# Electronics and Communication Engineering - Microwave Communication

### Exercise :: Microwave Communication - Section 3

21.

Roughly the time required for microwave cooking as compared to conventional cooking is

Explanation:

Time required for microwave cooking is much less than the time required for conventional cooking.

22.

If the minimum range of a radar is to be doubled, the peak power has to be increased by a factor of

 A. 2 B. 4 C. 8 D. 16

Explanation:

Range = (power)025.

23.

Which mode has the lowest cut off frequency in circular wave guides?

 A. TE01 B. TE11 C. TE20 D. TE21

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.

24.

Assertion (A): Klystron amplifiers use one or more intermediate cavities in addition to buncher and catcher cavity.

Reason (R): When one or more intermediate cavities are used the bandwidth can be increased.

 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:

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.

25.

A wave Em cos (bx - ωt) is a backward wave.

 A. True B. False