uwave man labcopy

7/31/2019 Uwave Man Labcopy

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Microwave and Radar Lab

Department of Electronics and Communication Engineering

To study about waveguide components

Wave-guide is a hollow metallic tube that acts as a high pass

filter, having uniform cross section for transmitting electromagnetic waves by successive

reflections from the inner walls of the tube. Wave-guide sections of specified length can besupplied with flanges, painted outside and silver or gold plated inside to protectfrom

oxidation. US standardise WG-90 waveguide is used in the lab that works in X-band with

inner dimensions as 2.286x1.016 sq. cms and outer dimension as 2.54x1.27 sq. cms.

These wave guides work under10 mode.

It provides sufficient power to the microwave klystron tube. For

the reflex klystron it operates in X-band(8-12GHz). Its specifications are:

Operating Voltage: mains, 230V, 50 Hz

Beam Voltage: 180-400V (+or-20V) with nearly 270 for max safety limit.

Repeller Voltage: 5 to 220V(+or- 10V) with max. safety limit between 40-90V

Current MOD Supply: 50mA


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Freq. Amplitude

For AM: 50-2500Hz 10-15V

For FM: 50-5000Hz 10-15V

Modulated signal is is an AM MOD square wave/ FM saw tooth wave.

it is a single cavity resonator, that delivers low power microwave(10-

100mW) power output in a microwave frequency range of 1-30GHz with an maximum

efficiency of 30% max. A small hole in the broad of the wave guide is provided, through

which output coupling the klystron tube enters into the waveguide. This hole is exactly

located in the centre in of the board walls as the electric field is maximum at the centre of

TE10 mode. Maximum power is obtained by tuning the plunger of the klystron cavity.

It consists of the GUNN power supply and requires more stabilized

voltage as it is sensible to fluctuation in voltage or temperature. GUNN oscillator uses a

GUNN diode that works on a negative resistance as a high frequency oscillator.

Operating Voltage: Mains 220V, 50Hz.

Voltage range: +/-(0-12)V

Regulation: 1 mV rms (ripple).

0.2% for +/- 10% variation in noise.

MOD square wave: FM is between 900-2599 Hz variable and for AM is 10-25 V variable.

Fixed attenuators are used for inserting a known attenuation

in a waveguide system. These consist of a lossy-vane inserted in a section of waveguide,

flange on both sides or can be a short section of waveguide with a tapered plug of

absorbing material at the ends which is made up of dielectric like glass slab coated with

aquadog or a carbon film. These are useful for isolation of waveguide circuits.


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Variable attenuators provide continuous or step wise

variable attenuation. These consist of a movable lossy-vane inside a section of waveguide

by means of a micrometer. The configuration of lossy vane is so designed to obtain the

low VSWR characteristics over the entire frequency band.These are meant for adjusting

power levels and isolating a source and load.

These are made of tunable resonant cavity (having high Q) of

particular size. This cavity is connected to the source of energy through a section of

waveguide. The cavity absorbs some power at resonance, which is indicated as a dip in

the output power. The tuning of the cavity is achieved by means of a plunger connected to

a micrometer. The reading of micrometer at resonance gives frequency from the

calibration-chart provided.


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Direct reading frequency meter is used to give direct frequency on

the dial provided. These are recommended for use whenever quick determination of

frequency and easy reading are desired in laboratory and production testing.

Slotted sections consists of a precision waveguide

slotted line and the probe-carriage. The waveguide slotted line, comprises of an

accurately machined section of waveguide in which a small longitudinal slot(2mm) has

been cut which is a basic means for monitoring wave-patterns inside the waveguide

system. For dominant mode traveling inside the waveguide, the slot does not radiate any


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power. A precision built probe carriage has a centimeter-scale with a vernier reading of 0.1

mm least count and a dial gauge can be mounted easily if precise readings are required.

