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- Question
A coaxial line has L = 500 nH/m and C = 50 pF/m. The characteristic impedance is
Options- A. 500 ?
- B. 250 ?
- C. 100 ?
- D. 50 ?
- Correct Answer
- 100 ?
Explanation
.
Microwave Communication problems
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- 1. A transmission line has Z0 = 300 ? and ZL = (300 - j300) ohm. The transmission coefficient is
Options- A. 1.265 ? - 18.43°
- B. 1.01 ? - 10°
- C. 1.14 ? 66.68°
- D. 1.09 ? 66.68° Discuss
Correct Answer: 1.265 ? - 18.43°
Explanation:
Transmission coefficient
= 1.265 ?- 18.43°.
- 2. The directive gain of a transmitting antenna is
Options- A. proportional to wavelength
- B. inversely proportional to wavelength
- C. proportional to square of wavelength
- D. inversely proportional to square of wavelength Discuss
Correct Answer: inversely proportional to square of wavelength
Explanation:
- 3. In a TWT the axial component of electric field advances along the tube
Options- 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 Discuss
Correct Answer: at a velocity that is a small fraction of speed of light
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.
- 4. The cut off wavelength in circular waveguide
Options- A. guide diameter
- B. square of guide diameter
- C. cube of guide diameter
- D. square root of guide diameter Discuss
Correct Answer: guide diameter
Explanation:
Some applications require dual polarization capability. Circular waveguide has this capability.These analysis uses cylindrical coordinates.
In circular waveguide TE11 mode has the lowest cut off frequency and is the dominant mode.
If D is diameter of waveguide
?c = 1.706 D for TE11 mode
?c = 1.029 D for TE21 mode
?c = 0.82 D for TE01 mode
?c = 1.306 D for TM01 mode.
- 5. The directivity in a receiving antenna
- increases the intercept area in forward direction
- reduces the noise picked up from other sources
- provides a means of discriminating against undesired signals originating in directions other than in which the desired transmitter lies.
Options- A. 1 only
- B. 1 and 2 only
- C. 1, 2 and 3
- D. 2 and 3 only Discuss
Correct Answer: 1, 2 and 3
Explanation:
All the three are correct.- 6. Assertion (A): A half wavelength line can be used as a 1 : 1 transformer.
Reason (R): The input impedance of a half wavelength line is equal to load impedance.
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:
Since a half wavelength line has an input impedance equal to load impedance, the impedance transformation ratio is 1:1.- 7. Consider the following statements
- Dissipative attenuator has a fixed value of attenuation.
- Reflective attenuator has a fixed value of attenuation.
- Both dissipative and reflective attenuators are available only with fixed attenuation.
- Both dissipative and reflective attenuators are available with either fixed or variable attenuation.
Options- A. 1 only
- B. 1 and 2
- C. 1, 2 and 3
- D. 4 only Discuss
Correct Answer: 4 only
Explanation:
Both dissipative and reflective attenuators are available with either fixed or variable attenuation.- 8. Consider the following statements
- Bunching of electrons occurs in two cavity klystron amplifier.
- Bunching of electrons occurs in multi cavity klystron amplifier.
- Bunching of electrons occurs in reflex cavity klystron amplifier.
Options- A. 1, 2, and only
- B. 1, 2 and 3 only
- C. 1 and 3 only
- D. 2 and 3 only Discuss
Correct Answer: 1, 2 and 3 only
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.
- 9. A coaxial RF cable has a characteristic impedance of 50 ? and C equal to 40 pF/m. The inductance is
Options- A. 1 ? H/m
- B. 10 ? H/m
- C. 0.1 ? H/m
- D. 0.01 ? H/m Discuss
Correct Answer: 0.1 ? H/m
Explanation:
.
- 10. The noise figure of multicavity klystron amplifier is very low
Options- A. True
- B. False Discuss
Correct Answer: False
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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