Exercise: Microwave Communication

Questions for: Microwave Communication

Microwave links are typically 50 km apart
A:
because of atmospheric attenuation
B:
because of output tube power limitation
C:
because of earth's curvature
D:
to ensure that applied dc voltage is not excessive
Answer: C

Earth's curvature limits the distance between microwave links.

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Which one of the following is not a negative resistance device?
A:
Gunn diode
B:
Tunnel diode
C:
Impatt diode
D:
Varactor diode
Answer: D

Gunn diode, Tunnel diode and Impatt diodes are negative resistance devices.

The limitation of solid state devices at high frequencies include those associated with transit time and junction capacitances.

The devices used are : Transferred electron oscillators (Gunn diode), Avalanche diode oscillators (Impatt diode, Trapatt diode, Masters, Lasers, Tunnel diode, Varactor etc).

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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
Answer: B

PIN diode is not used in oscillators.

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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
Answer: B

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

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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
Answer: B

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.

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