Vidyavardhini’s College of Engineering & Technology
Department of Electronics and Telecommunication Engineering
Experiment. Introduction to Microwave Components.
No:8
Aim To study Microwave Components
Theory Microwaves are electromagnetic waves (E.M. Waves) having wavelength in
the micron range. Though microwave frequencies refer to those from 1GHz
to 106GHz but generally used for those wavelengths measured in
centimeters, roughly from 10cm to 1cm (3 to 30 GHz) and the waves having
wavelengths less than 1cm corresponds to higher frequencies (> 30 GHz) are
called millimeter waves (mm waves).
MICROWAVE FREQUENCIES
Relationship between the frequency (f) and the wavelength (λ)
of an E.M. wave is c = f λ
Where, c is velocity of electromagnetic radiation, usually called the
speed of light.
TABLE 1.1 IEEE MICROWAVE FREQUENCY BANDS
DESIGNATION FREQUENCY RANGE IN GHZ
L-Band 1.0 to 2.0
S-Band 2.0 to 4.0
C-Band 4.0 to 8.0
X-Band 8.0 to 12.0
Ku-Band 12.0 to 27.0
K- Band 18.0 to 27.0
Ka-Band 27.0 to 40.0
Millimeter 40.0 to 300
Sub-Millimeter 300 and above.
MICROWAVE SYSTEMS:
Usually, a microwave system consists of a transmitter sub-system, consisting
of a microwave oscillator, waveguide, transmission antenna, and a receiver
sub-system; that includes a receiving antenna, waveguide microwave
detector, power and frequency meter. The intermediate region between the
transmitter and receiver, as well as the inner hollow space of the waveguide,
may be filled up with air or dielectric medium. The electromagnetic wave
traverses with the speed of light through air.
Vidyavardhini’s College of Engineering & Technology
Department of Electronics and Telecommunication Engineering
Description Of Microwave Bench:
The Measurement Techniques in Microwave frequencies is vastly
different from that of the more conventional techniques. At Low
Frequency, it is convenient to measure voltage and current and use them
to calculate power. However, at Microwave frequencies, they are
difficult to measure since they vary with position in a transmission line
and hence, they are of little value in determining power. Therefore, at
microwave frequencies, it is more desirable and simpler to measure
power directly. At microwave frequencies, quantities measured are
relative and is not necessary to know their absolute values, i.e., it is
sufficient to know the ratio of two power rather than exact input or output
powers. The parameter that can be conveniently measured at microwave
frequencies are Frequency, Power, Attenuation, Voltage Standing Wave
Ratio (VSWR), Phase, Impedance, Insertion Loss, Dielectric Constant
Noise Factor.
The general set up for measurement of any parameter in microwaves is
called microwave test bench. The microwave test bench incorporates a
range of instruments capable of allowing all types of measurements that
are usually required for a microwave engineer. The bench is capable of
being assembled or disassembled in a number of ways to suit individual
Experiments. A general block diagram of the test bench comprising of
different components is shown below.
Vidyavardhini’s College of Engineering & Technology
Department of Electronics and Telecommunication Engineering
Klystron
Mount + CRO
power
Isolator Frequency Slotted Attenuato Detector
Meter waveguid r
e
Matched
Load
Figure 1. Microwave Bench
Klystron Power Supply:
Klystron Power Supply generates the voltage required for driving the
Reflex Klystron Tube 2k25. It is a stable, regulated, and short-circuit-
protected power supply. It has built in facility of square wave and saw
tooth generators for amplitude and frequency modulation. The beam
voltage ranges from 200V to 450V with a maximum beam current, of 50
mA. The provision is given to vary repeller voltage continuously from
270V DC to – 10V.
Gunn Power Supply:
Gunn Power Supply comprises of an electronically regulated power
supply and a square wave generator designed to operate the Gunn
oscillator and Pin Modulator. The supply voltage ranges from 0 to 12 V
with a maximum current, of 1A.
Reflex Klystron Oscillator:
At high frequencies, the performance of a conventional vacuum tube is
impaired due to transit time effects, lead inductance and inter-electrode
capacitance. Klystron is a microwave vacuum tube employing velocity
modulation and transit time in achieving its normal operation. The reflex
type, known as reflex Klystron, has been most used source of microwave
power in Laboratory. It consists of an electron gun producing a
collimated electron beam. The electron beam is accelerated towards the
reflector by a dc voltage V0, while passing through the positive resonator
grids. The velocity of the electrons in the beam will be
Vidyavardhini’s College of Engineering & Technology
Department of Electronics and Telecommunication Engineering
Where e and m being electronic charge and mass respectively. The
repeller, which is placed at a short distance from the resonator grids, is
kept at negative potential with respect to cathode, and consequently it
retards and finally reflects the electrons which then turn back through the
resonator grids.
Gunn Oscillator:
Gunn oscillator utilizes Gunn diode which works on the principle that
when a d.c. voltage is applied across a sample of n type Gallium Arsenide
(GaAs), the current oscillates at microwave frequencies. This does not
need high voltage as it is necessary for Klystrons and therefore solid state
oscillators are now finding wide applications. Normally, they are capable
of delivering 0.5 watt at 10GHz, but as the frequency of operation is
increased the microwave output power gets considerably reduced. Gunn
oscillators can also be used as modulated microwave sources. The
modulation is generally provided by means of a PIN diode. PIN diode is
a device whose resistance varies with the bias applied to it. When
waveguide line is shunted with PIN Diode and the diode is biased
positively, it presents very high impedance thereby not affecting the line
appreciably. However, it is negatively biased it offers a very low
impedance; almost short-circuit thereby reflecting the microwave power
incident on it. As impedance varies with bias, the signal is amplitude
modulated as the bias varies. Since heavy power is reflected during
negative biasing of PIN diode, so an isolator or an attenuator should
invariably be used to isolate PIN diode avoid overloading of the latter.
