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EC Department SUBJECT NAME: Microwave Engineering
SUBJECT CODE: 2171001
B.E. 7th
SEMESTER
List of Practicals
Sr
No.
Name of Practical
1 Introduction to various components of microwave workbench and
measuring equipment.
2 To study Gunn diode V-I characteristics.
3 To study about characteristics of the Reflex Klystron Tube and to
determine its electronic range.
4 To determine the frequency and wavelength in a rectangular waveguide
working on TE10 mode.
5 To determine the standing wave ratio and reflection co-efficient.
6 To measure an unknown impedance using Smith chart.
7 To measure coupling factor, insertion loss and directivity of a Directional
coupler.
8 To study power division measurement in a Magic Tee.
9 To study measurement of vswr, insertion loss, isolation of Isolator and
Circulator.
10 To study Fixed and Variable attenuator.
EXPERIMENT: 1
AIM: Introduction to various components of microwave workbench and measuring
equipment.
THEORY:
MICROWAVE TEST BENCH:
Microwave Test Benches are precision made microwave systems, which use
standard type rectangular wave-guide components to illustrate the essential elements
of this field for study.
The equipment consist of:
1) A selection of wave-guide components
2) The power supply for the microwave source
3) A detector
4) A meter, which monitors the detector output
The components are explained below.
Klystron Power Supply:
Klystron Power supply is a state of art solid state regulated power supply for
operating low power klystron such as 2k25, 723 MB. Reflex Klystron uses only a
single cavity resonator and operates as an oscillator.
The various part included as electron gunn; Resonator, repeller and o/p coupling.
The reseller electrode is at a (-ve) potential and sends the partially bunched electrode
beam back to the resonator cavity. This provides a (+ve) feedback mechanism which
supports oscillation. If the voltage difference between resonator and repeller is V
and the distance is„d‟, the retardation experienced by the electron beam may be
obtained as
A= Force/Mass
= e/m
Gunn oscillator:
The Gunn Oscillator is based on negative differential conductivity effect in
bulk semi-conductor, which has two conductor bands minima separated at cathode
give rise to high field region, which travels towards anode. It disappears and another
domain is formed at cathode and starts moving towards anode and so on. The time
required for domain to travel from cathode to anode gives oscillation.
In a given oscillator, gunn diode is place in a resonant cavity. In this case the
oscillator frequency is determined by cavity dimension than by diode itself.
VSWR meter:
The meter indicates the signal level in propagation to the i/p which is
calibration directly in VSWR and db
Pin modulator:
Pin diode modulators are used to provide amplitude or pulse modulation in wide
range of microwave to study many applications.
These modulators uses PIN diode which is mounted across the waveguide
line with RF isolated DC bias lead passing to an external TNC(F).
Isolator:
This port circular may be converted into isolator by terminating one of its port
in\to momental load.
The isolator allows the waves to pass in one direction but stops them in
opposite direction.
Frequency meter:
It gives direct frequency on the dial provided These are recommended for used
Whenever quick determination of frequency and easy reading are desired.Direct
Reading frequency meters are used to measure the microwave frequency accurately.
There long scale length and numbered calibration marks provide high resolution
which is particularly useful when measuring frequency difference of small frequency
changes.
Variable attenuator Waveguide:
This is provided at least 20db of continuous variable attenuator. This Consist of
movable loosywave inside the section of a guide.
Magic Tee:
TEE consist of a section of waveguide with both series and shunt wave guide
arms mounted at the exact midpoint of main arm.
Fixed attenuator:
These are used for inserting a known attenuator to a waveguided. These of
attenuation to a waveguide.
These of a loosy inserted in a section flanged on the both ends.
H and E plane Tee:
Tee and series the type T junction and consist of three section of wave guide
joined together in order to divide and compare power levels.
E plane: The signal entering the first port of this junction will be equally divided of
second and third ports of the some magnitude but in \opposite phase
H Plane: the signal fed through the first port of h plane tee will be equally divided
in waveguide at 2nd
and 3rd
ports but in some phase.
Multiple Directional Coupler:
These are useful for sampling a port of microwave energy for monitoring
purpose and for measuring reflections and impedance.
Slotted Line:
Slotted section is used to measure various measuring parameter in microwave.
for example to determine VSWR, phase and impedances. These consist of a slot in
center of waveguide in which we can connect a probe and probe can be moved in
slot and position of probe can be measured by its Varnier scale. The travel of probe
carriage is more than three times of half wavelength.
Detector Mount:
The crystal detector can be used for the detection of microwave signal. At low level
of microwave power, the response of each detector approximates to square law
characteristics and may be used with a high gain selective amplifier having a square
law meter calibration.
EXPERIMENT: 2
AIM: To study Gunn diode V-I characteristics.
