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- Question
A cavity resonator is
Options- A. a hollow metallic enclosure
- B. a hollow enclosure having magnetic material as its walls
- C. a hollow enclosure having dielectric material as its walls
- D. either (b) or (c)
- Correct Answer
- a hollow metallic enclosure
ExplanationHollow metallic enclosures exhibit resonance behaviour when excited by electromagnetic field.These enclosures are called cavity resonators.
Microwave Communication problems
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- 1. Power gain of a multicavity klystron amplifier is
Options- A. more than 30 dB
- B. causes retardation of all electrons
- C. causes acceleration of some electrons and retardation of others
- D. none of the above Discuss
Correct Answer: more than 30 dB
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. A line has an attenuation of 0.054 Np/m. The attenuation in decibels is
Options- A. 4.7 dB/m
- B. 0.47 dB/m
- C. 0.54 dB/m
- D. 5.4 dB/m Discuss
Correct Answer: 0.47 dB/m
Explanation:
0.054 x 8.68 = 0.47 dB/m.- 3. The semiconductor diode which can be used in switching circuit in microwave range is
Options- A. PIN diode
- B. Varactor diode
- C. Tunnel diode
- D. Gunn diode Discuss
Correct Answer: PIN diode
Explanation:
A PIN diode has an intrinsic (i) layer between p and n layers. When reverse bias is applied depletion layers are formed at p-i and i-n junctions.The effective/width of depletion layer increases by the width of i layer. It can be used as a voltage controlled attenuator.
At high frequencies the rectification effect ceases and impedance of diode is effectively that of i layer.
This impedance varies with the applied bias. It is used in high frequency switching circuits, limiters, modulators etc.
- 4. Assertion (A): Klystron amplifiers use one or more intermediate cavities in addition to buncher and catcher cavity.
Reason (R): When one or more intermediate cavities are used the bandwidth can be increased.
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 and R is correct explanation of A
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.
- 5. If the minimum range of a radar is to be doubled, the peak power has to be increased by a factor of
Options- A. 2
- B. 4
- C. 8
- D. 16 Discuss
Correct Answer: 16
Explanation:
Range = (power)025.- 6. To couple a coaxial line to a parallel wire line it is best to use
Options- A. slotted line
- B. balun
- C. directional coupler
- D. ?/4 transformer Discuss
Correct Answer: balun
Explanation:
A balun gives 4 : 1 impedance transformation.- 7. In a microwave coaxial line, the maximum operating frequency is f max and breakdown strength of dielectric is Ed' Then maximum power handling capacity is proportional to
Options- A.
- B.
- C.
- D.
Discuss
Correct Answer:
Explanation:
Power handling capacity ? (voltage)2 and is inversely proportional to f2 max.- 8. If a line is open circuited Zin = Z0 tanh (gl).
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
The quantities required to be measured in microwave circuits are frequency, power and impedance.Microwave frequency measurement is done by slotted line, resonant cavities and transfer oscillator.
Microwave power measurement uses bolometers and micromave power meters.
A self balancing bridge working on the principle of power substitution is commomly used.
Microwave impedance measurement is done by measurement of reflection coefficient and VSWR.
A slotted line and probe is a basic tool for these measurements.' Network analyser enables rapid impedance measurement over a broad frequency range.
- 9. Assertion (A): PIN diode is used as a fast switch.
Reason (R): PIN diode has very high resistance when reverse biased and very low resistance when forward 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 and R is correct explanation of A
Explanation:
A PIN diode has an intrinsic (i) layer between p and n layers. When reverse bias is applied depletion layers are formed at p-i and i-n junctions.The effective/width of depletion layer increases by the width of i layer. It can be used as a voltage controlled attenuator.
At high frequencies the rectification effect ceases and impedance of diode is effectively that of i layer.
This impedance varies with the applied bias. It is used in high frequency switching circuits, limiters, modulators etc.
- 10. Consider the following applications
- Switch
- Attenuator
- Phase shifter
- Oscillator
Options- A. 1 2 3 and 4
- B. 1 2 and 3 only
- C. 1 and 2 only
- D. 2, 3 and 4 only Discuss
Correct Answer: 1 2 and 3 only
Explanation:
PIN diode is not used in oscillators.
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