As the tunable detector mount detects maxima and minima, hence can calculate the (1)

load impedance, (2) Standing wave ratio, (3) waveguide frequency (4)reflection coefficient

(5)guide wave length and (6)power

Tunable probes are used with slotted sections. A small probe is inserted

through the slot senses the relative field strength of the standing wave pattern inside the

waveguide. The depth of penetration into a waveguide section is adjustable by the knob

of the probe. The tip (central wire projection of the probe) pickup the RF power from the


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line and this power is rectified by crystal detector, which is then fed to the VSWR meter or

indicating instrument.

Circulators are matched three-port devices and these are

meant for allowing microwave energy to flow in clockwise direction with negligible loss but

almost no transmission in the anti-clockwise direction.

These are well-matched uni-directional devices offering low forward

insertion loss and high reverse isolation.


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Directional couplers are flanged, built in waveguide assemblies

which can sample a small amount of microwave power for measurement purposes. Cross

Directional coupler consists of two sections of waveguide joined together at right angles.

The coupling is provided by star slots made in broad wall of wave-guide. It is used in

monitoring signal frequency, power etc. in a microwave system.

Matched terminations are useful for VSWR measurement of various

wave-guide components. These are also employed as dummy and as a precise reference

loads with Tee junctions, directional Couplers and other similar dividing devices. These

consist of a small and high dissipative taper flap mounted inside the center of a section of

wave-guide.


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Magic Tee consists of a section of waveguide with both series

and shunt waveguide arms, mounted at the exact mid-point of main arm. Both ends of the

section of waveguide and both arms are flanged on their ends. These Tees are employed

in balanced mixers, automatic frequency control circuits and impedance measurement

circuits etc. This becomes a four terminal device where one terminal is isolated from the

input terminal.

Movable short consists of a section of a waveguide, flanged on one end

and terminated with a movable shorting plunger on the other end. By means of this non-

contacting type plunger a reflection co-efficient of almost unity may be obtained.


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These are the waveguides used for twiting or bending the

communication paths at required places. If the bend is in the direction of the wider

dimension, H lines are affected and if, the bend is in the direction of the narrow dimension,

the E-lines are affected(E-bend). The bending radius must be atleast 2 times the guide

wave length for a gradual bending as abrupt(for example 90 degree) may lead ti infinite

VSWR.


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To study the mode characteristics of the reflex klystron tube and to determine its

electronic tuning range.

MICROWAVE SETUP BENCH FOR REFLEX KLYSTRON

1) Klystron power supply2) Klystron tube 2k-25 with klystron mounts3) Isolator4) Frequency meter5) Detector mount6) Variable Attenuator7) Wave guide stand8) VSWR meter9) Oscilloscope10) BNC Cable


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The reflex klystron is a single cavity variable frequency microwave generator of low

power(1-2.5Watt) and low efficiency(22.48%). This is most widely used in applications

where variable frequency is desired as

1. In radar receivers

2. Local oscillator in w receivers

3. Signal source in micro wave generator of variable frequency

4. Portable micro wave links.

5. Pump oscillator in parametric amplifier

It is a cavity resonator which makes use of velocity modulation to transfer a

continuous beam ( AM, FM, CW noise) into modulated microwave power for delivering 10-

100mW output in a frequency range of 1-30GHz with a maximum efficiency of 30% in X-

band. In this scheme, an electron is emitted from cathode which is accelerated and

passed through a negative reflector at the other end, which retards its forward motion and

reflect it back to the resonator, thus operating as an oscillator. The cathode beam intensity

of electron and the repeller voltage is controlled by adjusting the beam current and the

repeller knob within the safety limit of the power supply. An continuous message signal

can be provided using a grid in order to modulate it with a high frequency carrier wave,

which in this case is the electron beam generated in resonator.

: The frequency is primarily determined by the dimension of

resonant cavity. Hence by changing the volume of resonator, mechanical tuning range of

Klystron is possible. Also a small frequency change can be done by adjusting the reflector

voltage. This is called Electronic Tuning Range

: The mode curves and frequency characteristics. The

frequency of resonance of the cavity decides the frequency of oscillation. A variation in

repeller voltages slightly changes the frequency.


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1) Connect the equipments and components as shown in thefigure.

2) Set the variable attenuator at maximumPosition.

