Microwave Communication problems
- 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:
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. 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 Discuss
Correct Answer: power transmission is most efficient when there are no standing waves on line
Explanation:
- 3. 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:
and VSWR 
. Therefore ZL = Z0(VSWR).
- 4. 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)
- 5. 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:
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.
- 6. 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 ?
- 7. 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:
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.
- 8. Assertion (A): Impatt diode is an avalanche diode.
Reason (R): Avalanche breakdown phenomenon occurs when a p-n junction is reverse biased.
Options- 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 Discuss
Correct Answer: Both A and R are correct but R is not correct explanation of A
Explanation:
It exhibits negative resistance and operates on the principle of avalanche breakdown.
Impatt diode circuits are classified as broadly tunable circuit, low Q circuit and high Q circuit.
The impedance of Impatt diode is a few ohms. The word Impatt stands for Impact Avalanche Transit Time diode.
The features of Impatt diode oscillator are : frequency 1 to 300 GHz, Power output (0.5 W to 5 W for single diode circuit and upto 40 W for combination of several diodes), efficiency about 20%.
Its applications include police radar systems, low power microwave transmitter etc.
- 9. A TE10 rectangular waveguide is to be designed for operation over 25-35 GHz and the band centre is 1.5 times the cutoff frequency. The dimension of broadside is
Options- A. 15 mm
- B. 10 mm
- C. 9 mm
- D. 7.5 mm Discuss
Correct Answer: 7.5 mm
- 10. In the given figure a short circuited transmission line resonator If n = 1, 2, 3...
Options- A. For series reasonance ?l = np and for parallel resonance
- B. For series resonance
and for parallel resonance ?l = np - C. ?l = np for both series and parallel resonance
- D.
for both series and parallel resonance
Discuss
Correct Answer: For series reasonance ?l = np and for parallel resonance
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