# Electronics and Communication Engineering - Microwave Communication

### Exercise :: Microwave Communication - Section 3

41.

The expression HZ = C cos(Bx) cos(Ay)yz is for

 A. TE wave B. TM waves C. both TE and TM waves D. some TE and some TM waves

Explanation:

In TM mode Hz = 0.

42.

Assertion (A): Impedance matching can be done by using stubs.

Reason (R): A double stub is used for impedance matching when frequency of signal varies.

 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:

Hollow metallic enclosures exhibit resonance behaviour when excited by electromagnetic field.

These enclosures are called cavity resonators.

43.

1. It uses thermionic emission.
2. It uses an attenuater.
3. It is inherently a resonant device.
Which of the above are correct?

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

Explanation:

In a klystron the resonant structure limits the bandwidth.

A TWT is a broadband device. Its main components are electron gun (to produce the electron beam) and a structure supporting the slow electromagnetic wave.

The velocity of wave propagation along the helix structure is less than velocity of light.

The beam and wave travel along the structure at the same speed.

Thus interaction occurs between beam and wave and the beam delivers energy to the RF wave.

Therefore the signal gets strengthened and amplified output is delivered at the other end of tube.

The main features of TWT are :

1. Frequency range - 0.5 GHz to 90 GHz

2. Power output - 5 mW at low frequencies(less than 20 GHz) 250 kW (continuous wave) at 3 GHz 10 MW (pulsed) at 3 GHz

3. Efficiency - about 5 to 20%

4. Noise - about 5 dB for low power TWT 25 dB for high power TWT

TWT is used as RF amplifier in broadband microwave receivers, repeater amplifier in broad band communication systems, communication satellites etc.

44.

If a line having Z0 = 300 ∠ 0 W is short circuited at far end, VSWR is

 A. 0 B. 1 C. ∞ D. 2

Explanation:

If line is s.c |rv| = 1 and VSWR = ∞.

45.

A klystron amplifier generally uses Pierce gun

 A. True B. False

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.