# Electronics and Communication Engineering - Microwave Communication

### Exercise :: Microwave Communication - Section 3

31.

If a short line is terminated in characteristic impedance it behaves as an infinite line.

 A. True B. False

Explanation:

In a line terminated by Z0, there are no reflections.

32.

Ga As exhibits negative differential mobility

 A. True B. False

Explanation:

A Gunn diode uses GaAs which has a negative differential mobility, i.e., a decrease in carrier velocity with increase in electric field.

This effects is called transferred electron effect. The impedance of a Gunn diode is tens of ohms.

A Gunn diode oscillator has a resonant cavity, an arrangement to couple Gunn diode to cavity, biasing arrangement for Gunn diode and arrangement to couple RF power to load.

33.

Consider the following statements

1. Bunching of electrons occurs in two cavity klystron amplifier.
2. Bunching of electrons occurs in multi cavity klystron amplifier.
3. Bunching of electrons occurs in reflex cavity klystron amplifier.
Which of the above statements are correct?

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

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)

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

34.

Assertion (A): For high frequency lines inductive reactance is very high as compared to ac resistance.

Reason (R): Due to skin effect ac resistance of line is higher than dc resistance.

 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

Explanation:

XL = ωL. As frequency increases line resistance increases slightly but XL increases directly as per frequency.

35.

Consider the following applications

1. Switch
2. Attenuator
3. Phase shifter
4. Oscillator
In which of the above can a PIN diode be used?

 A. 1 2 3 and 4 B. 1 2 and 3 only C. 1 and 2 only D. 2, 3 and 4 only