
Directors are placed before/ in front of the folded dipole (driven element). Its length is less
than λ/2.
The minimum number of directors required for any YAGI UDA antenna are 3.
Reflector acts like a mirror.
Directors act like a lens.
Folded dipole acts like a source.
Reflector have inductive effect.
Folded dipoles have resistive effect.
Directors have capacitance effect.
Design Equations:
1. Reflector = 152mts / f(MHz)
2. Driven element = 143mts / f(MHz)
3. Directors = 137mts / f(MHz)
4. Spacing between reflector and driven element,
S1 = 0.2 - 0.25 λ
5. Spacing between directors,
S2 = 0.3 - 0.4 λ
6. Length of the directors,
L1 = 0.3 to 0.4 λ
7. Length of the folded dipoles,
L2 = 0.4 to 0.45 λ
8. Length of the reflectors,
L3 = 0.5 to 0.55 λ

Folded Dipole:

It is an antenna consists of two or more parallelly and closely spaced dipoles connected together
at their edges / ends which forms a folded dipole.
In the folded dipole, one dipole must be center fed (connecting signal source) as shown in
figure.

Element ‘1’ is directly connected to source voltage V and the other element ‘2’ is inductively
coupled at their ends.
The radiation pattern at folded dipole is same as half wave dipole and it is like figure of 8.
The folded dipole is mainly used in YAGI UDA antenna as a main element (source element).
The input independence of folded dipole is much higher than half wave dipole.
For ‘n’ element folded dipole, its input independence is given as
Z1n = n2Rr

Where,
Rr -> the radiation resistance of half wave dipole
Rr = 73Ω
Where, ‘n’ is the number of elements in the folded dipole.
3 MARKS:
2.Design a yagi-uda antenna at frequency 200Mhz and number of elements are 5?

A:
Given frequency = 200Mhz
Reflector = 152mts / f(MHz) = 152/200 =0.76 mts
Driven element = 143mts / f(MHz) = 143/200 = 0.715mts
Directors = 137mts / f(MHz) = 137/200 = 0.685 mts
Spacing between reflector and driven element,
S1 = 0.2 - 0.25 λ
S1 = 0.2(c/f)
= 0.2(3*10^8/200*10^6)
=1.5(0.2)
=0.3
 Spacing between directors,
S2 = 0.3 - 0.4 λ

S2 =0.3(c/f)
= 0.3(3*10^8/200*10^6)
=0.3(1.5)
=0.45
2 MARKS:
2. A 3 element Yagi antenna is designed for GSM1800 then finds it director length?

A:Frequency for GSM1800 ranges from 1710-1785MHz

Assume frequency = 1750 MHz

Director length = 0.44 * λ

λ = 3*10^8/1750*10^6

λ =0.174

Director length = 0.44* λ

=0.44*0.17

=0.075
 UNIT-3
ASSIGNMENT-3
D.S.V.S.G. AMULYA
19331A0443
ECE- ‘A’
7-MARKS :

3. Find the length of reflector, director and driven element operating at 145MHz.
A. Given that frequency(f) = 145MHz.
Reflector length (feet) = 500/f (MHz)
= 500/145
= 3.44 feet
Driven element length (feet) = 475/f (MHz)
= 475/145
= 3.27 feet
Director element length (feet) = 455/f (MHz)
= 455/145
= 3.13 feet
3-MARKS :

3. Prove how the longer antenna behind the main antenna of a Yagi behaves as a
reflector ?
A.
• A basic Yagi-Uda antenna consisting of a driven element, one reflector and
director.
• The driven element infront of driven element is called director which is of lower
length of all the three elements.
• The length of director is comparatively small
• The length of main antenna in Yagi-Uda antenna is 0.46𝜆 and length of director
is 0.475 𝜆.
• Hence the longer antenna behind the main antenna of a Yagi behaves as a
reflector.
2-MARKS :

3. Draw the radiation pattern of folded dipole used in Yagi antenna?

A. Half-wave folded dipole antenna,which is omni-directional pattern.Half-wave
folded dipole antennas are used where optimum power transfer is needed and where
large impedances are needed.This folded dipole is the main element in Yagi-Uda
antenna.
7-Mark
4. Design a six element Yagi array operating at 500MHz and explain?
Given Operating frequency (f) = 500MHz
We know that 𝑐 = 𝑓𝜆
Therefore 𝜆 = 𝑐/𝑓
3 ∗ 108 300
𝜆= = ( ) = 0.6𝑚
500 ∗ 106 500
The design parameters of 6 element Yagi-Uda antenna is given by:
• The Length of the driven element 𝐿𝑎 = 0.46𝜆 = 0.46 ∗ 0.6 = 0.276𝑚
• The Length of the reflector 𝐿𝑟 = 0.475𝜆 = 0.475 ∗ 0.6 = 0.285𝑚
• The Length of the director 𝐿𝑑1 = 0.44𝜆 = 0.44 ∗ 0.6 = 0.264𝑚
𝐿𝑑2 = 0.44𝜆 = 0.44 ∗ 0.6 = 0.264𝑚
𝐿𝑑3 = 0.43𝜆 = 0.43 ∗ 0.6 = 0.258𝑚
𝐿𝑑4 = 0.40𝜆 = 0.40 ∗ 0.6 = 0.240𝑚
• Spacing between reflectors and driven element 𝑆𝑙 = 0.25𝜆 = 0.25 ∗
0.6 = 0.15𝑚
• Spacing between director and driven element 𝑆𝑑 = 0.31𝜆 = 0.31 ∗
0.6 = 0.186𝑚
• The diameter of the element is 𝑑 = 0.01𝜆 = 0.01 ∗ 0.6 = 0.006𝑚
• The Length of Yagi-Uda array is 𝐿 = 1.5𝜆 = 1.5 ∗ 0.6 = 0.9𝑚

0.186m

0.264m

0.276m

0.285m 0.240m

0.264m 0.258m

0.15m

3-Mark
 4. Prove how the shorter antenna in front of main antenna of a Yagi
acts as a director?