APPARATUS: Gunn Power Supply, Microwave Bench –Gunn oscillator, Gunn
diode, Isolators, Pin modulator, Variable attenuator (20 db),
Frequency meter, Slotted section, Detection mount, SWR meter.
THEORY: Gunn diodes are negative resistance devices which are normally used as
low power oscillator at microwave frequencies in transmitter and also as local
oscillator in receiver front ends.J.B. Gunn (1963) discovered microwave oscillation
in gallium arsenide (GaAs), Indium phosphide (InP) and Cadmium telluride
(CdTe).These are semiconductors having a closely spaced energy valley in the
conduction band as shown in figure for GaAs.
When dc voltage is applied across the materials, most of the electrons
will be locatedin the lower energy central valley T. At higher E-field, most of the
electrons will be transferred into the high energy satellite L & X valleys where the
effective electron mass is larger & hence electron mobility is lower than that in the
lower energy T valley.
Since the conduction conductivity is directly proportional to the mobility,
the conductivity & hence the current decreases with an increase in E-field or voltage
in an intermediate range, beyond a threshold value Vetch as shown in figure. This is
called transferred electron effect & the device is also called “Transfer Electron
Device”or “Gunn Diode”.
Thus, the material behaves as negative resistance device over a range of
applied voltages & can be used in Microwave oscillators.
The basic structure of a Gunn diode is shown in fig (a), which consists of
n-type GaAs semiconductor with regions of high doping (n+). Although there is no
junction this is called a diode with reference to the +ve end (anode), and negative
end (cathode) of the dc voltage applied across the device. If voltage or an electric
field at low level is applied to the GaAs, initially the current will increase with a rise
in the voltage. When diode voltage exceeds a certain threshold value Vth, a high
electric field is produced across the device where they become virtual immobile. If
the rate at which electrons are transferred is very high the current will decrease with
increase in voltage; resulting in an equivalent negative resistance effect. Since GaAs
is a poor conductor, Considerable heat is generated in the diode. The diode should be
well bounded into heat sink.
The electrical equivalent circuit of a gun diode is shown in figure where
Cj and -Rj are the diode capacitance and resistance respectively. Rs includes the total
resistance of a lead; ohmic contacts, and bulk resistance of the diode, Cp and Lp are
the package capacitance and inductance respectively. The negative resistance has a
value that typically lies in the range -5 to 20 ohms.Thegunn oscillator is based on –
ve differential conductivity effect in bulk semiconductor, which has two conduction
bands minima separated by an energy gap (greater than thermal agitation energies)A
disturbance at the cathode gives rise to high field region, which travels towards
anode & so on. The time required for domain to travel from Cathode to anode
(Transit time) gives oscillation frequency. In a gunn oscillator, Gunn diode is placed
in resonant cavity. In this case oscillator‟s frequency is determined by cavity
diminution than by diode itself.Althoughgunn oscillator can be amplitude modulated
with the bias voltage. We have used separate pin modulator through pin diode for
square wave modulation capability. A message of square wave modulation capacity
is modulation depth i.e. Output Ratio between on & off state.
PROCEDURE:
1. Set the component and equipment as shown in fig.
2. Initially set variable attenuation for max. Attenuation.
3. Keep control knob of gunn power supply as below: gunn bias knob: fully
anti clockwise.
4. Keep control knob of VSWR meter as below: meter switch: normal input
switch: low impedance, range db switch: 40 db, gunn control knob: fully
clock wise.
5. Set micrometer of the Gunn oscillator for required frequency of oscillation.
6. Switch on the Gunn power supply to voltage position.
7. Turn the meter switch of Gunn power supply to volt position.
8. Measure Gunn diode voltage corresponding to various current values
controlled by gunn bias knob through panel meter and meter switch, do not
exceed bias bolt above 10v.
OBSERVATION TABLE:
Sr.No CURRENT (Amp.) VOLTAGE (Volt)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
EXPERIMENT: 3
AIM: To study about characteristics of the Reflex Klystron Tube and to determine
its electronic range.
APPARATUS: Klystron power supply, Klystron tube with Klystron mount, Isolator
,Frequency meter, Variable attenuator, Detector mount, Wave guide
stand, VSWR meter, Oscilloscope
THEORY: The Reflex Klystron makes the use of velocity modulation to transform
a continuous electron beam into microwave power. Electron emitted from the
cathode are accelerated & passed through the positive resonator towards negative
reflector, which retards and finally, reflect the electrons and the electron turn back
through the resonator. Suppose an RF-field exists between the resonator the electron
travelling forward will be accelerated or retarded, as the voltage at the resonator
changes in amplitude.