3) Set the MOD switch of Klystron Power Supply at CW position, beam voltagecontrol knob to fully anti clock wise and repeller voltage control knob to fully

clock wise and meter switch to OFF position.

4) Rotate the Knob of frequency meter at one sidefully.

5) Connect the DC microampere meter atdetector.

6) Switch ON the Klystron power supply, CRO and cooling fan for the Klystrontube.

7) Put the meter switch to beam voltage position and rotate the beam voltage knob


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clockwise slowly up to 300 Volts and observe the beam current on the meter by

changing meter switch to beam current position. The beam current should not

increase more than 30 mA.

8) Change the repeller voltage slowly and watch the current meter, set the maximumvoltage on CRO.

9) Tune the plunger of klystron mount for the maximumoutput.

10) Rotate the knob of frequency meter slowly and stop at that position, where there isless output current on multimeter. Read directly the frequency meter between two

horizontal line and vertical marker. If micrometer type frequency meter is used

read the micrometer reading and find the frequency from its frequency calibration

chart.

11) Change the repeller voltage and read the current and frequency for each repellervoltage.


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Beam Voltage:V

Beam Current: mA.

Repeller

Voltag

Current

(mA)

Power

(mW)

Dip

Frequency

(GHz)


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1) Never keep repeller zero.2) All connections should be tight.3) Before Switching on the klystron power supply, set the beam voltage control knob

to fully anticlockwise, reflector voltage control knob to fully clockwise and meter

switch to off position.

4) Set the beam voltage to 300V and observe the beam current. The beam currentshould not increase more than 30mA.


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To study the V-I characteristics of Gunn diode.

1) Gunn power supply2) Gunn oscillator3) PIN Modulator4) Isolator5)

Frequency Meter

6) Variable attenuator7) Slotted line8) Detector mount and CRO.

MICROWAVE TEST BENCH FOR GUNN DIODE EXPERIMENT.

Gunn diode oscillator normally consists of a resonant cavity, an arrangement for

coupling diode to the cavity, a circuit for biasing the diode and a mechanism to couple


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the RF power from cavity to external circuit load. A co-axial cavity or a rectangular

wave guide cavity is commonly used.

The circuit using co-axial cavity has the Gunn diode at one end of cavity along with the

central conductor of the co-axial line. The O/P is taken using an inductively or

capacitivety coupled probe. The length of the cavity determines the frequency of

oscillation. The location of the coupling loop or probe within the resonator determines

the load impedance presented to the Gunn diode. Heat sink conducts away the heat

due to power dissipation of the device.

1) Set the components and equipments as shown in Figure.2) Initially set the variable attenuator for minimum attenuation.3) Keep the control knobs of Gunn power supply as below

1. Meter switchOFF2. Gunn bias knobFully anti clock wise PIN bias knob Fully anti clock wise

PIN mode frequencyany position

3. Set the micrometer of Gunn oscillator for required frequency of operation.4) Switch ON the Gunn power supply.5) Measure the Gunn diode current for corresponding various Gunn bias voltages

through the digital panel meter and meter switch. Do not exceed the bias voltage

above 10 volts.


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6) Plot the voltage and current readings on the graph.7) Measure the threshold voltage which corresponding to max

current.

Volts(v)


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>> UPS should be used for the power supply.

>> Do not keep Gunn bias knob position at threshold position for more than 10-15

sec.

>> Readings should be obtained as fast as possible. Otherwise due to excessive

heating Gunn diode may burn


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To study about insertion loss and attenuation measurement of attenuator.

1) Microwave source Klystron tube (2k25)2) Isolator (xI-621)3) Frequency meter (xF-710)4) Variable attenuator (XA-520)5) Slotted line (XS-651)6) Tunable probe (XP-655)7) Detector mounts (XD-451)8) Test Attenuator

1) Fixed2) Variable3) Matched termination (XL-400)4) Klystron power supply & Klystron mount5) Cooling fan6) BNC-BNC cable7) VSWR or CRO

The attenuator is a two port bidirectional device which attenuates some power when

inserted into a transmission line.