• A basic Yagi-Uda antenna consisting of a driven element, one

reflector and director.
• The element in front of driven element is called director which is
of lower length of all three elements
• The length of director is about 0.44𝜆 which is comparatively
smaller than that of main antenna 0.46𝜆
• The number of directors determines how many lobes appear on the
radiation pattern of this antenna. The more lobes there are, the
narrow they become, which means that they can focus on one
particular direction.
• Hence the shorter antenna in front of main antenna of a yagi acts as
a director.

2-Mark
4. Draw the equivalent model of Yagi antenna?

Director

Driven Element

Reflector
 UNIT-3A 5th QUESTION (19331A0403)
7 MARKS
5.Explain about the importance of folded dipole and derive its input impedance?

Ans:A folded dipole is the dipole antenna with ends folded back around and connected to each
other, forming a loop

Typically, the width d is far less than the length L

Because the folded dipole forms the loop, one might expect input impedance to depend on the
input impedance of a short circuited transmission line of length L. However a folded dipole
antenna can act as two parallel short circuited transmission lines of length L/2 seperated at the
midpoint . It turns the output of the input impedance of the folded dipole antenna will be
function of a transmission line of length L/2

Also, Folded dipole id “folded” back on itself, the currents can reinforce each other instead of
cancelling each other out, so the input impedance will also depend on the impedance of a dipole
antenna of length L
Let Zd is the impedance of the dipole antenna of length L and Zt is the input impedance of the
transmission line off length L/2,

ZA is the input impedance of the folded dipole given by:

The folded dipole is resonant and radiates well at odd integer multiples of half wavelengths
(0.5λ,1.5λ…..),when antenna is fed I n the center.The input impedance of the folded dipole
antenna is higher than for a regular dipole antenna

Derivation for 2 element folded dipole:

If t is very small

I1=I2=I

If S=Small

Z11= Z12
Z22=Z21

V/2=Z11I1+Z12I2

V/2=Z21I1+Z22I2

V/2 =I(Z11+Z12)

V/2=2IZ11

V/I= 4Z11=4*73Ω=292Ω (since Z11=73Ω)

For 3 element folded dipole is 657Ω

3 MARKS
5.Find the gain of a paraboloid of 2m diameter operating at 5 GHz when Half-wave dipole
feed is used?

Ans: The parabolic reflector antenna gain is calculated as the gain over an isotropic source,
i.e. relative to a source that radiates equally in all directions. This is a theoretical source that
is used as the benchmark against which most antennas are compared. The gain is quoted in
this manner is denoted as dBi.

G=10log10 k(πD/λ)2

G is the gain over an isotropic source in dB

k is the efficiency factor which is generally around 50% to 60%, i.e. 0.5 to 0.6
D is the diameter of the parabolic reflector in metres
λ is the wavelength of the signal in metres
λ =c/f
wavelength=0.06
Gain=10log10*0.65(3.14*2/0.06)2
=10 * 3.8525
=38.525dB
 2 MARKS
5.Draw YagUda array of circular loops and indicate reflector, feed and directors?

Ans:

R=reflectors

D1,D2,D3,D4,D5,D6= Directors
19331A0447
6. With a neat sketch explain the principle of parabolic reflector?
A) Type of reflector which has a reflecting surface having the shape of a
paraboloid that is used to collect and re-radiated the electromagnetic energy is
known as Parabolic Reflector. It is regarded as the simplest and popular form
of reflector antenna.
It is regarded as a reflective device that is used both at transmitting as well as

receiving antenna.
Basically, it transforms the plane wave from the axis towards the focus into the
spherical wave. While the spherical wave from a point source present at focus is
transformed into the plane wave and is parallelly propagated along the axis.
While at the receiving end, the electromagnetic wave from the transmitting
antenna when reaches the reflector antenna, then is converted into the spherical
wave and is further directed to change to electrical form which is required at the
respective circuit.
Working Principle of Parabolic Reflector:
Consider having a parabolic reflector in receiving mode and where the feed
element is present at the focus. The crucial function of the parabolic reflector is
to change the spherical wave into a plane wave.
So, at the focus when a feed antenna is placed which is nothing but an isotropic
source then the waves are emitted from the source.

The radiating element used at the focus is generally dipole or horn antenna, which
are used to illuminate the reflecting surface.
Thus, the waves emitted from the source, incident on the surface of the reflector
and are further reflected back as a plane wave of circular cross-section. This is

represented below:

As we can see clearly in the above figure that the waves from the feed incidents
at different points on the parabolic surface. However, all the waves after reflection
are collimated and the plane waves travel in the direction parallel to the axis.

It is to be noted here that, if there is any deviation of the surface of the reflector
from an actual paraboloid then it must not be more than some fraction of
wavelength.
Thus, all the collimated waves from the reflecting surface have equal path length
i.e., twice the focal length with a similar phase.

This will lead to provide very high radiation in the direction of the parabolic axis.
In this way, the spherical wave from the feed is converted into a plane wave.
Radiation Pattern of Parabolic Reflector

The figure shown below represents the radiation pattern of a paraboloid:

6. Determine the FNBW of 2.5m parabolic reflector used at 5GHz?

A) The first null beamwidth of the parabolic reflector is given as:

FNBW = 140* λ / D

Given:

F = 5 GHz, D = 2.5m

We know that,

C=λf

λ = C/f

λ = (3*10^8) / (5*10^9)

λ = 0.06m

FNBW = 140 * λ / D = (140 * 0.06) / 2.5

FNBW = 3.36°

3) How unidirectional pattern is formed in Yagi-Uda antenna from the bi

directional pattern?
A) The bidirectional pattern is changed in to unidirectional pattern in Yagi-Uda
antenna. The bidirectional radiation pattern generated from the feed antenna
touches the reflector called secondary antenna. Then one side of the radiation
pattern will be reflected back to the feed antenna, which leads to the generation
of unidirectional pattern.
 UNIT-3(a)
7MARKS

7.Explain about different Parabolic reflector antennas?

