Introduction of
Microwave Test
Bench Components
1
Microwave Test Bench
It is a setup shown below which consists of some microwave devices. This whole setup
with few alterations is able to measure guided wavelength, free space wavelength, cut-off
wavelength, impedance, VSWR, power etc.
2
Klystron Power Supply
The klystron power supply is shown in below figure. Klystron power supply is compact and regulated
power source for reflex klystron tubes. It is generally used to generate microwave power in
laboratories. Three power sources are required for reflex klystron operation: (1) positive voltage
(often referred to as beam voltage), (2) negative voltage (often referred to as repeller voltage), and
(3) filament power. It supplies regulated DC voltage to beam electrode, negative regulated voltage to
reflector and 6.3 V AC to filament. Beam and repeller voltages can be continuously varied according
to the requirement and can be monitored along with beam current regularly by a meter mounted on
the front panel.
The reflector supply can be internally modulated with a saw tooth wave for FM or with a square
wave for obtaining AM. Square and saw tooth wave generators used for modulating the klystron
output are integral part of the klystron power supply. The frequency of 1 KHz is provided.
Typical Specifications:
Beam Supply:
Voltage: 250 to 300 volts
Current : 50 mA (maximum)
Reflector Supply:
Voltage: 15 to 300 volts negative w.r.t cathode.
Current: 10 μA (maximum)
Filament Supply:
Voltage: 6.3 volts DC ± 0.5 volts w.r.t cathode.
Current: 10 A (maximum)
3
Reflex Klystron
• The reflex klystron tube is a single cavity
microwave tube that makes use of the principle
of velocity modulation to generate microwave
power. It is also known as electron beam
modulated tube or cavity modulated tube or
linear beam tube.
• It is used mainly as a microwave signal source
for general laboratory use and as a local
oscillator in microwave receivers.
• Efficiency of the reflex klystron is 20 to 30% and
output power is 10 to 500 mW in frequency
range of 1 to 25 GHz.
• The frequency of operation can be changed by
mechanical rotation of tuning screw – On doing
so the dimension of resonant cavity changes and
hence the frequency changes.
4
Klystron Mount
Klystron mount is a waveguide of suitable
length having octal base on the broad wall
of waveguide and used to transmit
microwave power from reflex klystron tube.
It is designed with movable plunger at one
end of the waveguide to direct the
microwave power generated by reflex
klystron tube. A small hole on the broad wall
of the waveguide is provided through which
coupling pin of reflex klystron tube enters
into the waveguide. Maximum power
transfer from reflex klystron tube can be
achieved by tuning of movable plunger.
5
Waveguide Isolator
• Isolator is a two port non-reciprocal
microwave component used to isolate
oscillators (reflex klystron in this case) from
load i.e. to reduce load mismatch.
• It means that it passes microwave signal in
one direction and blocks signal going in the
other direction.
• A simple waveguide isolator contains a
magnetized ferrite bar to which a resistive
plate is attached. Microwave energy
travelling in the forward direction is very
little attenuated. In the reverse direction the
energy is absorbed in the resistive plate.
This effect is due to the non-reciprocal
distortion of the RF field caused by the
magnetized ferrite.
6
Variable Attenuator
• An attenuator is a two port device that reduces the
level of transmitted microwave power allowed to
pass though it by either reflecting or absorbing the
power. Waveguide attenuators consists of an
attenuation plate (resistive surfaces known as
resistive cards or thin metallic film or glass
substrate) mounted within the waveguide by
means of metal rods or by moving the plate in and
out of the guide at a fixed location.
• Attenuators are required to adjust the power
flowing in a waveguide. Variable attenuators
provide a convenient means of adjusting power
level very accurately.
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Frequency Meter
• It consists of a cylindrical tunable cavity mounted with its
axis perpendicular to the main waveguide and is coupled to
the waveguide through small hole as depicted in Fig. (a).
• The resonant frequency of the cavity can be varied by
varying the position of plunger in the cavity.
• A micrometer is attached to the plunger to indicate the
frequency directly.
• Output power versus frequency characteristics of the cavity
frequency meter is shown in Fig. (b). The output of the
cavity frequency meter decreases when the cavity is tuned
through resonance. At frequency far from resonance the
cavity presents an effective short circuit to the waveguide
wall.
• The frequency measurement would, therefore, consists of
tuning the cavity of the frequency meter until maximum
‘dip’ occurs on the indicating VSWR meter and then reading
the micrometer position and the corresponding frequency
from the calibration chart provided with the frequency
meter.
(a)
(b)
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10.47 GHz
9.195 GHz
9
Waveguide Slotted Section
• This system consists of a waveguide, a travelling
probe carriage and facility for attaching
instruments. The precession built probe carriage
having centimeters scale with a vernier reading of
0.1 mm least count is used to measure the
position of the probe.
• The longitudinal slot is cut along the center of the
waveguide broad wall. The probe (crystal
detector) is made to move along the slotted wall
which measures the electric field strength inside
the waveguide.
• In slotted line a small part of the electric field is
fed to a crystal detector via a probe inserted in the
waveguide. SWR or VSWR can be read directly on
VSWR meter connected to the crystal detector
output. This method is accurate only if:
(a) the probe depth is small enough not to
disturb the field in the waveguide.
