Electronics and Communication Engineering - Microwave Communication

16. 

Which of the following lines is non-radiating?

A. Open two wire
B. Coaxial
C. Both
D. None of the above

Answer: Option B

Explanation:

Since fields are confirmed within a coaxial cable it is non-radiating.


17. 

Assertion (A): The impedance of a matched load is equal to characteristic impedance of line.

Reason (R): A matched termination absorbs all the power incident on it.

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 A

Explanation:

Matched load means no reflections.


18. 

Skin effect is more pronounced at high frequencies.

A. True
B. False

Answer: Option A

Explanation:

Skin effect increases as frequency increases.


19. 

Assertion (A): The phenomenon of differential mobility is called transferred electron effect.

Reason (R): GaAs exhibits transferred electron effect.

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 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.

Applications of Gunn diode oscillator include continuous wave radar, pulsed radar and microwave receivers.


20. 

Which of the following devices uses a helix?

A. Klystron amplifier
B. Klystron oscillator
C. TWT
D. Both (a) and (b)

Answer: Option C

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.