Microwave Engineering Lab Manual-4 5j662o

Microwave Engineering Laboratory Instructions 1. Microwave Laboratory will consist of 10 Experiments and will be held in the High-Frequency Lab (COMMUNICATION LAB-II). 2. Each group have 6 students . 3. Students will get a week’s time to complete an experiment 4. When coming for the next experiment, students are bound to bring and submit their previous experiment report before the start of the laboratory. 5. Laboratory performance is evaluated based on the successful completion of the experiment, the results, regularity and sincerity in the lab. 6. Students are required to come prepared for the corresponding experiment. 7. Evaluation for Lab comprises of performance during each session and a laboratory report . 8. Lab performance (5 marks) plus laboratory report (5 marks). 9. Lab record should be submitted with hard cover physics record. When you come for the next experiment. Lab report should be written in the following manner: Expt. Number: Title: Date: Objective: Set-up Diagram: Observations (In tabular form): Results (Give graphs): Conclusion (Draw your own conclusions from the results): Precautions: Attach the result sheets signed by the concerned faculty . 10. Discipline: It is essential that the students follow strict discipline in the Microwave lab. Some marks are set aside for discipline. Understand the maximum allowable values of the equipment and components and never exceed them. Avoid careless connections that will cause damage to the equipments or components. Before leaving switch off power connection to all the instruments on your table. No extra lab will be given under normal conditions.

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EXPERIMENT LIST 1. 2. 3. 4. 5. 6.

EXPERIMENTS Study of Reflex Klystron Characteristics. Study of Gunn Diode Characteristics. Study of Directional Coupler Characteristics. Measurement of Voltage Standing Wave Ratio. Radiation Pattern Measurement of a Horn Antenna. Impedance, Wavelength and Frequency

PAGES 17-20 21-22 23-24 25-27 28-33 34-37

Measurement. 7. Determination of Polarization of Horn antenna. 8. Coupling Measurement of H-plane, E-Plane and

38-42 43-44

Magic Tee junctions. 9. Measurement of Phase shift. 10. Scattering parameters of Circulator /Isolators.

45-46 47

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LABORATORY EQUIPMENTS (a) Slotted Waveguide Section with Carriage and Probe The equipment consists of a W/G section having a narrow longitudinal slot milled at the centre of the broad wall of the W/G. A tuneable probe carriage with detector is attached on the W/G section and by moving the probe over the slotted section the field variation along the length in the W/G guide can be recorded. Scales (including VS) is attached with the frame of slotted guide so that the positions of minimum and maximum along the line can be read.

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(b) Attenuators and matched terminators In microwave circuits, we require elements, which can absorb either all or a portion of the power falling on them without any appreciable reflections. Such elements are known as Attenuators. The first type of attenuators which absorbs all the power falling on them , are also known as Matched Terminations which are used to terminate a W/G section. These are used in the measurement of reflection coefficient and where the matched load is required.

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(c)Short Circuit Terminations Wave guide short circuit terminations are used to create standing wave patterns so that the maximum/minimum positions are sharply defined so that their position can be recorded precisely. Shorts are used as fixed or as variable.

(d) Direct Reading Frequency Meter 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. There are three types of wave meters (1) Transmission type; there the signal to which frequency they are tuned;(2) Absorption type which attenuate only the resonant frequency of signal and (3) Reaction type which absorbs energy from the transmission line at resonant frequency. Transmission type and absorption types are used with matched systems with suitable isolators.

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(e) Isolators This microwave device permits un-attenuated transmission in one direction (forward)but provides very high attenuation in the reverse direction(backward).Normally Isolators are made using ferrite materials.

(f) Directional Coupler A directional coupler is a useful hybrid waveguide junction that couples power in an auxiliary waveguide arm in one direction . It is a four port device commonly used for coupling a known fraction of the microwave power to a port (coupled port) in the auxiliary line while flowing from input port to out port in the main line . The remaining port is ideally isolated port and matched terminated. We consider a simple two hole directional coupler. In port 3 the signals enforces where as in port 4 signals cancel. For larger BW multi-hole directional couplers are used. In which the desired coupling response vs frequency can be achieved by proper selection of the number of holes and size of the holes. Waveguide couplers are forward coupler since the coupled power in the ancillary guide flows in the same direction as the input power in the main guide.

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To understand its operation we consider the block diagram in figure (a) and (b). The performance of a directional coupler is measured in of four basic parameters, i.e, Coupling(C), Transmission loss(T), Directivity(D) and Return loss(R) when all the ports are matched. These defined as

Where P 's are the powers at the ports. These are named according to the axis of the side arm which is parallel to the E-field or the H-field in the collinear arms, respectively. (g) Waveguide Tees Waveguide tees are three port components. They are used to connect a branch or section of the waveguide in series or parallel with the main waveguide transmission line for providing means of splitting, and also of combining power in a waveguide system. The two basic types, E-plane series) Tee and H-plane (shunt) Tee, are constructed as shown in figure. These are named according to the axis if the side arm which is parallel to the E-field or the Hfield in the collinear arms, respectively. If both arms are present then it is called Hybrid tee or magic tee. Because of the junction, waveguide tees are poorly matched devices. Adjustable matching reactance can be introduced by means of a tuning screw at the center. Because of symmetry and absence of non-linear elements in the junction, the S-matrix is symmetric: Sij=Sji;i=1,2;j=1,2. The general matrix for a tee junction is with no coupling to port3 (E-arm). Thus S14=S41=0.707=S24=S42 and S34=0. A wave fed into one collinear port 1 and 2, will not appear in the other collinear ports 2 and 1. Hence two collinear ports 1 and 2 are isolated from each other, making S12=S21=0

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(h) Circulators A circulator is a ive non-reciprocal three- or four-port device, in which a microwave or radio frequency signal entering any port is transmitted to the next port in rotation (only). For a three-port circulator, a signal applied to port 1 only comes out of port 2; a signal applied to port 2 only comes out of port 3; a signal applied to port 3 only comes out of port 1, so to within a phase-factor, the scattering matrix for an ideal three-port circulator is

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Applications 

Isolator



Duplexer

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Reflection amplifier

(h) Waveguide bends and twists In measurements it is often necessary to bend a waveguide by some angle. Waveguide bends in E and H plane of 90° is normally available. Waveguide bends designed by a section of rectangular waveguide and flange. Waveguide Twist is used to change the plane of Polarization of a wave Guide transmission line. Twist is made from a section of waveguide which has been precisely twisted.

