Code: A-20                                               Subject: MICROWAVE THEORY & TECHNIQUES

Time: 3 Hours                                                                                                     Max. Marks: 100

NOTE: There are 11 Questions in all.

· Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1       Choose the correct or best alternative in the following:                                           (2x8)

a. A lossless transmission line of  is terminated by an unknown impedance. The termination is found to be at a maximum of the voltage standing wave and the VSWR is 5. The value of the terminating impedance is 

(A)  500 .                                        (B)  50 .

(C)  600 .                                        (D)  60 .

             b.   A transmission line less than long short circuited at the far end acts like a

(A) pure inductance.                           (B)  pure capacitance.

(C)  series resonant circuit.                   (D)  parallel resonant circuit.

             c.   When a particular mode is excited in a wave guide there appears an extra electric component in the direction of propagation. The resulting move is

(A) Longitudinal electric.                     (B)  Transverse electromagnetic.

(C) Transverse electric.                        (D)  Transverse magnetic.  

             d.   A cavity resonator simulates

(A) a series resonant circuit.                 (B) a parallel resonant circuit.

(C) an iris inserted in a waveguide.       (D) a free running osillator.

             e.   The following solid-state device can operate at high power levels of the order of 100 W

(A) TRAPATT                                   (B)  IMPATT

(C)  GUNN                                        (D)  PIN

             f.    A pyramidal horn has an opening of 58 mm in the E-plane and 78 mm in the H-plane and if operates at 10 GHz. The gain of the horn antenna is

(A) 47.7                                              (B)  37.7

(C) 57.7                                              (D)  27.7

             g.   A loop is used to couple a waveguide and is placed in a location which for the TE₁₀ mode occurs close to the end wall of the guide.

(A) Maximum electric field.

(B) Minimum magnetic field.

(C) Maximum magnetic field.

(D) Maximum electric and magnetic fields.

             h.   Suppose a microwave transmitter and a receiver towers have equal height with respect to Mean Sea Level (MSL), in order to communicate over a distance of 40 km, their MSL height should be

(A) 33.5 m                                         (B)  43.5 m

(C) 23.5 m                                          (D)  13.5 m

PART I

Answer any THREE Questions. Each question carries 14 marks.

Q.2     a.   Distinguish between two wire parallel transmission lines and waveguides.             (4)

             b.   Determine the attenuation loss, reflection loss, transmission loss, return loss and insertion loss in dBs given a lossless transmission line with a characteristic impedance of 200  fed by a generator of 1 volt and impedance of 100 . The line is 300 m long and terminated by a resistive load of 300 .             (10)

Q.3    a.   Derive the wave equation for a TM wave and obtain all the field components in a rectangular waveguide.                                                           (10)

             b.   A Ku band rectangular waveguide has dimensions of a =1.067 cm,               b =0.432 cm. Calculate cut off frequency for the TE₁₁ mode if the guide is filled with Teflon .                              (4)

Q.4     a.   What do you understand by the term quality factor of a cavity resonator? Obtain an expression for the Q-factor of a rectangular cavity resonator.                                                            (10)

             b.   Write a note on re-entrant cavities.                                                                     (4)

 Q.5      a.   Obtain the scattering matrix of a Magic Tee. Mention at least three of its applications.                     (9)

             b.   The collinear ports of a magic tee are terminated by impedances of reflection coefficient  and . The difference port is terminated by an impedance with reflection coefficient of 0.8. If 1 watt power is fed at the sum port, calculate the power reflected at the sum port and power division at other ports.                                                                                                                         (5)

  Q.6     a. What are ferrites? Why are they useful at microwave frequencies? Mention their properties.                        (6)

b.   Explain the action of an isolator using ferrites.                                                      (4)

     c.   Distinguish between;

(i) E-bend and H-bend

(ii) E-corner and H-corner                                                                          (4)

PART II

Answer any THREE Questions. Each question carries 14 marks.

Q.7     a.   Explain the methods of measuring an unknown impedance at microwave frequencies.                      (6)

b.   With reference to a microwave bench, explain briefly the following:                         

(i) Wavemeters.

(ii) Vector network analyser.                                                                      (8)

Q.8     a.   Distinguish between TWT amplifier and Klystron amplifier.                                 (4)

b.   Explain how amplification is achieved in a TWT amplifier.                                   (6)

c.   What is frequency pushing and pulling in a magnetron? List the performance characteristics of a magnetron.                                                            (4)

Q.9     a.   Explain the doping profiles of a varactor diode as well as its electrical equivalent circuit.                 (6)

b. With reference to parametric amplifier, explain

(i) Parametric up converter.

(ii) Negative resistance parametric amplifier.

(iii) Degenerate parametric amplifier.

(iv) Broadband parametric amplifier.                                                            (8)

Q.10           a.                                                        Distinguish between

(i) Delayed domain mode and Quenched domain mode in a GUNN device.

(ii) IMPATT and TRAPATT.

(iii) MESFETS and Bipolar microwave transistors.                                       (9)

             b.   Explain the Gunn effect. A Gunn diode-has a drift length of 5 . What minimum voltage is required to initiate Gunn effect?                              (5)

Q.11                                                                      Write explanatory notes on (Any FOUR):-

(i) Microwave antennas.

(ii) Fading.

(iii) Microwave applications.

(iv) Microwave repeaters.

(v) Strip lines.

(vi) Phase shifters.                                                                       (3.5 x 4 = 14)