Types of parabolic reflector antennas:
1. Truncated parabolic reflector
2. Pill box parabolic reflector
3. Cylindrical parabolic reflector
4. Offset feed parabolic reflector
5. Cassegrain parabolic reflector

1.Truncated parabolic reflector:

• It is not circular in appearance when we view from focal point.

• If we cut upper and lower part of the paraboloid then it is called truncated parabolic
reflector.
• It is used for military applications.

2. Pill box parabolic reflector:

• This is a reflector antenna which is enclosed by two parallel conducting plates enclosed
like a box with inner parabolic structure.
• The resultant antenna is called as pill box.
• Feed is placed at focal point of pill box.
• It is used for marine radar applications (ship to ship communication).

3. Cylindrical parabolic reflector:

• A plane sheet is curved in one-dimension to obtain a parabolic cylinder or by cutting the

cylinder exactly at the center which forms a parabolic cylinder with a focal point placing
a feed antenna.
 • It is used in marine radar applications (ship to shore communication).

4.Offset feed parabolic reflector:

• If the feed antenna is placed outside the focal point without offset distance (d) and offset
angle (𝜃) then it is called Offset feed parabolic reflector antenna.
• By using this type of antenna, the reflected and collimated rays will be produced without
any interference.
• This type of feed antenna is used to reduce the aperture blockage (in this case feed
antenna will not block the reflected rays).

5.Cassegrain parabolic reflector:

• In this parabolic reflector, two reflectors are used. They are :

1. Hyperbolic reflector
2. Parabolic reflector
• In Cassegrain feed mechanism, the focal point of the hyperbola should coincide
with the focal point of parabolic reflector.
• The aperture size of the hyperbola is very small when compared to the aperture
size of parabola.
• In this Cassegrain field, the feed antenna is placed at back side i,e. base of the
parabolic reflector.
• The most popularly used feed antenna is cassegrain horn antenna.
3MARKS
7.Determine the HPBW of 2.5m parabolic reflector used at 5GHz?
Given:
f = 5 GHz, D = 2.5 m
We know that,3×108
C=f𝜆
𝑐
λ=
𝑓

λ = 3×108/5×109
λ = 0.06m
70𝜆
HPBW = 𝐷
70×0.06
=
2.5
HPBW = 1.68°
2MARKS
7.Define Aperture blockage?

the effect of antenna parts lying in the path of rays arriving at or departing from a radiating
element or the aperture of an antenna.

For example, the feed, sub-reflector, or support structure may produce aperture blockage for
a reflector antenna.
 UNIT-3
MICRO WAVE ANTENNAS
19331A0464
7 marks:
8. Draw and explain about Cassegrain feed antennas?
➢ In this parabolic reflector, two reflectors are used. They are :
1. Hyperbolic reflector
2. Parabolic reflector
➢ In Cassegrain feed mechanism, the focal point of the hyperbola should coincide with the
focal point of parabolic reflector.
➢ The aperture size of the hyperbola is very small when compared to the aperture size of
parabola.
➢ In this Cassegrain field, the feed antenna is placed at back side i,e. base of the parabolic
reflector.
➢ The most popularly used feed antenna is cassegrain horn antenna.
OPERATION:
➢ The Electromagnetic wave radiated from the horn antenna first falls on the hyperbolic
reflector and reflects back again by the parabolic reflector.
➢ The primary reflector is hyperbola and the secondary reflector is parabola.
➢ By using this type of parabolic reflector spill over radiation is almost zero and aperture
blockage is very less.
➢ This is mainly used at microwave frequency applications.
➢ This type of antennas have high gain and directivity.

3 marks:
8. Find the gain of a paraboloid of 2m diameter operating at 5GHz when Half-
Wave dipole feed is used?
Ans. Given,
D = 2m
Frequency = 5GHz
G = 10 log10 𝑘( 𝜋D/𝜆)^2
Where G is the gain over an isotropic source in db
K is the efficiency factor which is generally around 50% to 60%, i.e.0.5 to 0.6
D is the diameter of the parabolic reflector in meters.
𝜆 is the wavelength of the signal in meters.
𝑐
We know that, 𝜆 = 𝑓

3∗108
𝜆 = 5∗109

𝜆 = 0.06m
G = 10 log10(0.5)( 𝜋(2)/(0.06))^2
= 10log10 5,477.55
= 10(3.7385)
= 37.385
2 mark:
8. Define Spill-over Radiation?
Ans. This measures the amount of radiation from the feed antenna that is reflected by the
reflector. Due to finite size of the reflector, some of the radiation from the feed antenna will
travel away from the main axis at an angle greater than, thus not being reflected.
 19331A0456

UNIT-3

7 MARKS
9. Derive f/D ratio of parabolic reflector?

• It is one of the Important parameters for a parabolic reflector

• It simply defined as ratio of focal length to the mouth diameter of
parabola.