(i) VSWR Meter The SWR meter or VSWR (voltage standing wave ratio) meter measures the standing wave ratio in a transmission line. The meter can be used to indicate the degree of mismatch between a transmission line and its load (usually a radio antenna), or evaluate the effectiveness of impedance matching efforts.

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A VSWR meter is a sensitive high gain , high Q , low noise voltage amplifier tuned normally at a fined frequency of 1 KHz at which the microwave signal is modulated . The input to the VSWR meter is the detected signal output of the microwave detector and the output of the amplifier is measured with a square law calibrated voltmeter which directly gives the VSWR reading Vmax/Vmin for an input of Vmin, after the meter is adjusted to unity VSWR for an input corresponding to Vmax . A gain control can be used to adjust the reading to the desired value. The overall gain is of the order of 100 Db which can be altered in steps of 10 dB. There are three scales on the VSWR meter. When the VSWR is between 1 and 4, reading can be taken from the top SWR NORMAL scale. For VSWR between 3.2 and 10 , bottom of SWR NORMAL scale is used. When the VSWR is less than 1.3 , a more accurate reading can be taken by selecting the expanded scale graduated from 1 to 1.3 . The third scale at bottom is graduated in dB.

Technical Specifications (NV 103) Sensitivity: 0.1mV for 200W input impedance for full scale deflection. Noise Level: Less than 0.02mV Range: 0- 60dB in 10dB steps. Input: Un-biased low and high impedance crystal, biased crystal (200 and 200K) Meter Scale: SWR 1-4, SWR 3-10, dB 0-10, expand S W R 1 - 1.3, dB 0-2. Gain Control: Adjusts the reference level, variable range 0-10 dB(approx.)

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Input Connector: BNC (F) Input Frequency: 1000Hz ±10% Power Supply: 220 V ±10%, 50 Hz / 60 Hz on request Power consumption: 2 VA (approx.) Dimension (mm): W 262 × D 316 × H 130 Audio Output: Inbuilt speaker for audio communication. (J) Klystron There are two basic configurations of klystron tubes , one is called Reflex klystron used as a low power microwave oscillator and another is called Multi-cavity klystron, used as a low power microwave amplifier. In this lab, we are using Reflex klystron as low power microwave oscillator. It has a single reentrant microwave cavity as resonator. The electron beam emitted from the cathode is accelerated by the grid and es through the cavity anode to the repeller space between the cavity anode and the repeller electrode.

Reflex Klystron Mechanism:  The cathode emits electrons which are accelerated forward by an accelerating grid with a positive voltage on it and focused into a narrow beam.  The electrons through the cavity and undergo velocity modulation, which produces electron bunching and the beam is repelled back by a repeller plate kept at a negative potential with respect to the cathode.  On return, the electron beam once again enters the same grids which act as a buncher, therby the same pair of grids acts simultaneously as a buncher for the forward moving electron and as a catcher for the returning beam.  The necessary for electrical oscillations is developed by reflecting the electron beam, the velocity modulated electron beam does not actually reach the repeller plate, but is repelled back by the negative voltage.  The point at which the electron beam is turned back can be varied by adjusting the repeller voltage.

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 Thus the repeller voltage is so adjusted that complete bunching of the electrons takes place at the catcher grids, the distance between the repeller and the cavity is chosen such that the repeller electron bunches will reach the cavity at proper time to be in synchronization.  Due to this, they deliver energy to the cavity, the result is the oscillation at the cavity producing RF frequency.

Mode of Oscillation

(k) Gunn Oscillator A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a twoterminal ive semiconductor electronic component, with negative resistance, used in highfrequency electronics. Its largest use is in electronic oscillators to generate microwaves, in applications such as radar speed guns and microwave relay data link transmitters. GUNN Diodes (Transferred Electron Devices): 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 cium telluride (CdTe). These are semiconductors having a closely spaced energy valley in the conduction band as shown in Figure for GaAs. When a dc voltage is applied across the material, an electric field is established across it. At low E-field in the Page | 12

material, most of the electrons will be located in the lower energy central valley Γ. At higher Efield, most of the electrons will be transferred in to the high-energy satellite L and X valleys where the effective electron mass is larger and hence electron mobility is lower than that in the low energy Γ valley. Since the conductivity is directly proportional to the mobility, the conductivity and hence the current decreases with an increase in E-field or voltage in an intermediate range, beyond a threshold value Vth as shown in Figure. This is called the transferred electron effect and the device is also called ‘Transfer Electron Device (TED) or Gunn diode’. Thus the material behaves as negative resistance device over a range of applied voltages and can be used in microwave oscillators.

Multi-valley conduction band energies of GaAs

Current-voltage characteristics of GaAs The basic structure of a Gunn diode is shown in Fig. 5.2 (a), which is 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 positive 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 the diode voltage exceeds a certain threshold value, Vth a high electric field (3.2 KV/m for GaAs) is produced across the active region and electrons are excited from their initial lower valley to the higher valley, where they become virtually immobile. If the rate at which electrons are transferred is very high, the current will decrease with increase in voltage, resulting in equivalent negative resistance effect. Since GaAs is a poor conductor, considerable heat is generated in the diode. The diode will be bonded into a heat sink (Cu-stud). The electrical equivalent circuit of a Gunn diode is shown in Figure where Cj and – Rj are the diode capacitance and resistance, respectively, Rs includes the total resistance of lead, ohmic s, and bulk resistance of the diode, and Lp are the package capacitance and inductance, respectively. The negative resistance has a value that typically lies in the range –5 to –20 ohm.

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Constructional details and the electrical equivalent circuit of a Gunn Diode

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Experiment 1 Objective: To study the characteristics of the Reflex Klystron Tube and to determine it's electronic tuning range. (For NV 9000) Equipments Required: 1. Klystron Power Supply 2. Klystron tube with Klystron mounts 3. Isolator 4. Frequency meter 5. Variable attenuator 6. Detector mount, Wave guide stand 7. SWR meter and oscilloscope 8. BNC cable Theory: The Reflex Klystron makes the use of velocity modulation to transform a continuous electron beam into microwave power. Electrons emitted from the cathode are accelerated& ed through the positive resonator towards negative reflector, which retards and finally, reflects the electrons and the electrons turn back through the resonator. Suppose an RF-field exists between the resonators the electrons travelling forward will be accelerated or retarded, as the voltage at the resonator changes in amplitude.