The accelerated electron leaves the resonator at an increased velocity and
the retarded electrons leave at the reduced velocity. The electrons leaving the
resonator will need different time to return, due to change in velocities. As a result,
returning electrons group together in bunches, as the electron bunches pass through
resonator, they interact with voltage at resonator grids. If the bunches pass through
the grids at such a time that electrons are slowed down by the voltage then energy
will be delivered to the resonator; and Klystron will oscillator.
The frequency is primarily determined by the dimensions of resonator
cavity.Hence, by changing the volume of resonator, mechanical tuning of Klystron is
possible. Also, a small frequency change can be obtained by adjusting the reflector
voltage. This is called Electronic tuning.
PROCEDURE:
1. Connect the components and equipments as shown in figure.
2. Set the Variable Attenuator at the maximum position.
3. Set the mode switch of klystron power supply to CW position ,beam voltage
control knob to full anti-clock wise and reflector voltage control knob to
fully clock wise and the meter switch to OFF position.
4. Rotate the knob of Frequency meter at one side fully.
5. Put VSWR meter in 50 db att. and coarse find on mid direction and put in
low crystal impedance position.
Square wave operation:
1. Connect the equipments and components as shown in the fig.
2. Set Micrometer of variable attenuator around highest position.
3. Set Micrometer of variable attenuator around highest position.
4. Set the range switch of VSWR meter at 40db position, input selector switch
to crystal impedance position, meter switch to normal position.
5. Set Mod-selector switch to AM-MOD position. Beam voltage control knob
to fully anticlockwise position and meter switch to „OFF‟ position.
6. Switch „ON‟ the Klystron Power Supply, VSWR meter and cooling fan.
7. Change the meter switch of Klystron Power Supply to beam voltage knob
clockwise up to 300v.
8. Keep the AM-MOD amplitude knob and AM-FREQ knob at the mid-
position.
9. Rotate the reflector voltage knob to get the deflection in VSWR meter.
10. Rotate the AM-MOD amplitude knob to get the maximum output in VSWR
meter.
11. Maximize the deflection with & frequency control knob of AM-MOD.
12. If necessary, change the range switch of VSWR meter 50 db to 30 db if the
deflection in VSWR meter is out of scale or less than normal scale
respectively. Further the output can be also reduced by Variable Attenuator
for setting the output for any particular position.
13. Find the oscillation by frequency by Frequency Meter as described in the
earlier setup.
14. Connect oscilloscope in place at VSWR and see square wave across detector
mount.
Mode Study on Oscilloscope:
1. Set up component and equipments as shown in fig.
2. Keep position of variable attenuator at maximum position.
3. Set Mod selector switch to FM-MOD position with FM amplitude and FM
frequency knob at mid position. Keep beam voltage control knob fully
anticlockwise and reflector voltage knob to fully clockwise with meter
switch to „OFF” position.
4. Keep the time/division scale of oscilloscope around 100Hz frequency
measurement and volt/div to lower scale.
5. Switch On the klystron Power Supply and oscilloscope.
6. Change the meter switch of Klystron power supply to beam voltage position
and set beam voltage to 300V by beam voltage control knob.
7. Keep amplitude knob of FM modulator to maximum position and rotate the
reflector voltage anti-clockwise to get modes as shown in fig. As well as on
the oscilloscope. The horizontal axis represents voltage axis and vertical axis
reprents output power.
8. By changing the reflector voltage and amplitude of FM modulation any
mode of Klystron tube can be seen on an oscilloscope.
CONCLUSION:
EXPERIMENT-4
AIM: To determine the frequency and wavelength in a rectangular waveguide
working on TE10 mode.
APPARATUS: Gunn Power Supply (NV101), Microwave bench -Gunn oscillator,
Gunn diode, Isolator, Pin modulator, Variable attenuator (20 db),
Frequency Meter, Slotted section, Detection mount, SWR Meter,
CRO, Cable.
THEORY: A Gunn diode oscillator can be designed by mounting the diode inside a
waveguide cavity formed by a short circuit termination at one end and by an iris at
other end as shown in figure. The diode is mounted at the centre perpendicular to the
broad wall where the electric field component is maximum under the dominant TE10
mode. The intrinsic frequency fo of oscillation depends on the electron drift velocity
Vd due to high field domain through the effective length L.
fo= Vd / L
For GaAs, Vd ≈ 107 cm/sec. Normally, the cavity is tuned to resonate at
the intrinsic frequency fo by adjusting the short position. A tuning screw is inserted
perpendicularly at the centre of the broad wall for fine frequency tuning.
The total resistive loading from the cavity and the external load should be
around 20 per cent higher than the Gunn device resistance –Rj so that the circuit
resistance - RLRj / (RL- Rj) will be negative. The cavity has an impedance
transforming property between the high impedance of the output waveguide and the
required low value for the Gunn diode. The Gunn diode is placed on a metal post.