Attenuation A (db) = 10 log (P1/P2)

Where, P1 and P2 is the power detected by the load without and with the attenuator in

the line.

Insertion Loss(db)=P1-P2

The attenuators consist of a rectangular wave guide with a resistive vane inside it to


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absorb microwave power according to their position with respect to side wall of the wave

guide. An electric field is maximum at centre in TE10 mode. The attenuation will be

maximum if the vane is placed at centre of the wave guide. Moving from centre towards

the side wall, attenuation decreases. In fixed attenuator, the vane position is fixed where

as in variable attenuator, its position can be changed with the help of micrometer or by

other methods.

FIG: MEASUREMENT OF ATTENUATION

1) Connect the equipment as shown in the above figure.2) Energize the microwave source for maximum power at any frequency of operation3) Connect the detector mount to the slotted line and tune the detector mount also

for max deflection on VSWR or on CRO

4) Set any reference level on the VSWR meter or on CRO with the help of variableattenuator (Say P1)

5) Carefully disconnect the detector mount from the slotted line without disturbingany position on the setup, place the test variable attenuator to the slotted line

and detector mount to O/P port of test variable attenuator. Keep the micrometre


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reading of texts variable attenuator to zero and record the readings of VSWR

meter or on CRO. Let it to be P2. Then the insertion loss of test attenuator will be

P1-P2 db.

6) For measurement of attenuation of fixed and variable attenuator. Place the test

attenuator to the slotted line and detector mount at the other port of test attenuator.

Record the reading of

7) VSWR meter or on CRO. Let it be P3 then the attenuation value of variableattenuator for particular position of micrometer reading of will be P1-P3 db.

8) In case the variable attenuator changes the micro meter reading and record theVSWR meter or CRO reading. Find out attenuation value for different position of

micrometer reading and plot a graph.

9) Now change the operating frequency and all steps should be repeated for findingfrequency sensitivity of fixed and variable attenuator.


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Micrometer

reading

P1

(dB)

P2

(dB) Attenuation=10logP1/P2(db)

Insertion

Loss=P1-P2

(db)

For measuring frequency sensitivity of variable attenuator the position ofmicrometer reading of the variable attenuator should be same for all frequencies of

operation.


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To determine the frequency and wavelength in a rectangular wave guide working

in TE10 mode.

1. Klystron tube

2. Klystron power supply 5kps610

3. Klystron mount XM-251

4. Isolator XI-621

5. Frequency meter XF-710

6. Variable attenuator XA-520

7. Slotted section XS-651

8. Tunable probe XP-655

9. VSWR meter SW-115

10. Wave guide stand XU-535

11. Movable Short XT-481

12. Matched termination XL-400

The cut-off frequency relationship shows that the physical size of the wave guide will

determine the propagation of the particular modes of specific orders determined by

values of m and n. The minimum cut-off frequency is obtained for a rectangular wave

guide having dimension a>b, for values of m=1, n=0, i.e. TE10 mode is the dominant

mode since for TM mn modes, where n and m are the multiples of the half wave length

of the EM wans in the dimension of the waveguide which is the lowest-order mode

possible in TE10, called the dominant mode in a rectangular wave guide for a>b.

For dominant TE10 mode rectangular wave guide o, g and c are related as below.

Where ,


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o is free space wave length ,

g is guide wave length ,

c is cut off wave length

For TE10 mode c =2a where a is broad dimension of wave guide.

1) Set up the components and equipments as shown in figure.2) Set up variable attenuator at minimum attenuation position.3) Keep the control knobs of klystron power supply as below:

1. Beam voltageOFF2. Mod-switchAM3. Beam voltage knobFully anti clock wise4. Repeller voltageFully clock wise5. AMAmplitude knobAround fully clock wise6. AMFrequency knobAround mid position

4) Switch ON the klystron power supply, CRO and cooling fan switch.5) Switch ON the beam voltage switch and set beam voltage at 300V with help


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of beam voltage knob.

6) Adjust the repeller voltage to get the maximum amplitude in CRO7) Maximize the amplitude with AM amplitude and frequency control knob of power

supply.