1 θ₀
F/D = 4 cot ( 2 )

Derivation:

FP+PQ = FS+SR = K
 19331A0456

For parabola,

OP+PQ = constant = 2f

PQ = rꞋ cos θꞋ

rꞋ + rꞋ cos θꞋ = 2f

rꞋ (1+ cos θꞋ) = 2f

2𝑓
rꞋ =
(1+ cos θꞋ)

𝐷/2
first find, θ₀ = tan−1 [ ]
𝑍

D²
where Z₀ = f - 16𝑓

𝐷/2
θ₀ = tan−1 [ 𝐷² ]
𝑓−16𝑓

By simplifying,
𝐷 θ₀
f = 4 cot( 2 )

1 θ₀
F/D = 4 cot ( 2 )
 19331A0456

3 MARKS

9. find the gain of a paraboloid of 2.5 m diameter operating at 5GHZ when

half wave dipole is used?
Given,
D = 2.5m
Frequency = 5GHz
G = 10 log10 𝑘( 𝜋D/𝜆)^2
Where G is the gain over an isotropic source in dB
K is the efficiency factor which is generally around 50% to 60%, i.e.,0.5 to 0.6
D is the diameter of the parabolic reflector in meters.
𝜆 is the wavelength of the signal in meters.
𝑐
We know that, 𝜆 = 𝑓

3∗108
𝜆=
5∗109

𝜆 = 0.06m
G = 10 log10 (0.5)( 𝜋(2.5)/(0.06))²
= 10(3.932)
G = 39.328
 19331A0456

2 MARKS
9.Draw the image diagram for a square corner reflector?

If the corner angle is 90 degrees, then that reflector is said to be as square corner
reflector.
 19331A0456

19331A0425
ANTENNAS AND WAVE PROPAGATION
ASSIGNMENT-3
Question 10 (7 Marks) :
Explain the operation of corner reflector antenna. Mention its advantage and
disadvantage.
Answer :
• If two planes connected together at their ends with an angle 𝛼 (edge angle)
and length of two planes 𝑙1 and 𝑙2 with a height ‘h’, then it is called corner
reflector.

• It uses the law of optics that implies, the signal after reflection travels in
the same direction from which it was received.
• The principle of its operation is such that when an electromagnetic wave
strikes the corner reflector then the incoming ray gets reflected from each
electrically conductive surface once.
• This means for a dihedral structure the wave is reflected twice while in case
of trihedral structure the wave is reflected thrice.
 19331A0456

• Thus, the direction of propagation of the wave gets reverse.

• It reflects the wave in the direction from where these are originated and are
regarded as a passive device.
• The corner shaped reflector facilitate confining the radiated energy within
the metallic plate. This provides an improvement in the directivity by
reflecting the received energy in the desired direction.
• A corner reflector is a reflecting object which consists of two or three
mutually interesting conducting planes. If the two planes are mutually
perpendicular to each other then they are called square corner reflectors.

Square Corner Reflector :

• If the corner angle 𝛼 is 900 then it is called square corner reflector.
• In square corner reflector there are 3 imaginary sources and 1 real source.
• All these sources are separated with an angle 900 .
• The height of square corner reflector is 1.5 times to the height of feed
element.
• The distance between source and vertex point (d) must vary from 0.3𝛌 to
0.5 𝛌.
• The aperture distance between two planes (𝐷𝑎 ) must be 1 𝛌 to 3 𝛌.
1800
• The corner angle 𝛼 = , where ‘n’ is the number of planes.
𝑛
• If n=2, then the corner angle is 900 then it is called as square corner
reflector.
• Number of images depends on the corner angle of the reflector i.e, number
3600
of images is not equal to .
𝛼
• If 𝛼 is 900 then the number of sources are 4, in that 1 is real source placed
in front of the reflector and the remaining 3 are the image sources.

Advantages :
• If offers ease of construction.
• It possesses high directivity by reflecting the electromagnetic in the
direction of its source.

Disadvantages :
• Its presence makes the antenna arrangement quite bulky.
• The use of this reflector increase the cost of the corner reflector antenna.
 19331A0456

Question 10 (3 Marks) :
If f/d ratio of a parabolic reflector antenna is 0.65, the find aperture angle in
degrees?
Answer :
Given,
f/d = 0.65
𝑓 1 𝜃
= 4 cot ( 20 )
𝑑
1 𝜃
0.65 = 4 cot ( 20 )
𝜃
2.6 = cot ( 20 )
𝜃0
= cot −1 (2.6)
2

𝜃0 = 2cot −1 (2.6)
𝜃0 = 42.070

Question 10 (2 Marks) :
What is power gain of a parabolic reflector having mouth diameter 6λ?

Answer :

Given,

Diameter = 6λ
𝜋𝐷 2
Gain (G) = 10log10 𝑘 ( )
λ

k is efficiency factor which is generally around 0.5 to 0.6

𝜋(6λ) 2
G = 10log10 0.5 ( )
λ

G = 10log10 0.5 (6𝜋)2

 19331A0456

G = 22.49
 19331A0423
19331A0456

UNIT 3a
Microwave Antennas-I

11.Explain about different reflector antennas? (7M)

• A reflector is a passive element which is used to reflect the electromagnetic wave or
energy in a desired direction.
• A reflector is used to avoid back radiation or unwanted radiation.
• It is a passive element which is always placed back side of the source at a distance ‘d’.
• There are different types of reflectors based on frequency of operations and
applications.
(i) Rod reflector
(ii) Plane reflector
(iii) Corner reflector
(iv) Parabolic reflector

ROD REFLECTOR:

If the reflector shape is in the form of rod, then it is known as rod reflector. A rod
type of reflector is the one which is majorly used in Yagi uda antenna. The reflector is
located at a certain distance behind the driven element in that antenna arrangement
and has a length generally more than the length of the driven element i.e., half-wave
dipole.
PLANE REFLECTOR:
If the reflector is made up of plane metallic sheet, then it is known as plane reflector antenna.