The top of the post is insulated from the waveguide to form RF bypass capacitor and
dc bias voltage is applied to the post. The degree of coupling to the external
waveguide is adjusted by selecting the inductive iris dimension.
The power output of the Gunn diode oscillator is in the range of a few
watts for CW operation at biasing values 10V and 1A at 30- 40 GHz. A frequency
tuning range of nearly 2% can be achieved. For pulsed operation peak powers are
typically 100-200W. The power output of the Gunn diode is limited by the difficulty
of heat dissipation from the small chip. The advantages are small size, ruggedness
and low cost.
Mode represents in waveguides as either: TE m, n / TM m, n, Where, TE -
Transverse electric, TM- Transverse magnetic, m – Number of half wave length
variation in broader direction, n – Number of half wave length variation in shorter
direction.
λg/2 = (d1 – d2)
Here, d1 and d2 are the distances between two successive minima / maxima.
It is having lowest cut off frequency hence dominant mode.
For dominant TE10 mode in rectangular waveguide λo, λg and λc are related as
below:
For TE10 mode:
λc = 2a / m
Where, m =1 and „a‟ is broad dimension of waveguide, Where, λo – free space wave
length, λg – guide wave length, λc – cutoff wave length, Consider, C = f λ, Where,
C = 3 × 108 m/s is velocity of light, f = frequency
PROCEDURE:
1. Set up the components and equipment as shown in block diagram.
2. Set up the variable attenuator at maximum position.
3. Switch on the Gunn power supply, VSWR meter and cooling fan.
4. Adjust the voltage of power supply to get some deflection in VSWR meter.
5. Take the reading on Frequency meter at intersection of two horizontal and
one vertical red lines.
6. The value indicates the operating microwave frequency. At that frequency
the power becomes zero.
7. Take the reading from micrometer screw.
8. Turn the Gunn bias control knob to counter clockwise position.
9. Switch off the Gunn Power Supply.
OBSERVATION:
1) f =________________ 2) λ=________________
CALCULATION:
CONCLUSION:
EXPERIMENT: 5
AIM: To determine the standing wave ratio and reflection co-efficient.
APPARATUS: Gunn Power Supply (NV101), Microwave bench -Gunn oscillator,
Gunn diode, Isolator, Pin modulator, Variable attenuator (20 db),
Frequency Meter, Slotted section, Detection mount, Movable Short/
Termination or any unknown Load and BNC Cable, SWR Meter,
CRO, Cable, S-S Tuner.
THEORY: It is a ratio of maximum voltage along a transmission line is called
VSWR, as ratio of maximum to minimum current. SWR is measure of mismatch
between load and line.The electromagnetic field at any point of transmission line
may be considered as the sum of two traveling waves: the „Incident Wave‟
propagates from generator and the „Reflected Wave‟ propagates towards the
generator.
The Reflected Wave is set up by reflection of incident wave
fromdiscontinuity on the line or from the load impedance. The magnitude andphase
of reflected wave depends upon amplitude and phase of the reflectingimpedance.
The superposition of two traveling waves, gives rise to standing wave along with the
line. The maximum field strength is found where two waves are in phase and
minimum where the line adds in opposite phase. The distance between two
successive minimum (or maximum) is half the guide wavelength on the line. The
ratio of electrical field strength of reflected and incident wave is called reflection
between maximum and minimum field strength along the line.
Hence VSWR denoted by S is S= Emax / Emin = |E1|+|Er| / |E1|-|Er|, Where
E1 = Incident Voltage and Er = Reflected Voltage
Reflection Coefficient, ρ is ρ= Er / E1 = (Z-Z0) / (Z+Z0) Where, Z is the
impedance at a point on line, Z0 is characteristic Impedance.
The above equation gives following equation
| ρ| = S-1 / S+1
PROCEDURE:
1. Set up the equipment as shown in the block diagram.
2. Keep variable attenuator at maximum-position.
3. Keep the Control knob s of VSWR Meter as below:
Range : 40dB / 50 dB
Input Switch : Impedance Low
Meter Switch : Normal
Gain (Coarse-Fine) : Mid Position approx.
4. Keep the control knobs of Klystron power supply as below:
Mod-Switch : AM
Beam Voltage Knob : Fully Anticlockwise
Reflector Voltage Knob : Fully clockwise
AM frequency & amplitude
Knob : Mid Position
5. Switch ON the power supply, VSWR meter and cooling fan.
6. Turn the meter switch of klystron Power supply to beam. Voltage position
and set the beam voltage at 300V.
7. Rotate the reflector voltage knob to get deflection in VSWR Meter.
8. Tune the output by tuning the reflector voltage, amplitude and frequency of
AM modulation.
