Electronics and Communication Engineering - Microwave Communication

1. 

The directivity in a receiving antenna

  1. increases the intercept area in forward direction
  2. reduces the noise picked up from other sources
  3. provides a means of discriminating against undesired signals originating in directions other than in which the desired transmitter lies.
Which of the above statements are correct?

A. 1 only
B. 1 and 2 only
C. 1, 2 and 3
D. 2 and 3 only

Answer: Option C

Explanation:

All the three are correct.


2. 

Assertion (A): PIN diode can be used as attenuator and limiter.

Reason (R): PIN diode has a thin intrinsic layer.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R are correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

Answer: Option B

Explanation:

A PIN diode has an intrinsic (i) layer between p and n layers. When reverse bias is applied depletion layers are formed at p-i and i-n junctions.

The effective/width of depletion layer increases by the width of i layer. It can be used as a voltage controlled attenuator.

At high frequencies the rectification effect ceases and impedance of diode is effectively that of i layer.

This impedance varies with the applied bias. It is used in high frequency switching circuits, limiters, modulators etc.


3. 

The bunching action which occurs in multicavity klystron amplifier can be represented by Applegate diagram

A. True
B. False

Answer: Option A

Explanation:

A Klystron is a vacuum tube used for generation/amplification of microwaves.

An electron beam is produced by oxide coated indirectly heated cathode and is focussed and accelerated by focussing electrode.

This beam is transmitted through a glass tube. The input cavity where the beam enters the glass tube is called buncher.

As electrons move ahead they see an accelerating field for half cycle and retarding field for the other half cycle.

Therefore, some electrons are accelerated and some are retarded. This process is called velocity modulation.

The velocity modulation causes bunching of electrons. This bunching effect converts velocity modulation into density modulation of beam.

The input is fed at buncher cavity and output is taken at catcher cavity.

In a two cavity klystron only buncher and catcher cavity are used. In multi cavity klystron one or more intermediate cavities are also used.

The features of a multicavity klystron are :

1. Frequency range - 0.25 GHz to 100 GHz

2. Power output - 10 kW to several hundred kW

3. Power gain - 60 dB (nominal value)

4. Efficiency - about 40%.

A multicavity klystron is used in UHF TV transmitters, Radar transmitter and satellite communication.


4. 

The cut off wavelength in circular waveguide

A. guide diameter
B. square of guide diameter
C. cube of guide diameter
D. square root of guide diameter

Answer: Option A

Explanation:

Some applications require dual polarization capability. Circular waveguide has this capability.

These analysis uses cylindrical coordinates.

In circular waveguide TE11 mode has the lowest cut off frequency and is the dominant mode.

If D is diameter of waveguide

λc = 1.706 D for TE11 mode

λc = 1.029 D for TE21 mode

λc = 0.82 D for TE01 mode

λc = 1.306 D for TM01 mode.


5. 

In Reflex Klystron oscillator the focussing electrode is at a high potential

A. True
B. False

Answer: Option B

Explanation:

It uses a single cavity resonator for generating microwave oscillations.

Its parts are electron gun, resonator, repeller and output coupling.

It operates on the principle of positive feed back.

The repeller electrode is at negative potential and sends the partially bunched electron beam back to resonator cavity.

This positive feedback supports oscillations. Its feature are:

1. Frequency range - 2 to 100 GHz

2. Power output - 10 MW to about 2 W

3. Efficiency - 10 - 20 %

Its applications include radar receivers, local oscillator in microwave devices, oscillator for microwave measurements in laboratories etc.


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