Isolator:
The isolator is a two-port device. This device permits untenanted
transmission in one direction (forward direction) but provides very high
attenuation in the reverse direction (backward direction). This is
generally used in between the source and the rest of the setup to avoid
overloading of the source due to reflected power.
Variable Attenuator:
Vidyavardhini’s College of Engineering & Technology
Department of Electronics and Telecommunication Engineering
The attenuator is two-port device. The device that attenuates the signal
is termed as attenuator. Attenuators are categorized into two categories,
namely, fixed attenuators and variable attenuators. The attenuator used
in the microwave setup is of variable type. The variable attenuator
consists of a strip of absorbing material which is arranged in such a way
that its profusion into the guide is adjustable. Hence, the signal power to
be fed to the microwave setup can be set at the desired level. This type
of attenuator is called a flap attenuator.
Frequency Meter:
Frequency meter is basically a absorption cavity resonator. The cavity is
connected to a waveguide having been excited by a certain microwave
source. The Cavity can be made to resonate at source frequency by
adjusting its size by rotating the dial of frequency meter. At resonant
frequency it sucks up some signal from the guide to maintain its stored
energy. Thus if a power meter had been monitoring the signal power at
resonating condition of the cavity it will indicate a sharp dip. The
frequency can be read from the scale of direct reading frequency meter.
If it is indirect reading frequency meter tuning can be achieved by a
micrometer screw. The frequency can be obtained by using calibrating
chart.
Slotted Section:
To sample the field with in a waveguide, a narrow longitudinal slot with
ends tapered to provide smoother impedance transformation and thereby
providing minimum mismatch, is milled in the center of the top of
broader dimension of the waveguide. Such section is known as slotted
waveguide section. The slot is generally so many wave length long to
allow many minimum of standing wave pattern to be covered. The slot
location is such that its presence does not influence the field
configurations to any great degree. A probe is inserted through the slot
senses the relative field strength of the standing wave pattern inside the
waveguide. The probe is placed on a carriage plate which can be moved
along the waveguide. The probe is connected to a crystal detector and
the output is connected to indicating meter. For detector tuning a tuning
plunger is provided instead of a Stub.
Vidyavardhini’s College of Engineering & Technology
Department of Electronics and Telecommunication Engineering
Crystal Detector:
The simplest and the most sensitive detecting element is a microwave
crystal Diode. It is a nonlinear, nonreciprocal device that rectifies the
received signal and produces, a current proportional to the power input.
Since the current flowing through the crystal is proportional to the square
law detection property of a crystal is valid at a low power levels (<10
mw). However, at high and medium power level (>10mw), the crystal
gradually becomes a linear detector. Detector Mount is used for detection
in which Crystal Detector is shunted in waveguide.
VSWR Meter:
Direct- reading VSWR meter is a low-noise voltage tuned amplifier
calibrated in dB and VSWR for use with square law detectors. A typical
SWR meter has a standard tuned frequency of 1 KHz at which the
microwave signal is modulated. Clearly the source of power to be used
while using SWR meter must be giving us a 1 KHz square wave
modulated output. The band width facilitates single frequency
measurements by reducing noise while the widest setting accommodates
a sweep rate fast enough for oscilloscope presentation. The scale of
VSWR meter is calibrated in VSWR and VSWR in db. It has two normal
and one Expanded scale. In first normal scale VSWR from 1-3 can be
measured and in the second normal scale, VSWR from 3 to 10 can be
measured. To measure low VSWR (less than 1.3) Expanded scale is
used. The dB scale is present along with the Expanded dB scale. Using
the knob on the front panel choose the normal scale or Expanded scale.
Using the knobs on the front panel can change the gain in the VSWR
meter. Two knobs are present for changing gain from 0 to 10 db. Gain
can also be changed by 0 to 10 dB in steps of 10 dB using another knob.
While measuring VSWR, the gain should be either 50 or 60 db. for
accurate measurement of VSWR. Both crystal and bolometer may be
used in conjunction with the SWR meter. There is provision for high
(2,500 – 10,000 ohm) and low (50-200 ohm) impedance crystal inputs.
Input selector Switch is used to select the crystal or bolometer. This
instrument is the basic piece of equipment in microwave measuring
techniques and is used in measuring voltage peaks, valleys, attenuation,
gain, and other parameters determined by the ratio of two signals.
Waveguides:
Vidyavardhini’s College of Engineering & Technology
Department of Electronics and Telecommunication Engineering
A waveguide is a hollow metallic tube of a rectangular or circular cross-
section used to guide an electromagnetic wave. Waveguides are used
principally at microwave frequencies. In laboratories x band (8 to 12
GHz) range of frequencies are used. Therefore, standard X-band
rectangular waveguides are used having an inner width, 0.4 in and an
inner length, 0.9 in. In waveguides, the electric & magnetic fields are
confined to the space within the guides. Thus, no power is lost through
radiation, and even the dielectric loss is negligible, since the guides are
normally air filled. However, there is some power loss as heat in the
walls of the guides. It is possible to propagate several modes of
Electromagnetic waves within a waveguide. A given wave-guide has a
definite cutoff frequency for each allowed mode and behaves as a high
pass filter. The dominant mode in rectangular waveguides is TE10 mode.
Post
Experiment 1. Mention the frequency band for a millimeter wave.
Quiz 2. List some of IEEE microwave frequency bands.
3. List some of characteristic feature of microwave.
4. List some of the application of microwave technology.