CORNER REFLECTOR:
 19331A0456

• If two planes connected together at their ends with an angle of α ( wedge / corner
angle) and length of two planes 𝑙1 and 𝑙2 with height h.
• A corner reflector is a reflecting object which consists of 2 or 3 mutually intersecting
conducting planes. If the two planes are mutually perpendicular to each other, the it is
called as square corner reflector.

PARABOLIC REFLECTOR:

• It is a reflector antenna which has a shape of parabola and employees the properties of
a parabola.
• It is defined as plane of curve obtained by a locus of a point so that its distance from
focal point + its distance upto directrix is constant.
• A parabolic reflector is a 2D antenna.
• In a parabolic reflector primary antenna is called as feed antenna, which is placed at a
focal point.
• The electromagnetic energy that falls on the parabolic surface, it reflects back towards
the direction of propagation.
• Similarly, in the remaining case the electromagnetic energy falls on the parabolic
dish, it is confined to a point called focal point.
• If the parabolic antenna rotated along its axis in the direction of propagation, we will
get paraboloid.
• Paraboloid is a 3D antenna.
• If the base part of the parabola is trauncated then it is called as microwave dish
antenna.
 19331A0456

11. A paraboloid is operating at 6GHz has a beam width of 50

find mouth diameter? (3M)
A. Freq = 6GHz
BW=5°
𝑐 3∗108
Λ=𝑓 = 6∗109 = 0.05
70 λ
BW = = 5°
𝐷
70 λ
=D
5
70∗0.05
D= 5
7∗0.5
= 5

D = 0.7m
 19331A0456

11.For a corner reflector antenna with apex angle 400 then find
the number of images? (2M)
360°
A. Number of images n = 𝛼

Given α = 40°
360°
n= =9
40°

Number of images with apex angle 40° =9

 19331A0456

ASSIGNMENT -3(a)
19331A0448
7MARKS
12. Explain the operation of corner reflector antenna with image antenna
concept?
Ans:- If the two planes connected together at their ends with an angle 𝛼 (corner
angle α) and length of the two planes 𝑙1 and 𝑙2 with height ‘h’ then it is called
corner reflector.

A typical corner reflector is shown in fig.

l d

feed
 d Da h

l 
Da
(b) perspective view
(a) side view

Grid wires
dg

Supporting rod

(c) wire-grid corner reflector

 19331A0456

• A corner reflector is a reflecting object which consists of two or three

mutually intersecting conducting flat surfaces. Plane sheet forms with
mutually perpendicular surfaces are used as radar targets.
• If the two planes are mutually perpendicular to each other then it is called
square corner reflector.

The analysis for the field radiated by the source in the presence of a corner
𝜋
reflector is facilitated when the included angle (𝛼) of the reflector is α = 𝑛,
𝜋𝜋 𝜋
where n is an integer (𝛼 = 𝜋, , , , 𝑒𝑡𝑐. ) for those cases
2 3 4

(𝛼 = 180°, 90°, 60°, 45°, 𝑒𝑡𝑐 )

 19331A0456

3MARKS

12.A paraboloid is operating at 6GHz has a beam width of 50 find its gain?
Ans:- Given f = 6GHz
Beam width = 5°
4𝜋𝐾
Consider HPBW, we know that G = 𝐴𝑒 , K = 0.65
𝜆2
70𝜆
HPBW = 𝐷𝑎

70∗3∗108 𝐶
5° = 𝐷𝑎∗6∗109
, where λ = 𝑓 = 0.05
210
𝐷𝑎 = = 0.7m
300
4𝜋𝐾
G= 𝐴𝑒
𝜆2
𝜋𝐷𝑎2 𝜋(0.72)
𝐴𝑒 = = = 0.38
4 4
4𝜋(0.65)
G= ∗0.38
0.052

G = 1241.55
G(db) = 10𝑙𝑜𝑔10 G = 10log(1241.55)= 30.93
2MARKS
12. Find the power gain of a square horn antenna whose aperture size is 8λ?
4.5 𝑊𝑑
Ans: The power gain, 𝑔𝑝 = 𝜆2
4.5∗8𝜆∗8𝜆
= = 288
𝜆2
𝑔𝑝 = 24.59dB
 19331A0455

13.Explain the characteristics of an active square corner reflector with the

help of image principle(7m)
ANS:
• If the corner angle 𝛼 = 90° then it is called square corner reflector.
• In square corner reflector there are 3 image sources and one real source.
All these are seperated with an angle 90° .
• The height of the square corner reflector is 1.5 times the height of the
feed element.
• The distance between source and vertex point must be vary from 0.3λ to
0.5 λ.
• The aperture distance (𝐷𝑎 ) between the two planes must vary from

𝐷𝑎 = 1λ to 3λ
180
• The corner angle 𝛼 = where
𝑛
n = number of planes
if number of planes n = 2 then corner angle is 90° so it is called as
square corner reflector
• Number of images depends on corner angle of reflector
360
Number of sources = 𝛼
If 𝛼 = 90 then number of sources are 4 in that one is real source placed
infront of reflector and remaining three are image sources
 19331A0455

360
If 𝛼 = 90° that is =4
90
 19331A0455

360
If 𝛼 = 60° that is =6
60

360
If 𝛼 = 45° that is =8
45

360
If 𝛼 = 30° that is =6
30
 19331A0455

13.A paraboloid is operating at 6GHz has a beam width of 5 find

HPBW?(3m)
Sol:

Given Beam width = 5

3 𝑋 108
λ = 6 𝑋 109 = 0.05
140 λ
=5
𝐷𝑎

𝐷𝑎 = 70 X 0.05 = 3.5
70λ
HPBW =
𝐷𝑎
70 𝑋 0.05
3.5
 19331A0455

=1

13.Write differences between waveguide and horn antenna?