8) Tune the plunger of klystron mount for maximum Amplitude.9) Tune the repeller voltage knob for maximum Amplitude.10) Tune the frequency meter knob to get a dip on the CRO and note down the

frequency from frequency meter.

11) Replace the termination with movable short, and detune the frequency meter.12) Move the probe along with slotted line. The amplitude in CRO will vary .Note

and record the probe position, Let it be d1.

13) Move the probe to next minimum position and record the probe position again, Let itbe d2.

14) Calculate the guide wave length as twice the distance between two successiveminimum positions obtained as above.

15)Measure the wave guide inner board dimension a which will be around 22.86mm forx-band.

16)Calculate the frequency by following equation.

Where C = 3x108 meter/sec. i.e. velocity of light.

17. Verify with frequency obtained by frequency modes

18. Above experiment can be verified at different frequencies. fo = C/o => C =>

3x108 m/s (i.e., velocity of light)

For TE10 mode => c = 2a

Waveguide inner broad dimension a = 2.286cm (given)

c = 4.6cm


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1) All connections should be tight.2) Before Switching on the klystron power supply, set the beam voltage control knob



3) Set the beam voltage to 300V and observe the beam current. The beam currentshould not increase more than 30mA


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To study the function of multi-hole directional coupler by measuring

the following parameters.

1) The Coupling factor2) Insertion Loss and3) Directivity of the Directional coupler

1) Microwave Source (Klystron or Gunn-Diode)2) Isolator, Frequency Meter3) Variable Attenuator4) Slotted Line5) Tunable Probe6) Detector Mount Matched Termination7) MHD Coupler8 ) Waveguide Stand9) Cables and Accessories10) CRO

A directional coupler is a device with which it is possible to measure the incident and

reflected wave separately. It consist of two transmission lines the main arm and

auxiliary arm, electromagnetically coupled to each other Refer to the Fig.1. The power

entering, in the main- arm gets divided between port 2 and 3, and almost no power

comes out in port (4) Power entering at port 2 is divided between port 1 and 4.The

coupling factor is defined as

Coupling (dB) = 10 log10 [P1/P3]

Isolation (dB) = 10 log10 [P2/P3]


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Where P1 is matched and port 2 is terminated.

With built-in termination and power entering at Port 1, the directivity of the coupler is a

measure of separation between incident wave and the reflected wave. Directivity is

measured as follows:

Directivity D (dB) = I-C = 10 log10 [P2/P1]

Main line VSWR is SWR measured, looking into the main-line input terminal when the

matched loads are placed at all other ports. Auxiliary live 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 the transmission

line by insertion of coupler, it is defined as:

Insertion Loss (dB) = 10 log10 [P1/P2]


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1) Set up the equipments as shown in the Figure.2) Energize the microwave source for particular operation of frequency .3) Remove the multi hole directional coupler and connect the detector mount to

the slotted section.

4) Set maximum amplitude in CRO with the help of variable attenuator, Let it be X.5) Insert the directional coupler between the slotted line and detector mount.

Keeping port 1 to slotted line, detector mount to the auxiliary port 3 and matched

termination to port 2 without changing the position of variable attenuator.

6) Note down the amplitude using CRO, Let it be Y.7) Calculate the Coupling factor X-Y in dB.8) Now carefully disconnect the detector mount from the auxiliary port 3 and

matched termination from port 2 , without disturbing the setup.

9) Connect the matched termination to the auxiliary port 3 and detector mountto port 2 and measure the amplitude on CRO, Let it be Z.

10) Compute Insertion Loss= XZ in dB.11) Repeat the steps from 1 to 4.

12.Connect the directional coupler in the reverse direction i.e., port 2 to slottedsection, matched termination to port 1 and detector mount to port 3, without

disturbing the position of the variable attenuator.

13. Measure and note down the amplitude using CRO, Let it be Y0.

14. Compute the Directivity as Y-Y0 in dB.

1) Avoid loose connections.2) Avoid Parallax errors.


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To Study the operation of Magic Tee and calculate Coupling Co-efficient and

Isolation.