9. Tune for maximum deflection by tuning the plunger of Klystron Mount.
Then tune for maximum deflection by tuning the probe.
10. If necessary change the range dB-switch, variable attenuator position and
gain control knob to get deflection in the scale of VSWR Meter.
11. Move the probe along with slotted line, the deflection will change.
a. Measurement of low and medium VSWR:
o Move the probe along with slotted line to maximum deflection in VSWR
Meter.
o Adjust the VSWR Meter gain control knob or variable attenuator until meter
indicates1.0 on normal SWR scale (0-∞).
o Keep all the Control knobs as it is, move the probe to next minimum
position. Read the VSWR on scale and record it.
o Repeat the above step for change of S.S. Tuner probe & record the
corresponding SWR.
o If the VSWR is between 3.2 and 10, change the range dB switch to next
higher position and read the VSWR scale is 3 to 10.
b. Measurement of High VSWR (Double Minimum Method):
o Set the depth of S.S. Tuner slightly more for maximum VSWR.
o Move the probe along with slotted line until minimum is indicated.
o Adjust the VSWR meter gain control knob and variable attenuator to obtain
a reading of 3 dB of normal dB scale (0 to 10 dB) of VSWR Meter.
o Move the probe to the left on slotted line until full-scale deflection is
obtained i.e. „0‟ dB on 0-10 dB normal dB scale. Note and record the probe
position on slotted line. Let it be S1.
o Repeat the step 3 and 4 and then move the probe right along with slotted line
until full scale deflection is obtained on 0-10 dB normal dB scale. Let it be
S2.
o Replace the S.S. Tuner and terminator by movable short.
o Measure the distance between two successive minima position on probe.
Twice this distance is wave guide length λg.
o Calculate SWR by following equation:
SWR = λg / π (S1- S2).
For different SWR, calculate the reflection coefficient.
12. Turn the Gunn bias control knob to counter clockwise position.
13. Switch off the Gunn Power Supply.
OBSERVATION:
1) VSWR(S) =_________
2) REFLECTION COEFFCIENT(ƍ) =_________
CONCLUSION:
EXPERIMENT: 6
AIM: To measure an unknown impedance using Smith chart.
APPARATUS: Gunn Power supply , Microwave bench, Gunn Diode, Gunn
Oscillator, Isolator, Variable Attenuator 20db, Frequency Meter,
Slotted Section, Tunable Probe, S.S Tuner, Movable
short\termination or any unknown load, VSWR Meter
THEORY: The impedance at any point of a transmission line can be in the form
R+jX. For comparison SWR (S) and Reflection Coefficient (R) can be calculated as:
R = (Z-Z0)/ (Z+Z0)
S = (1+|R|)/ (1-|R|)
Where, Z0=Characteristics impedance of w/g at operating frequency, Z=Load
impedance at any point.
The measure is performed in following way: The unknown device is
connected to the slotted line and the SWR=S0 and the position of one minima is
determined. Then unknown device is replaced by movable sort to the slotted line.
Two successive minima positions are noted. The twice of the difference between the
minima position will be guide-wave length. One of the minima is used as reference
for impedance measurement. Find the difference of reference minima and minima
position obtained for load. Let it be „d‟. Take a smith chart taking „1‟ as center; draw
a circle of radius equal to S0. Make a point on circumference of chart towards load
side at a distance equal to d/g. Join the center with this point.
Find the point where it cut the drawn circle. The coordinates of this point
will show the normalized impedance of load.
PROCEDURE:
1. Set up the equipment as shown in the block diagram.
2. Set the variable attenuator at the maximum position.
3. Keep the control knobs of VSWR meter as below: Range dB: 50dB position,
Meter switch: Normal position, Gain (Coarse and Fine): Mid position, Keep the
Control knobs of Gunn power supply as below: Mod switch: AM, Gunn Bias
knob: Fully Counter clockwise position
4. Switch „ON‟ the Gunn power supply, VSWR meter and cooling fan.
5. Slowly vary the Gunn Bias Voltage and set it to 10V ( approx).
6. If necessary, change the range dB switch.
7. Vary attenuator position & Control knob to get deflection in the scale of VSWR
meter.
8. After getting maximum deflection on VSWR meter replace detector mount to
S.S. tuner and V match terminal.
9. Tune the tunable probe for maximum deflection in VSWR meter.
10. Keep the depth of pin of S.S. tuner to around 3-4mm and lock it.
11. Move the probe along with the slotted line to get maximum deflection.
12. Adjust VSWR meter gain control knob and variable attenuator unit such that the
meter indicates 1.0 on the normal upper SWR scale.