(2m)

Wave guide Horn antenna

• A waveguide is a hollow • The antenna which has the

metallic tube and in this shape of horn is called as ‘
metallic tube EM waves Horn Antenna ’. Its shape is
travels. as because of a waveguide
has one end of which is flared
out

• Operating frequency range • Operating frequency range

300mhz to 300 ghz 300mhz to 30 ghz

• The radiation pattern of wave • The radiation pattern of horn

guide is omni directional antenna is spherical wave front

• Applications: • Applications:
1.surface search radar 1.used for astronomical studies
applications • 2.used in microwave
2.used in microwave applications
applications
19331A0455
 19331A0455

20335A0401

7 marks:
14. Explain various types of feeds used in parabolic reflectors with neat diagrams?
A.Based on the feeling antenna used at the focal points, Parabolic reflectors are classified as:
i. Halfwave dipole feed parabolic reflector:
If a halfwave dipole antenna is used at focal point as the feeding antenna then it is called
as dipole feed parabolicReflector antenna. It has bidirectional radiation . So 50% of energy
radiated and 50% of energy reflected back.

ii. Horn feed parabolic reflector:

If a flared horn antenna is used at focal point as the feeding antenna then it is called as
horn feed parabolic Reflector antenna. Horn antenna has high directional characteristics. So
90% of energy is reflected by parabolic reflector antenna.
 19331A0455

iii. Yagi Uda antenna parabolic reflector:

If yagi uda antenna is used at focal point as the feeding antenna then it is called as yagi uda
feed parabolic Reflector antenna.

iv. Offset feed parabolic reflector:

If feed antenna is placed at an offset distance with an inclination angle. Aperture blockage
is completely eliminated but some spill over radiation problem exist.
 19331A0455

v. Casegrain feed Parabolic Refelector:

In Casegrain feed Parabolic Refelector there are two reflectors.
a. Hyperbolic Reflector.
b. Parabolic Reflector.

In casegrain feed, the feed antenna is placed at the back side i.e., Base of the parabolic
reflector. The aperture size of the hyperbola must be very small compared to aperture size of
the parabola. The focal point of Parabola should be coincide with the focal point of hyperbola.

3 marks:
14. A paraboloid reflector operates at 5GHz and diameter is 5m. It is required to measure far
field pattern so find minimum distance required?
A. Given frequency ,f =5GHz
Diameter of parabolic reflector = 5m
𝑐 3∗108
λ = 𝑓 =5∗109 = 0.06m
2𝐷2
minimum distance required ,R >
𝜆
2∗5∗5
R > 0.06
R > 833.3 m
2 marks:
14. What is the difference between parabolic reflector and paraboloid?

A. The main difference between parabolic reflector and paraboloid is parabolic reflector is 2

Dimensional and paraboloid is 3 Dimensional. Paraboloid have less attenuation compared

 19331A0455

to parabolic reflector.
 19331A0455

ASSIGNMENT - 3
DAKI.KOMALI
19331A0441
ECE-A
7MARKS:
15. Explain the effect of Spill over and Aperture blocking in parabolic reflectors?
Ans: FACTORS THAT INFLUENCE EFFICIENCY OF A PARABOLIC REFLECTOR:
1. Apeature blockage
2. Spillover radiation
3. Tappered illumination radiation
4. Polarization mismatch
SPILL OVER RADIATION:
It means some of the radiation from the feed antenna falls outside the edge of the
parabolic dish and doesnot contribute to main beam . This radiation is called spill over
radiation.

To overcome spill over radiation

• We have to use highly directional feed antenna

• The feed antenna should be placed exactly at focal point
• The outer edge of parabolic dish must be coated with ferrite absorbing material

G = ηD
𝟒𝝅
=η 𝛌𝟐
𝑨𝒆
 19331A0455

APERTURE BLOCKAGE:

• The feed structure and its supporting rods block some of the energy in the direction
of transmission or reception.
• The amount of energy blocked by primary antenna is called “Aperture blockage”. If
size of antenna increses amount of blockage increases.
• This can be avoided by “Offset feed mechanism”.

3MARKS:

15.Find the gain of a paraboloid of 3m diameter operating at 4 GHz when Half-wave dipole
feed is used?
Ans: Given diameter Da = 3m
Frequency (f) = 4GHz = 4 * 109 Hz
𝐷
Gain of paraboloid gp = 6.4( 𝛌𝑎 )2

f = 4 × 109 Hz
𝒄 𝟑 × 𝟏𝟎𝟗 𝟑
𝛌 = 𝒇 = 𝟒 × 𝟏𝟎𝟗 = 𝟒 = 0.75m
3
gp = 6.4( 3 )2
4

= 6.4(42)

= 102.4
Gain of paraboloid gp = 102.4
2MARKS:
15. Why corrugations are required for horn antenna?
Ans:
 19331A0455

• Corrugation means grooving inside horn antenna

• Corrugation is used to avoid internal reflector or interference which is occurred
inside a horn antenna
• It helps to avoid spill over radiation
 19331A0455

ASSIGNMENT-3

A Sowrya Deepika
19331A0405
ECE - 3A

7 Marks:-

16 Establish and explain gain and beamwidth relations for parabolic reflector.
Ans. Expression for Gain:-
The gain of a parabolic reflector depends on type of feed antenna placed at
focal point.
Relation between gain and aperture area is given by
4 𝜋𝑘
𝑔𝑝 = 𝐴𝑒
𝜆2

𝜋𝐷𝑎 2
Where 𝐴𝑒 = , 𝐷𝑎 = Mouth diameter of parabola
4

If the feed antenna is dipole antenna value of k is 0.65.

