Discussion
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- Question
In a line with finite attenuation
Options- A. power transmission is least efficient when there are no standing waves on the line
- B. power transmission is most efficient when there are no standing waves on line
- C. Load power depends on phase constant
- D. standing waves will always exist on the line
- Correct Answer
- power transmission is most efficient when there are no standing waves on line
ExplanationIf standing waves are not there, whole of the power is absorbed by load.
Microwave Communication problems
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- 1. In a klystron amplifier the force on all electrons
Options- A. causes acceleration of all electrons
- B. causes retardation of all electrons
- C. causes acceleration of some electrons and retardation of others
- D. none of the above Discuss
Correct Answer: causes acceleration of some electrons and retardation of others
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.
- 2. The number of PM radio channels which can be accommodated in a spectrum of 300 MHz is about
Options- A. 10
- B. 100
- C. 500
- D. 1500 Discuss
Correct Answer: 1500
- 3. The domain mode in rectangular waveguide is
Options- A. TE01
- B. TE02
- C. TE10
- D. TM11 Discuss
Correct Answer: TE10
- 4. In an infinite line the input impedance at every point is equal to characteristic impedance.
Options- A. True
- B. False Discuss
Correct Answer: True
- 5. The inductance of a twin feeder used to connect antenna to TV receiver is about
Options- A. 0.2 ? H/m
- B. 0.6 ? H/m
- C. 1.6 ? H/m
- D. 10 ? H/m Discuss
Correct Answer: 1.6 ? H/m
- 6. If load impedance ZL >> Z0, then
Options- A. Z0 = ZL (VSWR)
- B. ZL = Z0 (VSWR)
- C. ZL = Z0 (VSWR - 1)
- D. ZL = Z0 VSWR Discuss
Correct Answer: ZL = Z0 (VSWR)
Explanation:
If ZL >> Z0, |rv|
and VSWR 
. Therefore ZL = Z0(VSWR).
- 7. Varactor diodes use
Options- A. silicon
- B. GaAs
- C. either (a) or (b)
- D. neither (a) nor (b) Discuss
Correct Answer: either (a) or (b)
- 8. The electric field in a TWT due to applied signal
Options- A. is directed along the helix axis
- B. is directed radially from helix axis
- C. is inclined to the helix axis by about 60°
- D. is inclined to the helix axis by about 45° Discuss
Correct Answer: is directed along the helix axis
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.
- 9. The radiation resistance of a circular loop of one turn is 0.01 ohm. For 5 turn loop the radiation resistance is
Options- A. 0.002 ?
- B. 0.01 ?
- C. 0.05 ?
- D. 0.25 ? Discuss
Correct Answer: 0.25 ?
- 10. In a klystron amplifier the bunching effect
Options- A. converts velocity modulation into current modulation of beam
- B. converts current modulation into velocity modulation of beam
- C. both (c) and (b)
- D. neither (a) nor (b) Discuss
Correct Answer: converts velocity modulation into current modulation of beam
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.
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