13. Move the probe to next minima point note down the SWR=S0 on the scale. Also
note down the probe position, 14. Remove the S.S. tuner and Matched
Termination and place movable short at slotted line. The plunger of short should
be at zero.
15. Note down the position of two successive minimum positions. Let it be as d1
and d2. Hence g=2(d1-d2).
16. Calculate d/g.
17. Find out the normalization impedance as described in the theory section.
18. Repeat the same experiment for other frequency if required.
19. Turn the Gunn bias knob in counter clockwise direction.
20. Turn off the Gunn Power Supply.
OBSERVATION:
d=___________ S0=___________ d1=___________ d2=___________
g=2(d1-d2) = ___________
Unknown Impedance from Schmitt Chart (normalized) = ___________
CONCLUSION:
EXPERIMENT: 7
AIM: To measure coupling factor, insertion loss and directivity of a Directional
coupler.
APPARATUS: Gunn power Supply (NV101), Microwave bench - Gunn oscillator,
Gunn diode, Isolator, Pin modulator, Variable attenuator (20 db),
Frequency meter, Slotted section, Detection mount, Microwave
Power Meter, Directional Coupler, CRO, Connectors and cables as
required.
THEORY: A directional coupler is a device with which it is possible to measure the
incident and reflected wave separately online. It consists of two transmission line,
the main arm and auxiliary arm, electromagnetically coupled to each other. When
power is coupled at the input port P1, it gets split into 2 parts by the directional
coupler. Majority of power is obtained at port2 which is output power. The opening
of the main waveguide into the adjacent waveguide is through holes placed λ/4 apart
due to which they power is added in phase at the Forward Port (Port 3) and added
out of phase at the Backward Port (Port 4). However, any power obtained due to any
mismatch is terminated at port 4 by an internal terminator to avoid any reflection
from this port. When Directional coupler is fed from Port2, we obtain Backward
Power at Port 3 and Forward Power is terminated at Port 4.
Directional Coupler
Coupling (db) = 10 log10 [P1/P3] ,where port 2 is terminated
Isolation = 10 log10 [P1/P4], where P1 is matched.
With built-in termination and power is entering at port 1. The directivity of
the coupler is a measure of separation between incident and the reflected wave. It is
measured as the ratio of two power outputs from the auxiliary line when a given
amount of power is successively applied to each terminal of the main lines with the
port terminated by material loads.
Directivity 0(db) = Isolation – Coupling = 10 log10 [P3/P4]
Min line VSWR is SWR Measured looking into the main line input
terminal when the matched loads are placed at all other ports.
Auxiliary line VSWR is SWR measured in the auxiliary line looking into
the output terminal, when the matched loads are placed on other terminals.
Main line insertion loss is the attenuation introduced in transmission line
by insertion of coupler. It is defined as insertion loss.
Insertion Loss = 10 log10 [P1/P2], where power is entering at port 1
PROCEDURE:
Measurement of coupling, insertion loss and directivity:
1. Set up the equipments as shown in the block diagram below.
2. Klystron and gunn oscillator.
3. Energies the microwave source for particular frequency operation as
described operation of and Gunn Oscillator.
4. Remove the multi-hole directional coupler and connect the detector mount to
the frequency meter. Tune the detector for the maximum output.
1. Set any reference level of power on Microwave power meter with the help of
variable attenuator, gain control knob of Microwave power meter, and note
down the reading. (Reference level let it be P1).
2. Insert the directional coupler as shown in second fig. with detector to the
auxiliary port 3 and matched termination to port 2, without changing the
position of variable attenuator and gain control knob of Microwave power
meter.
3. Note down the reading on Microwave power meter on the scale with help of
range-db switch if required. (Let it be P3)
4. Calculate coupling factor, which will be P1-P3 in dB.
5. Now carefully disconnect the detector from the auxiliary port 3 and detector
to port 2 without disturbing the set-up.
6. Connect the matched termination to the auxiliary port 3 and detector to port
2 and measure the reading on Microwave power meter. Suppose it is P2.
7. Compute insertion loss P1-P2 in dB.
8. Connect the directional coupler in the reverse direction, i.e. port 2 to
frequency meter side, matched termination to port 1 and detector mount to
port 3, without disturbing the position of the variable attenuator and gain
control knob of Microwave power meter.
9. Repeat the same for other frequencies.
10. Turn the Gunn bias control knob to counter clock wise.
11. Switch off the Gunn power supply.
OBSERVATION:
1. Input Power at port 1, P1 =____________
2. Output Power at port 2, P2=____________
3. Output Power at port 3, P3=____________
4. Output Power at port 4, P4=____________
CALCULATION:
1) Coupling (dB) = 10log10 (P1/P4)
2) Isolation (dB) = 10log10 (P2/P3)
3) Directivity = Isolation (dB) - Coupling (dB) = 10log10 (P2/P1)
4) Insertion = 10log10 (P1/P2)
CONCLUSION:
EXPERIMENT: 8
AIM: To study power division measurement in a Magic Tee.