Figure: Schematics Diagram of Klystron The accelerated electrons leave 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 through resonator, they interact with voltage at resonator grids. If the bunches the grid at such a time that the electrons are slowed down by the voltage then energy will be delivered to the resonator; and Klystron will oscillate. Fig. 2 shows the relationship between output power, frequency and reflector voltages.

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Figure: Square Wave modulation of the Klystron The frequency is primarily determined by the dimensions of resonant 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 the same result can be obtained, if the modulation voltage is applied on the reflector voltage as shown in the fig. Procedure: Carrier Wave Operation: (1) Connect the components and equipments as shown in figure.

(2) Set the Variable Attenuator at no attenuation 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 select to Beam position. (4) Keep SWR meter at 50dB attenuation and coarse and fine potentiometers on mid position and crystal impedance at 200ohm. (5) Keep SWR/dB switch at dB position. (6) Set the multi-meter in DC microampere range. (7) Switch 'On' the klystron power supply & cooling fan for klystron tube. (8) Now in K.P.S set Mode select switch to AM- MODE position. Beam voltage control knob to fully anticlockwise position. Reflector voltage control knob to the maximum clockwise position (9) Change the reflector voltage slowly and observe the reading on the SWR meter. Set the voltage for maximum reading in the meter. If no reading is obtained, change the plunger position of klystron mount and detector mount. Select the appropriate range on SWR Meter. Now replace SWR meter to multi-meter. (10) Tune the plunger of klystron mount for the maximum output. (11) Rotate the knob of frequency meter slowly and stop at that position, when there is less output current on multi-meter. Read directly the frequency between two horizontal line and vertical line markers. If micro meter type frequency meter is used, read micrometer frequency and find the frequency from its calibration chart.

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Square Wave Operation: (1) Connect the equipments and components as shown in the figure.

Figure 4 (2) Set Micrometer of variable attenuator for no attenuation. (3) Set the range switch of SWR meter at appropriate position, crystal selector switch to 200ohm impedance position, mode select to normal position. (4) Now in KPS set Mode select switch to AM- MOD position. Beam voltage control knob to fully anticlockwise position. Reflector voltage control knob to the maximum clockwise position. (5) Switch ‘On’ the Klystron Power Supply, SWR meter and cooling fan. (6) Change the beam voltage knob clockwise up to 300V. (7) Keep the AM amplitude knob and AM frequency knob at the mid-position. (8) Rotate the reflector voltage knob to get reading in SWR meter. (9) Rotate the AM amplitude knob to get the maximum output in SWR meter. (10) Maximize the reading by adjusting the frequency control knob of AM. (11) If necessary, change the range switch of SWR meter if the Reading in SWR meter is greater than 0.0db or less than -10dB in normal Mode respectively. Further the output can also be reduced by Variable Attenuator for setting the output for any particular position. (12) Connect oscilloscope in place of SWR Meter and observe the square wave across detector mount. Mode Study on Oscilloscope: (1) Set up the components and equipments as shown in figure 7. (2) Set Mode selector switch to FM-Mode 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.

Figure 5

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Figure 6 (3) Keep the time/division scale of Oscilloscope around 100Hz frequency measurement and volt/ div to lower scale. (4) Switch ‘On’ the klystron power supply and oscilloscope. (5) Set beam voltage to 300V by beam voltage control knob. (6) Keep amplitude knob of FM modulator to maximum position and rotate the reflector voltage anti-clockwise to get modes as shown in figure 8 on the oscilloscope. The horizontal axis represents reflector voltage axis, and vertical axis represents output power. (7) By changing the reflector voltage and amplitude of FM modulation, any mode of Klystron tube can be seen on an Oscilloscope.

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Experiment-2 Objectives: Study of Characteristics of Gunn Diodes 1. V-I Characteristics 2. Output Power and frequency as a function of voltage. 3. Square wave modulation through PIN diode. Equipments: 1. Gunn oscillator 2. Gunn power supply 3. PIN modulator 4. Isolator 5. Frequency meter 6. Variable attenuator 7. Detector mount 8. Waveguide stands 9. SWR meter 10. Cables and Accessories. Theory: Gunn Oscillator: In a Gunn Oscillator, the Gunn Diode is placed in a resonant cavity. In this case the oscillation frequency is determined by cavity dimension than by the diode itself. Although Gun Oscillator can be amplitude-modulated with the bias voltage, we have used separate PIN modulator through PIN diode for square wave modulation. Procedure: 1. Set the components and equipments as shown in the Figure 2. Initially set the variable attenuator for maximum attenuation. 3. Keep the control knob of Gunn Power Supply as below: Meter Switch - ‘OFF’ Gunn bias knob - Fully anticlockwise Pin bias knob - Fully anticlockwise Pin Mod frequency - Any position 4. Keep the control knob of VSWR meter as below: Meter Switch - Normal Input Switch - Low Impedance Range db Switch - 40 db Gain Control knob - Fully clockwise 5. Set the micrometer of Gunn Oscillator for required frequency of operation. 6. ‘ON’ the Gunn Power Supply, VSWR meter and Cooling fan. A. Voltage-current characteristics 1. Turn the meter switch of ‘Gunn power supply to voltage position. 2. Measure the Gunn diode Current Corresponding to the various voltage controlled by Gunn bias knob through the meter and meter switch. Do not exceed the bias voltage above 10 volts. 3. Plot the voltage and current readings on the graph as shown in Figure. 4. Measure the threshold voltage which corresponds to maximum current.

NOTE: DONOT KEEP GUNN BIAS KNOB POSITION AT THRESHOLD POSITION FOR MORE THAN 10-15 SECONDS. READING SHOULD BE OBTAINED AS FAST AS POSSIBLE. OTHERWISE, DUE TO EXCESSIVE HEATING, GUNN DIODE MAY BURN. Page | 22

