Electronics and Communication Engineering - Microwave Communication

21. 

Which one of the following is transferred locking electron device C antenna?

A. BARITT diode
B. IMPATT diode
C. Gunn diode
D. Step recovery diode

Answer: Option C

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.


22. 

A magnetron requires an external magnetic field with flux lines parallel to axis of cathode

A. True
B. False

Answer: Option A

Explanation:

It is somewhat similar to TWT and can deliver microwave power over a wide frequency band.

It has an electron gun and a helix structure. However the interaction between electron beam and RF wave is different than in TWT.

The growing RF wave travels in opposite direction to the electron beam.

The frequency of wave can be changed by changing the voltage which controls the beam velocity.

Moreover the amplitude of oscillations can be decreased continuously to zero by changing the beam current.

It features are:

1. Frequency range - 1 GHz to 1000 GHz.

2. Power output - 10 mV to 150 mW (continuous wave) 250kW (pulsed).

It is used as signal source in transmitters and instruments.


23. 

When a line short circuited at far end, the minimum voltage occurs at

A. far end
B. source end
C. midway between source and far end
D. none of the above

Answer: Option A

Explanation:

When a line is short-circuited at far end, the voltage at far end is zero.


24. 

In a TWT the axial component of electric field advances along the tube

A. at a velocity that is almost equal to speed of light
B. at a velocity that is a small fraction of speed of light
C. at a velocity that is about 50% of speed of light
D. at a velocity that may be even more than speed of light

Answer: Option B

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.


25. 

If other parameter are constant, transmission loss at 5 GHz as compared to that at 0.5 GHz is

A. 20 dB more
B. 20 dB less
C. 700 dB more
D. 100 dB less

Answer: Option A

Explanation:

As frequency increases, transmission loss increases slightly.