APPARATUS: Microwave source, Isolator, Variable attenuator, Frequency meter,
Slotted line, Tunable probe, H-plane Tee, E-plane Tee and Magic
Tee, Matched termination, Wave guide stand, Detector mount,
Microwave Power meter, Accessories.
THEORY:
Microwave T-junctions:
A T-junction is an intersection of three waveguides in the form of English
alphabet „T‟. There are several types of Tee junctions. The following Tee junctions
will be discussed.
1. H-plane Tee junction
2. E-plane Tee junction
3. Magic-T junction
H-plane Teejunction:
An H-plane Tee junction is formed by cutting a rectangular slot along the
width of a main waveguide and attaching another waveguide-the side arm-called the
H-arm as shown in Fig. The port1 and port2of the main waveguide are called
collinear ports and port3 is the H-arm or side arm. H-plane Tee is so called because
the axis of the side arm is parallel of the planes of the main transmission line. As all
three arms of H-plane Tee lie in the plane of magnetic field, the magnetic field
divides itself in to the arms. Therefore this is also called a current junction. When
power is fed to port 3, it gets divided equally in port 1 and port 2 and both the
powers are in same phase.
E-plane Tee:
A rectangular slot is cut along the broader dimension of a long waveguide
and a side arm is attached as shown in Fig. Port1 and port2 are the collinear arms and
port3 is the E-arm.
When TE10 mode is made to propagate into port3, the two outputs at
port1 and port2 will have a phase shift of 180 degree as shown in Fig. Since the
electric field lines change their direction when they come out of port1 and port2, it is
called an E-plane Tee. E-plane Tee is a voltage or series junction
symmetrical about the central arm. Hence any signals that is to be split or any two
signal that are to be combined will be fed from the E arm. The scattering matrix of
an E-plane Tee can be used to describe its properties. In general, the power out of
port3 (side or E arm) is proportional to the difference between instantaneous powers
entering from port1 and port2.Also, the effective value of the power leaving the E
arm is proportional to the phase difference between the powers entering ports 1 and
2.when powers entering the main arms (ports 1 and 2) are in phase opposition,
maximum energy comes out of port 3 or E arm.
Magic Tee:
Here rectangular slots are cut both along the width and breadth of a long
waveguide and side arms are attached as shown in Fig. Ports 1 and 2 are collinear
arms, port 3 is the H-arm, and port 4 is the E-arm. Such a device became necessary
because of the difficulty of obtaining a completely matched three port Tee junction.
This four port hybrid Tee junction combines the power dividing properties of both
H-plane Tee and E-plane Tee as shown in Fig and has the advantage of being
completely matched at all its port. When power is input at port 3, there is no power
at port 4 and vice versa i.e. it provides isolation between the ports 3 and 4.
PROCEDURE:
(A): For H-Plane Tee:
1. Connect the apparatus as per block diagram given below
2. Remove the tunable probe and H-Plane Tee from the slotted line and connect the
detector mount to slotted line.
3. Energize the microwave source for particular frequency of operation and tune the
detector mount for maximum output.
4. Measure the input power to the Tee by placing the Microwave Power meter. This
is input power to the tee and this is the input Port
5. Without disturbing the position of variable attenuator and gain control knob,
carefully place the H-plane Tee after slotted line.
6. Determine the two output powers using Microwave Power meter.
7. Move the Gunn Power bias knob in counter clockwise direction
8. Turn off the Gunn Power Supply
(B): For E-Plane Tee
1. Connect the apparatus as per block diagram given in above procedure
2. Remove the tunable probe and E-Plane Tee from the slotted line and connect
the detector mount to slotted line.
3. Energize the microwave source for particular frequency of operation and
tune the detector mount for maximum output.
4. Measure the input power to the Tee by placing the Microwave Power meter.
This is input power to the tee and this is the input Port
5. Without disturbing the position of variable attenuator and gain control knob,
carefully place the E-plane Tee after slotted line.
6. Determine the two output powers using Microwave Power meter.
7. Move the Gunn Power bias knob in counter clockwise direction
8. Turn off the Gunn Power Supply
(C): For Magic Tee
1. Connect the apparatus as per block diagram given below
2. Remove the tunable probe and Magic Tee from the slotted line and connect
the detector mount to slotted line.
3. Energize the microwave source for particular frequency of operation and
tune the detector mount for maximum output.