For an isotropic source k is 1.
Therefore the gain for a half wave dipole feed parabolic reflector antenna is
4𝜋(0.65) 𝜋𝐷𝑎 2 (0.65)𝜋2 𝐷𝑎 2
𝑔𝑝 = × =
𝜆2 4 𝜆2
𝐷
𝑔𝑝 = 6.4( 𝜆𝑎 )2
 19331A0455

Beam Width:-

Parabolic reflectors are used to produce high directional beams like pencil
beams.
These parabolic reflectors are mainly used in satellite communication.
70𝜆
Half power beam width = 𝐷𝑎

140𝜆
First null beam width = 𝐷𝑎

3 Marks:-

16 Find the gain of paraboloid of 4m diameter operating at 4GHz when half wave
dipole is feed is used?
Ans:- Given diameter = 4m
Frequency = 3 GHz
𝑐 3 × 108
λ = 𝑓 = 3 × 109

λ = 0.1m
𝐷
Now, gain = 𝑔𝑝 = 6.4( 𝜆𝑎 )2
4
= 6.4(0.1)2

= 6.4 × 402
 19331A0455

𝑔𝑝 = 10240

The gain of paraboloid is = 𝑔𝑝 = 10240

Gain in dB = 10log 𝑔𝑝

= 10log(10240)
= 10(4.0102)
= 40.102dB

Gain in dB = 40.102 dB

2 Marks:-

16 What is optimum horn?

Ans:- An optimum horn means horn antenna with a very long horn (an aperture
limited horn). The optimum horn yields maximum gain for a given horn length.
 19331A0455

UNIT 3A

19331A0433

7 Marks
17.Calculate the beamwidth between first nulls & gain in dB for a 2.5m paraboloid reflector
used at 6GHz?
Paraboloid reflector:
It is a reflector antenna which has the shape of paraboloid and employs the properties of
parabola.
A paraboloid is a three dimensional surface obtained by revolving the parabola about the
axis. The paraboloid is called the parabolic reflector or dish antenna.
The geometry of a parabolic reflector in transmitting mode and its radiation pattern is shown
below.

CD = mouth diameter,Da

AB = axis of parabola

AF = focal length

A = vertex

F = focus

P = parabola

. Given diameter of parabolic reflector D=2.5m

Frequency f= 6GHz
Beamwidth between first nulls FNBW = 140 λ/D
 19331A0455

.λ=c/f
= 3 *10^8 / 6 * 10^9
= 0.05 m
FNBW = 140 λ / D
= 140 * 0.05 / 2.5
= 2.8 rad

Gain G = 10 log10 k (π D / λ)^2

Where G is gain of antenna in Db
k is efficiency factor which generally around 60% i.e 0.6
D is diameter of parabolic reflector in metres
. λ is wavelength of signal in metres
G = 10 log10 0.6 (3.14 * 2.5 / 0.05)^2
= 41.69 dB
°
Therefore First nulls bandwidth FNBW = 2.8

Gain in dB =41.69dB

3 marks

17.Draw the structure of corrugated horn and why it is used?

Corrugation mean grove

Ψc = half of the flare angle
 19331A0455

t = width of the groove

w = separation between adjacent grooves
bl = aperture dimension
d = groove depth
corrugated horns are used as feed antennas for paraboloids, in which spill over efficiency and
cross polarisation losses are reduced and aperture efficiency is increased. These system are
used in radio astronomy and satellite communications.

2marks
17. The length of an E-Plane Sectoral Horn is 15cms. Design the horn dimensions such that it
is optimum at 10GHz?

Diameter of horn antenna Da =( 3 / λ ) ^ ½

λ = c/f
= 3 * 10^8 / (10 * (10^9))
λ = 0.03m
Da = ( 3 / λ ) ^ ½
Da = (3 / 0.03 ) ^ ½
Da = 10m

𝛥 = (Da)^2 / 8
= 100 / 8 * 0.03
𝛥 = 416.6

α = 2 tan-1( Da/ 2* le)

α = 2 tan-1( 10 / 2* 15 * 10-2)
α = 0.38°
 19331A0455

UNIT-3A
MICROWAVE ANENNAS -1
7MARK:
18.An 8GHz pyramidal horn is with dimensions 16x8 cms, find its gain and
directivity assuming 90 percent efficiency?
A: Horn Antenna:
It is a radiating element which is in the form of a horn.It is a
waveguide one end of which is flared out.
➢ If flaring is done along both electric and magnetic field directions then it
is called pyramidal horn antenna.
Given,
Frequency (f) = 8GHz
d = 16cm
w = 8cm
𝑐
𝜆= 𝑓
3∗10^8
= 8∗10^9

= 0.0375
The directivity of a loss-less horn antenna is its gain and is given by
4𝜋𝐴𝑒 4𝜋դ𝐴𝑎
D= =
𝜆^2 𝜆^2
𝐴𝑒
դ = aperture efficiency =
𝐴𝑎

դ = 90% = 0.9
4𝜋(0.9)𝐴𝑎 (11.31)𝐴𝑎
D= =
𝜆^2 𝜆^2
11.31(𝑤𝑑)
= 𝜆^2
11.31(16∗8)
= 𝜆^2
 19331A0455

11.31(16∗8)
= (0.375)^2

= 100.25
Directivity (db) = 10log10 𝐷 = 10log10 (100.25) = 20db
Gain (db) = դ*directivity (db)
= 0.9*20
= 18
3 MARK:
18. A coaxial feed pyramidal horn antenna has the following rectangular wave guide
dimensions: Waveguide (WR340) a = 3.4 inches, b = 1.7 inches. What will be the
cut-off frequency (in GHz) for TE10 mode operation?
A:
Given, Waveguide dimensions: Waveguide (WR340)
a = 3.4 inches
= 8.636 cm
b = 1.7 inches
= 4.318 cm
The cut-off frequency for a rectangular waveguide can be calculated using the formula given
below
𝑐
fc =
2𝑎
3∗10^8𝑚
= 2(8.636)𝑐𝑚
3∗10^8𝑚
= 17.272𝑐𝑚
0.172∗10^8𝑚
= 0.01𝑚
= 0.172*10^10
= 1.72 GHz