1) Microwave source : Klystron tube (2k25)2) Isolator (XI-621)3) Frequency meter (XF-710)4) Variable Attenuator (XA-520)5) Slotted line (SX-651)6) Tunable probe (XP-655)7) Detector Mount (XD-451)8) Matched Termination (XL-400)9) Magic Tee (XE-345/350)10) Klystron Power Supply + Klystron Mount11) Wave guide stands and accessories

The device Magic Tee is a combination of E and H plane Tee. Arm 3 is the H-arm and

arm 4 is the E-arm. If the power is fed, into arm 3 (H-arm) the electric field divides

equally between arm1 and 2 with the same phase and no electric field exists in the arm

4. If power is fed in arm 4 (E-arm) it divides equally into arm 1 and 2 but out of phase

with no power to arm 3. Further, if the power is fed in arm 1 and 2 simultaneously it isadded in arm 3 (H-arm) and it is subtracted in E-arm i.e., arm 4.

The Isolation between E and H arm is defined as the ratio of the power supplied by the

generator connected to the E-arm (port 4) to the power detected at H-arm (port 3) when


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side arms

1 and 2 are terminated in

matched load.

Isolation3-4 (dB) = 10 log10 [P4/P3]

Similarly, Isolation between other ports may be

defined.

It is defined as Cij = 10 /20

Where is attenuation / isolation in dB when i' is input arm and j is output arm.

Thus, = 10 log10 [P4/P3]

Where P3 is the power delivered to arm i and P4 is power detected at j arm.


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Fig: Magic Tee

1) Setup the components and equipments as shown in figure.2) Energize the microwave source for particular frequency of operation and tune

the detector mount for maximum output.

3) With the help of variable frequency of operation and tune the detector mount formaximum output attenuator, set any reference in the CRO let it be V3.

4) Without disturbing the position of the variable attenuator, carefully place the MagicTee after the slotted line, keeping H-arm to slotted line, detector mount to

E-arm and matched termination to Port-1 and Port-2.

5) Note down the amplitude using CRO, Let it be V4.6) Determine the Isolation between Port-3 and Port-4 as V3-V4.7) Determine the coupling co-efficient from the equation given in theory part.8) The same experiment may be repeated for other Ports also.


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PORTS POWER(watt)

Isolation3-4 (dB) = 10 log10 [P4/P3]

Coupling Co-efficient:

= 10 log Pi/Pj

Therefore C=10-/20

All connections should be tight.2) Before Switching on the klystron power supply, set the beam voltage control knob

to fully

anticlockwise, reflector voltage control knob to fully clockwise and meter switch to of

position.

3) Set the beam voltage to 300V and observe the beam current. The beam currentshould not

increase more than 30mA.

4) Cooling Fan should be used


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To study the function of three port circulator by measuring the following parameters

by using source reflex klystron:

1) Insertion loss and2) Isolation

1) Microwave source : Klystron tube (2k25)2) Isolator (XI-621)3) Frequency meter (XF-710)4) Variable Attenuator (XA-520)5) Slotted line (SX-651)6) Tunable probe (XP-655)7) Detector Mount (XD-451)8) Matched Termination (XL-400)9) 3 port test circulator10) Klystron Power Supply + Klystron Mount11) Wave guide stands and accessories

Circulator is defined as device with ports arranged such that energy entering a port is

coupled to an adjacent port but not coupled to the other ports. This is depicted in fig.

Circulator can have any number of ports.

Port

Port

3Port

1


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The important circulator parameters are:

Insertion Loss is the ratio of power supplied by a source to the input port to the power

detected by a detector in the coupling port i.e output port with other ports

terminated in the matched load. It is expressed in decibels.

Isolation is the ratio of power fed to input port and the power detected at not coupled port

with other port terminated in the matched load. This ratio is expressed in db.

Set up for measurement of insertion loss and isolation of circulator is shown in fig


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1) Setup the components and equipments as shown in fig.3.2)

Energize the microwave source for maximum output for a particular frequency ofoperation. Tune the detector mount for maximum output in the VSWR Meter.

