Discussion
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- Question
The number of TV channels which can be accommodated in a spectrum of 300 MHz is about
Options- A. 5
- B. 10
- C. 50
- D. 200
- Correct Answer
- 50
ExplanationA TV channel has a bandwidth about 5 MHz. Hence 300 MHz spectrum can accommodate about 50 channels.
Microwave Communication problems
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- 1. Which mode has the lowest cut off frequency in circular wave guides?
Options- A. TE01
- B. TE11
- C. TE20
- D. TE21 Discuss
Correct Answer: TE11
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.
- 2. A wave Em cos (bx - ?t) is a backward wave.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
It is a forward wave because as t increases, x increases.- 3. If a short line is terminated in characteristic impedance it behaves as an infinite line.
Options- A. True
- B. False Discuss
Correct Answer: True
Explanation:
In a line terminated by Z0, there are no reflections.- 4. Which one of the following is not a negative resistance device?
Options- A. Gunn diode
- B. Tunnel diode
- C. Impatt diode
- D. Varactor diode Discuss
Correct Answer: Varactor diode
Explanation:
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).
- 5. 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.
- 6. In a multicavity klystron amplifier the signal to be amplified develops an ac voltage of signal frequency across the gap in buncher cavity
Options- A. True
- B. False Discuss
Correct Answer: True
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.
- 7. In the given figure the E and H lines in a coaxial cable
Options- A. The radial lines are E lines and circular lines are H lines
- B. Radial lines are H lines and circular lines are E lines
- C. The directions of E and H lines are wrong
- D. Radial lines may be E and H lines depending an direction of current Discuss
Correct Answer: The radial lines are E lines and circular lines are H lines
Explanation:
H lines are concentric circles around conductor and E lines are radial.- 8. Which device has internal positive feedback?
Options- A. Two cavity klystron amplifier
- B. Multi-cavity klystron amplifier
- C. Reflex klystron amplifier
- D. All of the above Discuss
Correct Answer: Reflex klystron amplifier
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. In the given figure the reflected current wave after first reflection is
Options- A. 0.18 A
- B. -0.18 A
- C. 0.3 A
- D. -0.3 A Discuss
Correct Answer: -0.18 A
Explanation:
- 10. Assertion (A): The velocity of electromagnetic waves on overhead lines and coaxial cables is the same.
Reason (R): Free space has an intrinsic impedance of 377 ohms.
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: A is wrong but R is correct
Explanation:
Velocity of em waves on caoxial cables is less than that on overhead lines.
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