Code: A-20                                               Subject: MICROWAVE THEORY & TECHNIQUES

Time: 3 Hours                                                                 Flowchart: Alternate Process: December 2005                                    Max. Marks: 100

 

NOTE: There are 9 Questions in all.

·      Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

·      Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

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

 

 

Q.1       Choose the correct or best alternative in the following:                                         (2x10)

       

a.       If a 50-ohm loss-less transmission line is terminated in a pure reactance of 25 ohms, then the VSWR produced on the line is

 

                   (A)  zero.                                            (B)  1/2.

(C)   2.                                                 (D)  infinity.

       

b.      Transverse electric wave travelling in Z-direction satisfies

 

(A)                               (B) 

(C)                             (D) 

            

             c.   Reflex klystron is a

                  

(A)     Low power oscillator                   (B)  High power oscillator

(C)  Amplifier used in Receiver            (D)  A mixer device

 

             d.   Which one is not ATTD, (Avalanche Transit Time Device)

 

(A)    BARITT                                     (B)  IMPATT

(C)  TRAPATT                                  (D)  PIN   

 

             e.   The expression for fo in a rectangular cavity resonator is given by

                  

(A)              

(B)    

(C)           

(D) 

 

                   where the parameters/symbols have usual meaning.

 

 

             f.    A T.V. transmitter antenna has a height of 144 meters  (AMSL) and the receiving antenna has a height of 25 m (AMSL).  What is the maximum distance along the earth through which the T.V. signal could be received by space propagation?

 

(A)     68 km                                          (B)  68 mt

(C)  680 km                                        (D)  680 mt

 

             g.   Bandwidth for a power-up converter is given by (have their usual meanings)

 

(A)                             (B) 

(C)          (D) 

 

             h.   Two identical directional couplers are used in waveguide to sample the incident and reflected powers.  The output of the two couplers is found to be 2.5 mw and 0.15 mw.  The value of VSWR in the waveguide is given by

 

(A)    1.64                                             (B) 1.63

(C) 10.64                                            (D) 10.63

 

             i.    When the phase velocity of an EM wave depend on frequency in any medium, the phenomenon is called

 

(A)   Scattering.                                    (B)  Polarisation.

(C) Absorption.                                   (D) Dispersion.

 

             j.    On the smith chart, a loss less line is represented by

 

(A)     the diameter along real axis.         

(B)     the diameter along imaginary axis.

(C)  the diameter equally inclined to the real and imaginary axis.

(D)  a circle with its centre at the origin.

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   A certain transmission line has a characteristic impedance of 75 + j0.01  and is terminated in a load impedance of 70 + j50 . Compute reflection coefficient and transmission coefficient.       (7)         

       

             b.   List the drawbacks of single-stub matching.                                                        (4)

                  

             c.   A Lossless transmission line with characteristic impedance 500 ohm is excited by a signal of voltage  volts at 1.2 MHz.  If the line is terminated by  at a distance 1 km, calculate input impedance of open circuit line and input impedance of short circuit line.                                             (5)

 

  Q.3     a.   Bring out the similarities and dis-similarities between waveguide and       2-wire transmission line.                                                               (8)

 

             b.   Derive the wave equation for a TE wave.                                                            (8)          

 

  Q.4     a.   Derive an expression for resonant frequency fo in a rectangular cavity resonator.                  (6)

 

             b.   A circular waveguide has a radius of 3 cms and is used as a resonator for  mode at 10 GHz by placing two perfectly conducting plates at its two ends.  Determine the minimum distance between the two end plates for resonance.                                                                                    (6)

 

             c.   Explain Loop coupling and aperture coupling.                                                     (4)

 

  Q.5     a.   Prove that it is impossible to construct a perfectly matched, lossless reciprocal three-port junction.                                                             (6)

       

             b.   In a H-plane T-junction, compute power delivered to the loads 40 ohm and 60 ohm connected to arm 1 and 2 when 10 mw power is delivered to the matched port.                                                      (6)

 

             c.   Explain how, magic-tee can be used as a balanced microwave mixer.                  (4)

 

  Q.6     a.   Write short notes on waveguide bend corners and twists.                                    (8)

 

             b.   A symmetric directional coupler with infinite directivity and a forward attenuation of 20 dB is used to monitor the power delivered to a load .  If Bolometer1 introduces a VSWR of 2.0 on arm 4.  Bolometer 2 is matched to arm 3.  If Bolometer1 reads 8 mw and Bolometer2 reads 2 mw, find the amount of power dissipated in .                                 (8)

                                                                             

  Q.7     a.   Discuss the high frequency limitation of conventional tubes.                                  (8)

 

             b.   By carrying out detailed mathematical analysis for reflex klystron, obtain the expression for maximum power output.                                                 (8)

 

  Q.8     a.   Explain in detail the operation of PIN diode.                                                       (6)

 

             b.   A negative resistance parametric amplifier has a signal frequency of 2 GHz, pump frequency of 12 GHz, output resistance of signal generator of   1 K Determine (i)  Power gain in dB (ii) Power gain if it work as USB converter.                                                                                                  (6)

 

             c.   Write a short note on varactor diode.                                                                 (4)          

 

 

 

 

 

 

  Q.9     a.   A typical SI BARITT diode has the following specifications.                                   

                   Relative dielectric constant = 12.5

                   Donor concentration =

                   Length = 8

                   Calculate (i) Critical voltage (ii) breakdown voltage (iii) breakdown electric field.                (8)       

 

             b.   Mention the performance characteristics, applications and disadvantages of MASERS.                   (8)