B. Output power and frequency as function of bias voltage 1. Turn the meter switch of Gunn power supply to voltage position. 2. Increase the Gunn bias control knob. 3. Rotate PIN bias knob to around maximum position. 4. Tune the output in the VSWR meter through frequency control knob of modulation. 5. If necessary change the range db switch of VSWR meter to higher or lower db position to get deflection on VSWR meter. Any level can be set through variable attenuator and gain control knob of VSWR meter. 6. Measure the frequency by frequency meter and detune it. 7. Reduce the Gunn bias voltage in the interval of 0.5V or 1.0V and note down corresponding reading of output at VSWR meter and frequency by frequency meter. 8. Use the reading to draw the power vs Voltage curve and frequency vs voltage and plot the graph. 9. Measure the pushing factor (in MHz/Volt) which is frequency sensitivity against variation in bias voltage for an oscillator. The pushing factor should be measured around 8 Volt bias. C. Square Wave Modulations 1. Keep the meter switch of Gunn Power Supply to volt position and rotate Gunn bias voltage slowly so that meter of Gunn Power Supply reads 10Volt. 2. Tune the PIN modulator bias voltage and frequency knob for maximum output on the oscilloscope. 3. Coincide the bottom of square wave in oscilloscope to some reference level and note down the micrometer reading of variable attenuator. 4. Now with the help of variable attenuator coincide the top of square wave to same reference level and note down the micrometer reading. 5. Connect VSWR to detector mount and note down the db reading in VSWR meter for both the micrometer reading of the variable attenuator. 6. The difference of both db reading of VSWR meter gives the modulation depth of PIN modulator.

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Experiment-3 Objective: Study the function of multi-hole directional coupler by measuring the following parameters: 1. To Measure main-line and auxiliary-line VSWR. 2. To Measure the coupling factor and directivity. Equipments: 1. Microwave source (Klystron or Gunn Diode type) 2. Isolator 3. PIN modulator 4. Frequency meter 5. Variable attenuator 6. Slotted line 7. Tuneable Probe 8. Detector mount 9. Matched Terminator 10. MHD coupler 11. Wave guide stand 12. Cables & accessories 13. VSWR meter Theory: A directional coupler is a device with it is possible to measure the incident and reflected wave separately. It consists of two transmission line, the main arm and auxiliary arm, electromagnetically coupled to each other. Refer to the fig. The power entering port 1 the main arm gets divided between port 2 and 3 and almost no power comes out in port 4. Power entering port 2 is divided between port 1 and port 4.

Figure : Directional Coupler Coupling (db) = 10 log10P1/P3 . where port 2 is terminated Isolation = 10 log10P2/P3 . 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. Hence Directivity 0 (dB) = Isolation - Coupling = 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 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 = 10 log10P1/P2 when power is entering at port 1

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Figure: Setup for measurement of VSWR of MHD Coupler Procedure: (1) Main Line SWR Measurement (a) Set up the equipments as shown in the fig. (b) Energize the microwave source for particular frequency operation as described. (Procedures given in the operation of klystron and Gunn oscillator) (c) Follow the procedure as described for SWR measurement experiment (Low and medium SWR measurement). (d) Repeat the same for other frequency. (2) Auxiliary Line SWR Measurement (a) Set up the components and equipments as shown in the figure. (b) Energize the microwave source for particular frequency operation as described operation of Klystron and Gunn Oscillator (c) Follow the procedure as described for SWR measurement experiment (Low and medium SWR measurement). (d) Repeat the same for other frequencies. (3) Measurement of Coupling Factor, Insertion Loss (a) Set up the equipments as shown in the fig. (b) Energize the microwave source for particular frequency operation as described operation of Klystron and Gunn Oscillator. (c) Remove the multi-hole directional coupler and connect the detector mount to the slotted line. (d) Set any reference level of power on SWR meter with the help of variable attenuator, gain control knob of SWR meter, and note down the reading. (Reference level let it be X) (e) 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 SWR meter. (f) Note down the reading on SWR meter on the scale with the help of range db switch if required. (Let it be Y) (g) Calculate coupling factor, which will be X-Y in dB. (h) Now carefully disconnect the detector from the auxiliary port 3 and match termination from port 2 without disturbing the set-up. (i) Connect the matched termination to the auxiliary port 3 and detector to port 2 and measure the reading on SWR meter. Suppose it is Z. Result and Analysis: Calculate the coupling factor, which will be X-Y in db Compute insertion loss X-Z db Compute the isolation Z-Y. Now Directivity = Isolation – Coupling

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Experiment-4 Objective: To determine the Standing Wave-Ratio and Reflection Coefficient. Apparatus: 1. Gunn power supply 2. Gunn oscillator 3. SWR meter 4. Isolator 5. PIN modulator 6. Frequency meter 7. Slotted line 8. Tuneable probe 9. S-S tuner 10. Matched termination Theory: It is a ratio of maximum voltage to minimum 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 travelling 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 from a discontinuity on the line or from the load impedance. The magnitude and phase of reflected wave depends upon amplitude and phase of .the reflecting impedance. The superposition of two travelling 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

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Where

EI = Incident Voltage Er= Reflected Voltage Reflection Coefficient, ρ is Where Z is the impedance at a point on line, Zo is characteristic Impedance. The above equation gives following equation.

Figure: Setup for VSWR measurement

Procedure: (1) Set up the equipment as shown in the fig. (2) Keep variable attenuator at no attenuation position. (3) Connect the S.S tuner & matched termination after slotted line. (4) Keep the control knobs of Gunn power supply as shown: 1. Gunn bias knob : fully anti- clockwise 2. PIN bias knob : fully anti- clockwise 3. PIN Mod freq. : mid position 4. Mode switch : Int. mode position (5) Keep the control knob of SWR as shown: 1. Range : 40dB/50dBposition 2. Crystal : 200 ohm 3. Mode switch : Normal 4. Gain (coarse & fine) : mid position 5. SWR/dB switch : dB position (6) Set the micrometer of Gunn oscillator at 10mm position. (7) Switch ON the Gunn power supply, SWR meter and cooling fan. (8) Observe the Gunn diode current corresponding to the various voltages controlled by Gunn bias knob through the LCD meter, do not exceed bias voltage above 10.5 volts. (9) If necessary change the range db-switch, variable attenuator position and gain control knob to get deflection in the scale of SWR meter. (10) Move the probe along with slotted line, the reading will change. (11) For low SWR set the S.S tuner probe for no penetration position. (a) Measurement of low and medium VSWR I. Move the probe along with slotted line to maximum deflection in SWR meter in dB. II. Adjust the SWR Meter gain control knob or variable attenuator until the meter indicates 0.0 dB on normal mode SWR for 0.0 dB is 1.0 by keeping switches at SWR we can read it directly. Page | 27

III. Keep all the Control knobs as it is, move the probe to next minimum position. Keep SWR/dB switches at SWR position. IV. Repeat the above step for change of S.S. Tuner probe path & record the corresponding SWR. Read SWR from display & record it. V. If the SWR is greater than 10, follow the instructions that follow. (b) Measurement of High SWR (Double Minimum Method) I. Set the depth of S.S tuner slightly more for maximum SWR. II. Move the probe along with slotted line until a minimum is indicated. III. Adjust the SWR meter gain control knob and variable attenuator to obtain a reading of 3 dB (or any other reference).at SWR meter. IV. Move the probe to the left on slotted line until maximum reading is obtained i.e. 0 db on scale. Note and record the probe position on slotted line. Let it be d1. (Or power should be increased by 3 db). V. Move the probe right along with slotted line until maximum reading is obtained on 0 db scale. Let it be d2. VI. Replace the S.S tuner and terminator by movable short. Result and analysis: VII. Measure the distance between two successive minima position or probe. Twice this distance is waveguide length. λg = 2(d1-d2) VIII. Now calculate SWR using following equation SWR= λg /Π (d1-d2) IX. For different SWR, calculate the refection coefficient.

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Experiment-5 Objectives: 1. To plot the radiation pattern in E- & H- planes. 2. To determine 3-dB beam width in both planes and compute directivity. 3. To determine gain using two identical horn antennas and compute radiation efficiency. Equipments: 1. Klystron tube 2. Klystron power supply 3. Isolator 4. Frequency meter 5. Variable attenuator 6. Klystron mount, 7. Waveguide stands 8. SWR meter 9. Detector mount 10. Two horn antennas 11. Turn table and accessories. Theory: In microwave communications, the transmission and reception of microwave signal through free space, is a must. Antenna acts as an impedance transformer between the free space and source in this communication. The fundamental antenna parameters are field patterns, directivity, bandwidth and gain. A waveguide may behave as an antenna if its open end is matched to free space intrinsic impedance. Such an antenna will have shapes like that of horns and are called as horn antenna.

RADIATION PATTERN The radiation pattern of an antenna is a diagram of electric field strength. Here, the directional characteristics of an antenna would ideally be shown as a three-dimensional graph in which, for each direction, the radius from a central point is proportional to the power density at a given distance. For practical reasons, the radiation pattern is normally shown by twodimensional graphs which show a section or sections of the three-dimensional pattern. The radiation pattern of an antenna is a 3-D graphical representation of the radiation properties of the antenna as a function of position (usually in spherical coordinates). If we imagine an antenna is placed at the origin of a spherical coordinate system, its radiation pattern is given by measurement of the magnitude of the electric field over a surface of a sphere of radius r. For a fixed r, electric field is only a function of q and f , so we can write E(q ,f ). 3-D radiation patterns are difficult to draw and visualize in a 2-D plane like on a piece of paper. So usually

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they are drawn in two principal 2-D planes which are orthogonal to each other (E- and Hplanes). E-plane (H-plane) is usually the plane in which there are maximum electric (magnetic) fields for a linearly polarized antenna.

A 2-D section of a 3-D radiation pattern (Beamwidth between first nulls: BWFN and Half power beamwidth: HPBW) Polar plot

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Linear plots Antenna: If a waveguide, which is propagating a signal, is left with an open end, some of the signal energy will escape into space. Some will be reflected because the end is not well matched to free space, so a VSWR of about 2 will typically result. Let us consider first the energy which does get radiated or transmitted into space. Suppose the transmitted power is Pt. If it were radiated in all directions equally, then at a distance r from the source the total power Pt would be spread evenly across the surface of a sphere of surface area 4πr2. A receiving antenna occupying area A of that sphere would receive a portion of the transmitted power,

When it is required to transmit energy efficiently into space, a device called an ‘aerial’ or ‘antenna’ is used. The horn is a very simple form of antenna, being no more than a flare-out shape of the waveguide walls. It improves the match between the waveguide and free space, and narrows the angle over which energy is radiated. By concentrating the radiation in a particular direction, the power radiated in that direction is increased (at the expense of reduced power in other directions). The factor by which it is increased is called the ‘gain’ of the transmitting antenna. Thus, the power received by the receiving antenna of area A becomes:

The gain G is often expressed in decibels as: 10 log10 (G) dBi, where the ‘i’ refers to an isotropic radiator; one which radiates equally in all directions. Fig. 3.4 shows the planes used for a rectangular waveguide, designated E-plane and H-plane because they contain the directions of the electric and magnetic field respectively.

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As shown in Figure, a radiation pattern usually has several ‘lobes’. Generally, most energy is concentrated into the main lobe. Radiation in side and back lobes represents a waste of power. 3-Db Beam Width: ‘3-dB beam width’ is often used as a measure of the directivity of an antenna. It is the angle (HPBW in Figure) between the two points on the main lobe at which the radiated power density is half the maximum. The gain is generally the highest if the beam width is narrow and the side lobes are small, so that all the power is sent in the desired direction. An antenna which has these entire characteristic will also generally be an efficient receiver of radiation. Far-Field Pattern: The radiation pattern differs when measured close to the antenna and at a distance. It is usually the latter condition which is of interest, referred to as the ‘far-field’. For practical purposes, and in the case of a simple horn antenna, the far-field may be taken to start at a distance 2D2/λo from the horn, where D is its larger dimension at the opening, and λ o is the free-space wavelength. Radiation measurements are easily disturbed by reflections from the ground and other objects. These problems are avoided as far as possible in practice by using clear areas out of doors, or by using ‘anechoic chamber’, rooms having walls specially designed to absorb radiation (pyramid shaped carbon-impregnated polyurathene foams).

Antenna test-bench set-up for radiation pattern measurement (a) Schematic and (b) Block diagram Procedure: A. Radiation Pattern Plotting: 1. Set the components and equipments as shown in Figure.

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2. Same type of transmitting and receiving antenna (horn antenna) are used, keeping the axis of both antennas in the same axis line. To satisfy the ‘Far-field Pattern’ a space of about 15 cm between antennas may be tried at the start. 3. The variable attenuator is set accordingly for maximum deflection at the VSWR meter. 4. The amplifier (Klystron or Gunn diode) is set for maximum sensitivity. 5. Align the antennas at 00 directions. Notice that antennas must be similarly ‘polarized’. 6. Attenuator is adjusted for deflection near maximum (possibly at 0-dB). 7. Using a protractor (similar to Fig. 3.6) to measure angles, rotate the receiving antenna about the centre of the broad edges of its aperture (opening). Set the angle to 100, 200, 300 and 400 in each direction. Record the meter reading in each case. They are plotted on a graph sheet like Fig. The 3-dB beam-width of the antenna for E- and H-planes are found out from the graph (as the meter reading is proportional to received power, consequently 3 dB, half power, means that the meter reading is half the maximum reading). 8. Calculate the directivity from the HPBW in E- and H-planes as

B. Gain measurement: 1. Set up the equipments like we used in the previous experiment (this is also known as two antenna method since we are using two identical horn antennas at the transmitter and receiver with same gain). 2. Keep the range dB switch of VSWR meter at 50 dB position with gain control full. 3. Energize the Gunn oscillator (or Klystron Amplifier) for maximum output at desired frequency. 4. Obtain full scale deflection at VSWR meter with variable attenuator. 5. Obtain the reading at the VSWR meter at the receiving antenna and record it. 6. Without touching gain control knob, replace the transmitting horn by detector mount and change the appropriate range dB position to get the deflection on scale. Note and record the range dB position and deflection of VSWR meter. 7. Calculate the difference in dB between the power measured in step 4 and 5. Then calculate the gain as explained in the following example. 8. Radiation efficiency can be calculated as

EXAMPLE Suppose that a deflection of 5 dB on 20 dB range dB position was obtained in step 5, the difference between 4 and 5 is 50 – (20 – 5) = 25 dB. Convert the dB into power ratio. As for above example, it will come out to be 316, which will be Pt/Pr. Calculate gain by following equation:

In our above example, suppose operating frequency is 9GHz. So, 0 l = 3.33cm. c is velocity of light and is 3 10 ×10 cm/sec. Suppose the distance between antennas is 15 cm. 9. Convert G into dB in above example G dB = 10 log 318 = 15.02 dB 10. The same setup can be used for other frequency of operation. Meter deflection reading for movement of antenna by respective degree

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Typical chart for plotting radiation patterns (choose proper scale for your radiation pattern plots)

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Experiment-6A Objective: To determine the frequency and wavelength in a rectangular waveguide working in TE10 mode Equipments: 1. Klystron tube 2. Klystron power supply 3. Klystron mount, Isolator 4. Frequency meter 5. Variable attenuator 6. Slotted section waveguide 7. Tuneable probe 8. VSWR meter 9. Waveguide stand 10. Movable short/matched termination. THEORY For dominant TE10 mode in rectangular waveguide λ0, λg, and λc are related as below:

Where λ0 is free space wavelength, λg is guide wavelength and λc is cut-off wavelength For TE10 mode, λc = 2a where 'a' is broad dimension of waveguide. PROCEDURE 1. Set up the components and equipments as shown in Fig. 1. 2. Set the variable attenuator at maximum position. 3. Keep the control knobs of VSWR meter as below: Range db 50 db position Input Switch Crystal low Impedance Meter switch Normal position Gain (Coarse & fine) Mid Position 4. 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 5. Switch ‘ON’ the Klystron power supply, VSWR Meter and cooling fan. 6. Rotate the meter switch of power supply to beam voltage position and set beam voltage at 300 V with help of beam voltage knob. 7. Adjust the reflector voltage to get some deflection in VSWR meter. 8. Maximize the deflection with AM amplitude and frequency control knob of power supply. 9. Tune the plunger of Klystron mount for maximum deflection. 10. Tune the reflector voltage knob for maximum deflection. 11. Tune the probe for maximum deflection in VSWR meter. 12. Tune the frequency meter knob to get a ‘dip’ on the VSWR scale and note down the frequency directly from frequency meter. 13. Replace the termination with movable short, and detune the frequency meter. 14. 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. 15. Move the probe to next minimum position and record the probe position again.

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16. Calculate the guide wavelength as twice the distance between two successive minimum position obtained as above. 17. Measure the waveguide inner broad dimension ‘a’ which will be around 2.286cm for Xband. 18. Calculate the frequency by following equation:

where c = 3 × 108 meter/sec. i.e. velocity of light. 19. with frequency obtained by frequency meter. 20. Above experiment can be verified at different frequencies. 21. Record the experimental results in a tabulated form as per format given below:

Setup for measurement of wavelength and frequency

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Experiment-6B Objective: To measure an unknown Impedance with Smith chart Equipments: 1. Gunn oscillator 2. Gunn power supply 3. Isolator 4. PIN modulator 5. Frequency meter 6. Variable attenuator, Slotted Line 7. Tuneable probe 8. SWR meter 9. Wave guide stand 10. S.S. Tuner 11. Matched Termination. Theory: The impedance at any point of a transmission line can be written in the form R + jX. For comparison SWR can be calculated as

Reflection Coefficient

Where Zo= Characteristics impedance of w/g at operating frequency Z = Load impedance at any point. The measurement is performed in following way: The unknown device is connected to the slotted line and the SWR = So and the position of one minima is determined. Then unknown device is replaced by movable short to the slotted line. Two successive minima positions are noted. The twice of the difference between 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 from unknown load. Let it be ’d’. Take a Smith chart taking '1' as center; draw a circle of radius equal to So. Mark a point on circumference of chart towards load side at a distance equal to d/λg. the center with this point. Find the point where it cut the drawn circle. The coordination of this point this will show the normalized impedance of load

Figure: Setup for Impedance measurement Procedure: (1) Set up the equipments as shown in the above figure. (2) Set the variable attenuator at no attenuation position. (3) Connect S.S tuner after slotted line. (4) Connect matched termination after S.S tuner.

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(5) Keep the control knobs of SWR meter as below. 1. Range dB : 40dB/50 dB position 2. Crystal : 200 ohm 3. Mode Switch : Normal Position 4. Gain (Coarse & fine) : Mid Position. 5. SWR/dB switch : dB Position (6) Keep the Control knobs of Gunn power supply as below 1. Gunn bias : Fully anti- clockwise 2. PIN bias supply : Fully anti- clockwise 3. PIN Mod frequency : Mid position 4. Mod Switch : Internal mod position (7) Set the micrometer of Gunn oscillator at 10mm position. (8) Switch "ON' the Gunn Power Supply, SWR meter and cooling Fan. (9) Observe the Gunn diode current corresponding to the various voltage controlled by Gunn bias voltage. (10) Tune the frequency meter knob to get dip on the SWR scale, and note down the frequency directly from frequency meter. Now you can detune the meter from dip position. (11) Measure the guide wavelength λg as previous experiment. λg = 2 (d1- d2). (12) Keep the depth of pin of S.S. Tuner to around 3-4mm and lock it. (13) Move the probe along with slotted line to get maximum reading. (14) Adjust SWR meter gain control knob and variable attenuator unit such that the meter indicates 1.0 on the normal upper SWR scale. (15) Move the probe to next minima point. (16) Select SWR/dB switch to SWR position. Record the SWR reading. (17) At this maximum position of the meter record the probe position from slotted line as X1. (18) Replace the load by fixed short/movable short & measure the new standing wave position i.e. shifts in minima. Record it as X2. (19) Calculate X2-X1, it will be positive if the minima shift is towards load & negative if it has shifted towards generator. (20) Calculate shift in wavelength. (d) = X2 – X1

Figure: Standing waves in impedance measurement (21) Use normalized chart (Smith Chart) & draw a circle with radius = 1/VSWR & take center of circle = 0.00 on the smith chart. (22) Locate a point at a distance d (shift in minima) from the 0.0 moving in clockwise or anti-clockwise direction (depends on getting minima towards generator or load). (23) the above point to the centre of smith chart. The intersection of VSWR circle & this line gives load, reactive component or reactive circle & resistive component on real circle. (24) Normalized impedance a+ib where a & b are the real and reactive components. (25) The multiplication with characteristic impedance will give you the load impedance.

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Experiments-7 Objective: Measuring the transverse and longitudinal field distribution of microwaves in front of a horn antenna. Equipments: 1. Gunn oscillator 2. Large horn antenna 3. Stand rod (245mm, with thread) 4. Gunn power supply with amplifier 5. E-field probe 6. Physics microwave accessories 7. Voltmeter (DC, U≤10V) 8. Saddle bases 9. BNC lead (2m long) 10. Cable (100cm, black) 11. Ruler Theory: Properties of microwaves Microwaves are electromagnetic waves with frequencies between 300 MHz and 300 GHz and wavelengths between 1 m and 1 mm. Although the frequencies are below those of visible light by more than 3 orders of magnitude, many properties of microwave radiation can be compared with those of visible light. For instance, microwaves can be polarized the same way as light waves. If the electrical field oscillates in a fixed plane, this is called linear polarization. Such a linear polarization can be created or analyzed by means of a polarizer. If a linearly polarized wave with the electric field amplitude E0 impinges on a polarizer that is rotated against the direction of polarization of the wave by the angle θ, the field component

will the polarizer. Therefore the intensity of the wave is

behind the polarizer. In optics, second equation is known as the Malus law There is a marked difference between the generation of microwaves and light waves. Microwaves are generated in a wave- guide and emitted into free space via an extended antenna. At a sufficiently large distance, the antenna can be regarded as a point like source. At this distance the electric and the magnetic fields of the microwave oscillate perpendicularly to each other and to the direction of propagation (far field). Both fields decrease inversely proportionally to the distance, their ratio being constant:

At distances below the limit

(D: greatest transverse dimension of the antenna, λ: wavelength) Page | 39

the field distribution of the radiated wave is more complex (near field). Only in waves radiated perpendicularly to the antenna, the direction of propagation and the electric and magnetic field are perpendicular to each other. Microwave source: In this experiment, a Gunn oscillator is used as a microwave source. It operates at a frequency of 9.4 GHz and releases a power of approx. 10 mW via a connected horn antenna (see Fig. 1). The oscillator is a short section of a rectangular waveguide with a small ceramics body being held by a brass post immediately in front of the back. In the ceramics body, there is a semiconductor element with a negative differential resistance. This so-called Gunn element plays the active role in generating oscillations of the electric and magnetic field. On the opposite side, the waveguide is closed by a pinhole diaphragm, through which part of the generated microwave power escapes. A horn antenna is coupled to the closed cavity via another waveguide section. The microwave power is emitted into free space by the antenna.

Fig. Internal structure of the microwave source and distribution of the electric field E in the dominant mode of the cavity oscillation (a) Gunn element, (b) cavity, (c) pinhole diaphragm, (d) waveguide, (e) horn antenna In the cavity, standing electromagnetic waves can arise, whose wavelengths are determined by the dimensions of the cavity. If the cavity is made smaller, the wavelength becomes shorter and the frequency is increased. The frequency can also be changed by introducing a dielectric pin. In the dominant mode, the resonance frequency is given by

c: velocity of light, b: cavity width s: cavity length (here: distance between the pinhole diaphragm and the Gunn element, see Fig. For s=22mm and b=23mm, f=9.4GHz is obtained and there from λ= 33 mm. If the relevant dimension of the antenna is D = 80 mm, the limit rD = 400 mm results for the far field.

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Measuring the field strength: An E-field probe (see Fig.2 ), which does not affect the field distribution, is used to measure the electric field strength in the microwave field in a single point. In the probe, short wires, which are soldered to a high-frequency diode, act as dipole antennas for microwaves. A highresistance layer made of graphite taps the received signal. The copper wires in the lower part of the probe are twisted so as to avoid magnetically induced voltages.

Strictly speaking, the E-field probe measures the electric field’s component that is parallel to the longitudinal axis of the probe and rectifies the signal. As the diode characteristic is not linear, the output signal is approximately proportional to the square of the field component. The Gunn power supply is equipped with an integrated amplifier for the output signal of the Efield probe.

Fig. Experimental setup : for measuring the field distribution in front of the horn antenna

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Setting up: Remarks: Measuring results may be distorted by reflection of the micro- waves from vertical surfaces of objects close to the experimental setup: Choose the direction of transmission of the horn antenna so that reflecting surfaces are at a distance of at least 4 m. If possible, use microwave absorbers to build up a reflection- free measuring chamber. If several experiments with microwaves are run at the same time, neighbouring Gunn oscillators can interfere: Try to find a suitable arrangement of the experiments. In this case, use of microwave absorbers is mandatory to set up separate reflection-free measuring chambers. The varying magnetic field of microwaves can induce voltages in cable loops: Avoid cable loops. The experimental setup is illustrated in Fig. For measuring distances, make an 800 mm long rule by sticking together scale paper, or use a ruler. Attach the Gunn oscillator to the horn antenna with the quick connectors. Align the horn antenna horizontally, screw the 245 mm long stand rod into the corresponding thread and clamp it in a saddle base. Connect the Gunn oscillator to the output OUT via a BNC lead. Connect the E-field probe to the amplifier input and the voltmeter to the output DC OUT of the Gunn power supply. Set up the E-field probe in front of the centre of the horn antenna. Set the modulation frequency with the frequency adjuster so that the multimeter displays maximum received signal. Procedure: I. Transverse field distribution 1. Set up the E-field probe in front of the horn antenna at the distance x0 = 100 mm. 2. Vary the position of the E-field probe between y = −200 mm and +200 mm in steps of 40 mm. For each case read the received signal U and take it down. 3. Repeat the measurement for the distance x0 = 200 mm.