3) Set any reference level of power in VSWR meter with the help of variable attenuatorand gain control knob of VSWR Meter, Let it be P1.

4) Carefully remove the detector mount without disturbing the position of the set up.Insert the circulator between slotted line and detector mount. Keep input port to

slotted line and detector to its output port. A matched termination should be placed

at third port

5) Record the reading in the VSWR meter, if necessary, change range (db) switch tohigh or lower position, and taking 10 db change for one step change of switch

position. Let it be P2.

6) Compute insertion loss given as P1P2 in db.7) For measurement of isolation, the circulator has to be connected in reverse i.e.

output port to slotted line and detector to input port with other port terminated by

matched termination after setting a reference level without circulator in the set up

described in insertion loss measurement .Let same P1 level is set.

8) Record the reading of VSWR meter after and let it be P 3.9) Compute isolation as P1P3 in db.10) Repeat the experiment for other ports of circulator.11) Repeat the above experiment for other frequency if needed.

Insertion loss(in db)= P1P2 Isolation (in db)= P1P3


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1) All connections should be tight.2) Before Switching on the klystron power supply, set the beam voltage control knob




4) Cooling Fan should be used


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To study waveguide horn and its radiation pattern and determine the gain.

5) Gunn power supply6) Gunn Oscillator7) PIN modulator8) Frequency meter9) Isolator10) Variable attenuator11) Detector mount12) Two-horn antenna13) Turntable14) VSWR meter and Accessories

MICROWAVE SET UP BENCH FOR MEARUREMENT OF RADIATION PATTERN


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If a transmission line propagating energy is left open at one end, there will be radiation

from this end .In case of rectangular waveguide this antenna presents a mismatch of

about 2 :1 and it radiates in many directions. The match will improve if the openwaveguide is a horn shape.

The Radiation pattern of an antenna is a plot of field strength or more often the power

intensity as a function of the aspect angle at a constant distance from the radiating

antenna. An antenna pattern is of course three- dimensional but for practical reasons it is

normally presented as a two-dimensional pattern in one or several planes. An antenna

pattern consists of several lobes, the main lobe, side lobes and the back lobe. The major

power is concentrated in the main lobe and it is required to keep the power in the side lobe

and back lobe as low as possible. The power intensity is maximum in the main lobe

compared to the power intensity achieved from an imaginary omni directional antenna

(radiating equally in all directions) with the same power fed to the antenna is defined as

gain of the antenna. Fig 1a and 1b shows the antenna radiation pattern. Fig 2 shows the

experimental set up.


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1) Set up the equipments as shown in the fig. 2 Keeping the axis of both antennas insame line.


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2) Energize the Gunn oscillator for maximum output at desired frequency with squarewave modulator by tuning square wave amplitude and frequency of modulating

signal of Gunn power supply and tuning the detector.

3) Also tune the S S tuner in the line for maximum output (if S S Tuner is in the set up)4) Obtain full-scale deflection (0db) on normal db scale (o-10db) at any convenient

range switch position of the VSWR meter by gain control knob of VSWR Meter by

gain control knob of VSWR meter or by variable attenuator.

5) Turn the receiving horn to the left in 20 or 50 steps up to 400500 and note thecorresponding VSWR reading in normal db range. When necessary, change the

range switch to next higher range and add 10 db to the observed value.

6) Repeat the above step but this time turn the receiving horn to the right and notedown the reading.

7) Plot a relative power pattern i.e. out put Vs angle.8) From diagram determine 3 db width (beam width) of the horn antenna.

Pr = (PtoG1G2)/(4S2)

Where Pt = transmitted power

Pr = received power

G1,G2 = gain of transmitting and receiving antenna

S = radial distance between two antenna

o=free space wavelength.

1) All connections should be tight.2) Do not exceed the Gunn bias voltage above 10V.3) Do not keep Gunn bias knob position at threshold position for more than 10-15

seconds

4) Reading should be obtained as fast as possible, otherwise due to excessive heatingGUNN Diode may burn.