4. Measure the input power to the Tee by placing the Microwave Power meter.
This is input power to the tee and this is the input Port
5. Without disturbing the position of variable attenuator and gain control knob,
carefully place the Magic Tee after slotted line keeping H-arm connected to
slotted line, detector to E-arm and matched termination to arm 1 and 2. Note
down the reading of Microwave Power meter. Determine the isolation
between port 3 and port 4 as P3-P4 in dB.
6. The same experiment can be repeated for other ports also.
7. Repeat the above experiment for other frequency.
8. Move the Gunn Power bias knob in counter clockwise direction
9. Turn off the Gunn Power Supply
CONCLUSION:
Experiment No. 9
AIM: To study measurement of vswr, insertion loss, isolation of Isolator and
Circulator.
APPARATUS: Micro-wave source, Klystron power supply 5KPS610, Klystron
Tube 2K25, Klystron mount XM251, Isolator XI621, Frequency
meter XF710, Variable Attenuator XA520, Slotted line X565,
Detector Mount XD450, Wave guide stand XU535, BNC Cable,
VSWR Meter SW115, Cooling fan.
CIRCUIT:
THEORY:
A circulator is a ferrite device (ferrite is a class of materials with strange
magnetic properties) with usually three ports. Circulators are non-reciprocal. That is,
energy into port 1 predominantly exits port 2, energy into port 2 exits port 3, and
energy into port 3 exits port 1. In a reciprocal device the same fraction of energy that
flows from port 1 to port 2 would occur to energy flowing the opposite direction,
from port 2 to port 1.The selection of ports is arbitrary, and circulators can be made
to "circulate" either clockwise (CW) or counter-clockwise (CCW).
Fig (a)-Circulators
By terminating one port, a circulator becomes an isolator, which has the
property that energy flows on one direction only. This is an extremely useful device
for "isolating" components in a chain, so that bad VSWRs don't contribute to gain
ripple, or lead to instabilities (unwanted oscillations). An isolator is a non-reciprocal,
passive network.
Fig (b) Isolators
PROCEDURE:
1. Connect the apparatus as shown in the above figure.
2. Switch on the power supply.
3. Connect circulator and measure the output voltage on VSWR meter or CRO.
4. Give input to circulator from port 1 and measure voltages at port 2 and port -
3.
5. Repeat the above steps for port 2 and port 3.
6. Connect isolator and measure the output voltage from detector.
7. Give input to isolator at port 1, measure output at any one of another port
and terminate the rest left over port.
8. Repeat steps for port 2 and 3.
CONCLUSION:
EXPERIMENT: 10
AIM: To study Fixed and Variable attenuator.
APPARATUS: Microwave source, Isolator, freq. meter, Variable attenuator,
Slotted line, Tunable probe, Detector mount, Matched termination,
VSWR meter, Fixed and variable type attenuator.
THEORY:
The attenuator is two port bi directional devices which attenuates some
power when inserted into transmission line. Attenuation A(dB)=10log(base 10)
P1/P2, Where P1 is power absorbed or detected by load without the attenuator in the
line and P2 is power absorbed / detected by the load line with attenuation in the line.
The attenuation consists of a rectangular waveguide with resistive vane
inside it to absorb microwave power according to their positions with respect to side
wall of the waveguide. As electric field is maximum at centre in TE10 mode, the
attenuation will be maximum if the vane side wall, attenuation decreases in the fixed
attenuator. The vane position is fixed where as in variable attenuator; its position
can be changed by help of micrometer or by other methods.
PROCEDURE:
A) Input VSWR Measurement
1. Remove the tunable probe attenuator and matched termination form the
slotted section in the above set up.
2. Connect the detector mount to the slotted line and tune the detector mount
also for maximum deflection on VSWR meter.
3. Set any reference Level on the VSWR meter with the help of variable
attenuation and gain control knob of VSWR meter. Let it be P1.
4. Carefully disconnect the detector mount from the slotted line, without
disturbing any position on the set up. Place the test variable attenuator to the
slotted line and detector mount to other port of test variable attenuator.
5. Keep the micrometer reading of test variable attenuator to zero and record
the reading of VSWR meter. Let it be P2, then the insertion loss or test
attenuator will be P1-P2 dB.
6. For measurement of attenuation of step 4 of above measurement. Carefully
disconnect the detector mount from the slotted line without disturbing any
position obtained up to step 3.
7. Place the test attenuator to the slotted line and detector mount to the other
port of test attenuator.
8. Record the reading of VSWR meter.
9. Let it be P3. Then the attenuation value of fixed attenuator for particular
position or micro meter reading will be P1-P3 dB.
10. In case of variable attenuator, change the micro meter reading and record the
VSWR meter reading. Find out Attenuation value for different position of
microwave reading.
11. Now change the operating frequency and whole step should be repeated for
finding frequency sensitivity for fixed and variable attenuator.
CONCLUSION: