**ANNA UNIVERSITY QUESTION PAPER **

**B.E. ELECTRONICS AND COMMUNICATION ENGINEERING **

**SEMESTER - 7 **

** EC432 MICROWAVE ENGINEERING **

**Time: Three Hours Maximum : 100 Marks **

**Answer All The Questions **

**PART - A (2 x 10 = 20 Marks) **

1. With the help of electronic admittance diagram explain the modes of a reflex

klystron oscillator.

2. A two-cavity klystron operates at 10 GHz with I0 = 3.6 mA, V0 = 10KV. The

drift space length is 2 cms, the output cavity total shunt conductance is Gsh = 20 Âµ

mho and the beam coupling coefficient ÃŸ2 = 0.92. Find the maximum voltage

gain.

3. Explain the use of a PIN device as a single switch and a double switch circuits.

4. Give the comparison between IMPATT, TRAPATT and BARITT diodes

considering their performance characteristics and applications.

5. Establish the relation between Z and S matrices of a two-port network.

6. Derive the scattering matrix of a H plane T.

7. Explain the concept of choke flange and how it is realized in microwave joint.

8. Explain the use of magic T in a balanced microwave mixer.

9. Explain the principle of the power meter double bridge circuit for microwave

power measurement.

10. A crystal detector generates a signal of 10 m V for an incident microwave power

of -25 dBm. What is the detector sensitive in mV/ mW ? Why is the microwave

signal amplitude modulated by a 1 KHz square wave before it is applied to a

crystal detector?

** PART - B (5 x 16 = 80 Marks) **

11. Derive the expressions for power output and efficiency of a reflex klystron

oscillator.

A reflex klystron operators at frequency 5 KHz with repeller space L =- 0.5 cm,

dc beam voltage V0 =300V, beam current I0 = 20 mA, repeller voltage - VR volt

for 1¾ mode. Calculate (i) maximum power output (ii) repeller voltage, (iii) the

bandwidth for the range of .VR = 1V, (iv) efficiency.

12.a) Explain with the help of neat diagram the construction and amplification process

in TWTA. Draw the rf power output and gain characteristic curves.

A helix TWT operates at 5 GHz under a beam impedance 8KV and beam current

400 mA. The helix impedance is 20 ohm and the interaction length is 25 cm. Find

the output power gain in dB.

** (OR ) **

12.b) Draw the schematic diagram of a cylindrical magnetron tube oscillator. If a & b

are outer & inner radii of the cathode and anode cylinders respectively, derive the

expression for the cut off magnetic field Bc. Give three important application of

magnetron with power and frequency specifications.

A cylindrical magnetron with 16 resonators is operated at 1 GHz with a = 3 cm,

b= 5 cm, V0= 20 KV, B0 = 0.1 Tesla. Find the Hartree voltage.

13.a) Draw the electrical equivalent circuit of a Gunn diode explaining the parameters.

Explain with diagram how a Gunn diode oscillator can be designed using

rectangular waveguide in X-band. Give typical power output and frequency

tuning range achievable. What is the difficulty in obtaining very high power?

The drift velocity of electrons is 3 x107 cm/s, through the active region of length

20 x 10-4 cm. Calculate the natural frequency of the diode and the critical voltage

for GaAs. Critical field for GaAS is 3.2 KV / cm.

** (OR) **

13.b) Explain how Varactor diode is used as frequency multiplier, State and explain

Man1ey Rowe relations with the help of the circuit diagram.

A sinusoidal input signal at frequency fs and pump signal at frequency fp are

applied across a varactor diode. If the output circuit is a bandpass filter with

resistive series load at frequency fs + fp, calculate the power gain.

14.a) What are the properties of S parameters of a perfectly matched, lossless,

reciprocal microwave network?

A matched magic-T is terminated at collinear ports 1&2 and difference port 4 by

impedances of reflection coefficients G1 = 0.5, G2 = 0.6 & G4 = 0.8 respectively. If

I-watt power is fed at a sum port 3, calculate the power reflected at port 3 &

power transmitted to other three ports.

**OR **

14.b) Explain with a schematic diagram. The working principle of a four part circulator

and how it is achieved. A three port circulator has an insertion loss of dB,

isolation 30 dB and VSWR = 1.5. Find the S-matrix.

15.a) Why VSWR> 20 is not measured directly by VSWR meter? Explain the theory

of double minimum method of experimentally determining high VSWR of a load

by using w/g-slotted line. What are the possible errors in the slotted line method?

** OR **

15.b) Define antenna test ranges. Explain with a schematic diagram the antenna gain

measurement with three-antenna method. How do you predict about the

polarization of the signal from the measured output voltage pattern?