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To measure VSWR and reflection coefficient by standing wave and double

minimum methods

1) Klystron tube2) klystron power supply3) VSWR meter4) klystron mount5) isolator6) frequency meter7) variable attenuator8) slotted line9) tunable probe10) waveguide stand11) movable short/termination or any unknown load12) BNC cable and S-S tuner

The electromagnetic field at any point of transmission line, may be considered as sum of

two traveling waves :- first is the incident wave which propagates from the source to the

load and the reflected wave which propagates towards the generator and another is

reflected wave which is set up by reflection of incident wave from a discontinuity in the line

or from the load impedance. The superposition of the two traveling waves gives rise to a

standing wave along the line. The maximum field strength is found where the waves are in

phase and minimum where the two waves add 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 co-efficient.

The voltage standing wave ratio (VSWR) is defined as ratio between maximum and

minimum field strength along the line

Hence VSWR denoted by S is as follows


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S=Emax / Emin

=(I Ei I + I Er I)/( I Ei I- I Er I)

Ei = Incident Voltage, Er = Reflected voltage

Reflection coefficient, p is

p =(Er/Ei)=(ZL - ZO)/( ZL + Zo)

Where ZL is the load impedance, Zo is characteristics impedance.

The above equation gives following equation

| p |=(S-1)/( S + 1)

The setup for VSWR measurement is shown in fig.


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\

1) Set up the equipment as shown in fig. 12) Keep variable attenuator in the minimum attenuation position.3) Keep the control knobs of VSWR meter as below:

1. Range db = 40 db/50db2. Input switch = Low impedance3. Meter switch = Normal4. Gain (coarse fine) = Mid position approx.

4) Keep the control knobs of klystron power supply as below:1. Meter switch = Off2. Mod switch = AM3. Beam voltage knob = Fully anticlockwise4. Reflector voltage knob = Fully clockwise5. AM-amplitude knob = Around fully clockwise6. AM-frequency = Mid position7. & amplitude knob

5) Switch ON the klystron power supply, VSWR meter and cooling fan.


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6) Turn the meter switch of klystron power supply to beam voltage position and setthe beam voltage at 300 V.

7) Rotate the reflector voltage knob to get deflector in VSWR meter.8) Tune the output by turning the reflector voltage, amplitude and frequency of AM

modulation.

9) Tune plunger of klystron mount and probe for maximum deflection in VSWR meter.10) If required, change the range db-switch, variable attenuator position and gain

control knob to get deflection in the scale of VSWR meter.

11) As you move probe along the slotted line, the deflection will change.

1) Move the probe along the slotted line to get maximum deflection in VSWR meter.fig2.

2) Adjust the VSWR meter gain control knob or variable attenuator until the meterindicates 1.0 on normal VSWR scale.

3) Keep all the control knobs as it is, move the probe to net minimum position. Readthe VSWR on the scale and record it.

4) Repeat the above step for change of S.S. tuner probe depth and record thecorresponding VSWR.

5) If the VSWR is between 3.2 and 10, change the range dB switch to next higherposition and read the VSWR on second VSWR scale of 3 to 10.

1) Set the depth of S.S. tuner slightly more for maximum VSWR.2) Move the probe along-with slotted line until a minimum is indicated.3) Adjust the VSWR meter gain control knob and variable attenuator to obtain a

reading of 3 db in the normal db scale (0 to 10 db) of VSWR meter.


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4) Move the probe to the left on slotted line until full-scale deflection is obtained, i.e.0db on 0-10 db scale. Note and record the probe position on slotted line let it be d1

fig3.

5) Repeat the step 3 and 4 and then move the probe right along the slotted line untilfull scale deflection is obtained on 0-10 db normal db scale. Let it be d2.

6) Replace the S.S. tuner and termination by movable short.7) Measure the distance between two successive minima positions of the probe.

Twice this distance is guide wave length g.

8) Compute SWR from the following equation.SWR = g/ (d1d2)

SWR = g/ (d1d2)

All connections should be tight.2) Before Switching on the klystron power supply, set the beam voltage control knob





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uwave man labcopy

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