2 MARK:
18. Find the power gain of the horn whose dimensions are 10*5 cm operating at
6GHz.
 19331A0455

A: Given, dimensions of horn are

d = 10cm, w = 5cm, f = 6GHz
𝑐
𝜆= 𝑓
3∗10^8𝑚
= 6∗10^9

= 0.05m
= 5cm
4.5(𝑤𝑑) 4.5(5∗10)
Power gain, gp = = = 9 db
𝜆^2 (5)^2
 19331A0455

REG NO: 20335A0404

AWP ASSIGNMENT-3
SET-19
7-MARK
19. Draw the structure of corrugated horn and explain?

A. The corrugated horn antenna has parallel slots or grooves along the inside
surface of the horn, transverse to the axis. These corrugations are small
when compared to the wavelength. Corrugated horns have several
advantages including a wider bandwidth, and smaller side-lobes that other
types. The corrugated horn provides a pattern that is nearly symmetrical,
with the E and H plane beam-widths being nearly the same.
• Corrugated horn antenna is mainly used to avoid spill over radiation
• Corrugation means grooving (teeth inside horn antenna).
This grooving is mainly used to avoid internal reflections or interference
occurred inside horn antenna.
• The efficiency of horn antenna can be increased from 75% to 90% by using
corrugations.
• Apart from flared angle, flared length and aperture length , we have other
three parameters : Width of the groove (w), distance between the
grooves(d), and height of the groove (h).
• Mainly used for military applications in high performance RADAR.
• Corrugated horn antennas are widely used as feed horns for satellite dishes
and radio telescopes.
 19331A0455

3-MARK
19. For a coaxial feed pyramidal horn antenna uses a rectangular waveguide
WR75.What will be the cut-off frequency for TE10 mode?

A. The dimensions of WR75 rectangular waveguide =

0.75 Inches [19.05 mm] x 0.375 Inches [9.525 mm]
 19331A0455

The cut-off frequency for a rectangular waveguide can be

calculated using the formula given below:
𝑐
𝑓𝑐 =
2𝑎
Where:
fc = rectangular waveguide cut-off frequency in Hz
c = speed of light within the waveguide in metres per second
a = the large internal dimension of the waveguide in metres
3 x 108
𝑓𝑐 = = 7.874 𝑘𝐻𝑧
2(0.01905)

2-MARK
19. Draw offset feed parabolic reflector?
A. An offset dish antenna or off-axis dish antenna is a type of parabolic antenna.
It is so called because the antenna feed is offset to the side of the reflector,
in contrast to the common "front-feed" parabolic antenna where the feed
antenna is suspended in front of the dish, on its axis.
19331A0455
 19331A0455

AWP ASSIGNMENT 3

20335A0407
7marks

20. What is an electromagnetic horn antenna? What are its applications? Explain about
different types of horn antennas

A. Horn antenna is a radiating element which has the shape of horn. It is a waveguide in
which one end is open and other end is flared out, to improve radiation efficiency,
radiation pattern and directivity.

• Horn antenna is basically used at microwave frequencies (0.3-30Hz).

• It has high directivity; hence called as high directional antenna.
• It is mainly used as a reference antenna in testing applications, antenna measurements.
• The gain of horn antenna is very high which is used to produce high directional narrow
beam.
• They are used as feed elements.

Types of horn antenna:

1) Sectorial horn antenna
a. E plane
b. H plane
c. Pyramidal
2) Conical horn antennas
3) Corrugated horn antennas

Sectorial horn is a horn in which flaring exists only in one direction.

- If flaring is along the direction of electric field, it is called sectorial E-plane horn.
- If flaring is along the direction of magnetic field, it is called sectorial H-plane horn.
- If flaring is along E and H, it is called as pyramidal horn.
 19331A0455

Conical horn antennas

The figure-3 depicts conical horn antenna type. It is made by flaring out one end of circular

waveguide.

Following are the applications of horn antennas:

• Used as feed elements in reflector antennas

• Used to provide moderate gain

• Used in lab measurements and experiments

 19331A0455

Corrugated Horn Antennas

In a corrugated horn the surface of the inner part is corrugated. The corrugations are typically
axial (that is, in the direction of the horn axis) or radial (i.e., perpendicular to the axis). The
depths of the corrugations usually vary between λ/4 and λ/2, where λ is the wavelength of the
electromagnetic field at the design frequency

3marks
20. If the efficiency of the horn antenna is 50% with aperture dimension 20cmX15cm then find
its gain in dB?
Sol: Given,
Efficiency = 50%
Dimensions of Aperture = 20cm×50cm
= 20×10-2 ×15×10-2 m2
For X band range 8 to 12.5 centre frequency is 9.5GHz
Gain G = ȠD
4𝜋𝐴𝑒
Directivity D =
𝜆2

𝑐 3×108
= 𝑓=
9.5×109
= 0.0315 mts
4𝜋×20×10−2 ×15×10−2
D=
(0.0315)2

= 379.93 dB
 19331A0455

G = 0.5×379.93
= 189.965 dB
Gain in dBi = 10log10 189.965
= 22.787 dBi

2marks

20. How directivity of horn antenna can be improved?

The directivity of the horn can be enhanced by using longitudinal strips and di
electric fillings (hard horn) or stepped tapers (multi-mode horn). An effective way to enhance the
directivity is to place a homogeneous dielectric lens at the antenna aperture.