(b)the crystal detector works in the square
law region i.e. the output voltage is
proportional to the input power.
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• The amount of coupling between waveguide and co-axial cable depends on
diameter and depth of penetration of the probe.
• The slotted section with tunable probe is used for monitoring standing wave
patterns (determination of location of voltage standing wave maxima and minima
along the line) into waveguide systems, measuring the VSWR and impedance of
the unknown load.
14.49 cm
9.5 cm
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Standing Waves and VSWR
The electromagnetic field at any point of a waveguide may
be considered as the sum of two travelling waves: the
incident wave which propagates from the generator and the
reflected wave which propagates towards the generator. The
reflected wave is due to discontinuity in the waveguide or
from a load impedance. The amplitude and phase of the
reflected wave depends upon the amplitude and phase of
the reflecting impedances and losses in the waveguide.
The presence of two travelling waves gives rise to the
standing waves along the waveguide. The electric and
magnetic field varies periodically with distance. The
maximum field strength is found where the two waves add in
phase and the minimum is observed where the two waves
add in opposite phase. Figure shows the standing wave
pattern for different load impedances. The distance between
two successive minima (or maxima) is half the guided
wavelength.
VSWR = Emax/Emin
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VSWR Meter
VSWR meter is essentially a low noise, high gain, tuned amplifier operating at fixed
frequency generally 1 kHz, having an output meter calibrated to indicate directly in
SWR (dB) when used with square law device. This device is primarily used for
measuring SWR in conjunction with slotted line with a suitable detector.
Specifications:
Frequency: 1 kHz ± 2%
Scale Selector: -5 dB, Normal
and Expanded
Meter Scale: SWR 1-4
SWR 3-10
Expanded SWR 1-1.3
Normal dB 0-10
Expanded dB 0-2
It reads more accurately on the
upper half of the scale than on the
lower half of the scale. Therefore, to
bring all readings on the upper half
of the scale a -5 dB attenuator is
provided.
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Matched Termination
• Matched terminations are designed to
absorb
incident
energy
without
appreciable reflections i.e. having VSWR
of the order of 1.05. It is also known as
matched load. It consists of a section of
waveguide in which a tapered resistive
card (absorbing material) is attached.
• The reflections arising from the end are
minimized by tapering the card. The card
is placed parallel to the dominant TE10
mode at a place where the electric field is
maximum to have maximum attenuation.
• As the card has finite thickness, the
reflections arising from it cannot be ruled
out. Moreover, it is located at Emax which
will give maximum reflected power. To
avoid this the resistive card is kept closer
to the side wall and its length is
increased.
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Procedure of Tuning Microwave Bench for Optimum Output Power
• Make the connections as required according to the setup given.
• Set the variable attenuator at maximum position.
• Keep the control knobs of klystron power supply as below:
Meter switch - ‘Off’
Mod-switch - AM
Beam voltage knob - Fully anticlockwise
Reflector voltage - Fully clockwise
AM-Amplitude knob - Around fully clockwise
AM-Frequency knob - Around mid-Position
• If you are using VSWR meter, keep the control knobs of VSWR meter below:
Range dB - 50 dB position
Input switch - Crystal low impedance
Meter switch - Normal position
Gain (coarse & fine) - Mid Position
• Switch ‘ON’ the klystron power supply, VSWR meter/CRO and cooling fan.
• Rotate the meter switch of power supply to beam voltage position and set beam
voltage at 250 V (you should not make beam voltage higher than 300V) with help of
beam voltage knob (you should not touch this knob till the end of the experiment).
• Adjust the reflector voltage to get some deflection in VSWR meter.
• Maximize the deflection with AM amplitude and frequency control knob of power
supply.
• Tune the reflector voltage knob for maximum deflection.
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• Tune the probe for maximum deflection in VSWR meter.
Procedure of Measurements
• Tune the frequency meter plunger to get a ‘dip’ on the VSWR scale and note down
the frequency directly from the frequency meter.
• Move probe along with the slotted line, the deflection in VSWR meter will vary.
Move the probe to a minimum deflection position, to get accurate reading; it is
necessary to increase the VSWR meter range dB switch to higher position. Note
and record the probe position.
• Move the probe to next minimum position and record the probe position again.
• Calculate the guided wavelength as twice the distance between two successive
minimum positions obtained as above.
• Finally, calculate the cutoff wave length and broader dimension of the waveguide.
Observations:
• Resonant frequency from frequency meter: f0 = ______ GHz
• For short circuit termination, note down the positions of four consecutive minima
with the help of VSWR meter.
Position of first minima, d1 = ______ cm
Position of second minima, d2 = ______ cm
Position of third minima, d3 = ______ cm
Position of fourth minima, d4 =_____ cm
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Calculations:
•
•
•
•
•
Take average, (let)A = [(d2-d1) + (d4-d3)] /2
Now λg can be calculated as 2*A
Calculate λo from f0.
From λg and λo, calculate λc with the help of equation (2).
At last, from λc, calculate the broader dimension of waveguide, a = λc/2.
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