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II. Longitudinal field distribution 1. Set up the E-field probe in front of the center of the horn antenna (y0 = 0 mm). 2. Measure the received signal U between x = 100 mm and 820 mm in steps of 40 mm and take it down.

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Experiments-8 Objective: Study of Magic Tee. Apparatus: 1. Microwave source 2. Isolator 3. Variable attenuator 4. PIN modulator 5. Frequency meter 6. Slotted line 7. Tuneable probe 8. Magic Tee 9. Matched termination 10. Wave guide stand 11. Detector mount 12. VSWR meter and accessories. Theory: The device magic Tee is a-combination of the E and H plane Tee. Arm 3, the H-arm forms an H plane Tee and arm 4, the E-arm forms an E plane Tee in combination with arm 1 and 2 a side or collinear arms. If power is fed into arm 3 (H-arm) the electric field divides equally between arm 1 and 2 in the same phase, and no electrical field exists in arm 4. Reciprocity demands no coupling in port 3 (H-arm). 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 from arm 1 and 2, it is added in arm 3 (H-arm), and it is subtracted in E-arm, i.e. arm 4.

Figure : Magic Tee The basic parameters to be measured for magic Tee are defined below. (1) Input VSWR Value of SWR corresponding to each port, as a load to the line while other ports are terminated in matched load. (2) Isolation The isolation between E and H arms 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 side arms I and 2 are terminated in matched load. Hence,

Similarly, isolation between other parts may also be defined. Page | 44

(3) Coupling coefficient. It is defined as

Where α = attenuation / isolation in dB 'i’ is input arm 'j’ is output arm. Thus

Where Pi is the power delivered to arm i Pjis power detected at j arm.

Figure: Setup for the study of Magic Tee (1) VSWR Measurement of the Ports (a) Set up the components and equipments as shown in fig. keeping E arm towards slotted line and matched termination to other ports. (b) Energize the microwave source for particular frequency of operation and tune the detector mount for maximum output. (c) Measure the SWR of E-arm as described in measurement of SWR for low and medium value. (d) Connect another arm to slotted line and terminate the other port with matched termination. Measure the SWR as above. Similarly, SWR of any port can be measured. (2) Measurement of Isolation and Coupling Coefficient. (a) Remove the tunable probe and Magic Tee from the slotted line and connect the detector mount to slotted line. (b) Energize the microwave source for particular frequency of operation and tune the detector mount for maximum output. (c) With the help of variable attenuator and gain control knob of SWR meter, set any power level in the SWR meter and note down. Let it be P3. (d) 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 SWR meter. Let it be P4. (e) In the same way measure P1 & P2 by connecting detector on these ports one by one. (f) Determine the isolation between port 3 and 4 as P3-P4 in dB. (g) Determine the coupling coefficient by P3- P1 for port P1 & P2. (h) Repeat the above experiment for other frequencies.

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Experiment-9 Objective: To Study the Phase Shifter Apparatus: 1. Microwave source 2. Isolator 3. Variable attenuator 4. Frequency meter 5. Slotted section 6. Tuneable probe 7. Phase shifter 8. Precision Movable short 9. SWR meter Theory: A phase shifter consists of a piece of Wave-guide and a dielectric material inside the waveguide placed parallel to Electric vector of TE10 mode. The phase changes as piece of dielectric material is moved from edge of wave-guide towards the centre of the waveguide. Procedure: (1) Set up the equipment as shown in the fig.

Figure: Setup for Study of Phase Shifter (2) First connect the matched termination at the end of slotted line. (3) Energize the microwave source for maximum output at particular frequency of operation. (4) Find out the λg (guide wavelength) with help of tuneable probe Slotted line and SWR meter. It is the twice the distance between two minima on the slotted line. (5) Now connect the precision movable short at the first place of matched termination and find out the minima for this step. (6) Note and record the reference minima position on the slotted line. Let it be X. (7) Remove carefully the movable shift from the slotted line without disturbing its current position. Place the phase shifter to the slotted line with its micrometer reading zero and then place the movable short to the other port. (8) The reference minima will short from its previous position, rotate the micrometer of movable precision short to get the minima at reference minima position and note the micrometer reading of movable short. (9) Open the phase shifter in suitable steps. Result and analysis: (10) Fill in the given table as per step 7 and 8. Record the corresponding micrometer reading of short. Measure the phase shift as per given example.

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(11) Precision movable short is rotated to get the minima, at reference minima position at different values or phase shift of micrometer. Calculation: We can calculate phase shift in of degree by λg = 360’ (One cycle). For example: If λg = 4.32 cm Phase shifter position or micrometer is moved to 2mm. Now the reference minima gets changed vary the precision movable short to get the reference minima position i.e 0.5 cm now the shift in phase is Since, 4.32 cm = 360’ Let it be Y 0.5 cm = Y

Plot the graph.

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Experiment-10 Objective: To find the characteristics of ferrite device – circulator. Apparatus: 1. Microwave source (klystron power supply) 2. Klystron Mount 3. Isolator 4. Variable Attenuator 5. Frequency meter 6. Circulator 7. Power Detector 8. Matched termination ----- 1 No Theory: 1. The S – matrix of 3 – port circulator is 0 0 1 S= 1 0 0 0 1 0 Where

1. 2.

S11 = S22 = S33 = 0 S12 = S23 = S31 = 0 S21 = 20log (V2 / V1) S13 = 20log (V1 / V3) Insertion loss = 10 log (p1/p2) Isolation = 10 log (p1/p3)

Procedure: 1.

Arrange the bench setup without connecting circulator and measure the input power. 2. Now connect the circulator and note down the output power at port 2 & port 3. 3. Substitute the values to estimate the S – matrix of circulator.

Result: Thus the characteristics of given 3 – port circulator was obtained and verified. Bench Setup Diagram Of Ferrite Devices: MICROWAVE SOURCE

ISOLATO R

VARIABLEA TTENUATO R

FREQUEN CY METER

DETECTOR OR MATCHED TERMINATIO N

CIRCULATOR

MATCHED TERMINATIO N OR DETECTOR

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