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- Question
If f is the frequency of electromagnetic wave, fc is cutoff frequency, then in a rectangular waveguide
Options- A. attenuation is low when f > fc
- B. attenuation is low when f < fc
- C. attenuation is high when f < fc
- D. either (b) or (c)
- Correct Answer
- attenuation is low when f > fc
ExplanationWavelength should be less than cutoff wavelengths.Therefore frequency should be higher than cutoff frequency for minimum attenuation.
Microwave Communication problems
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- 1. In a backward wave oscillator the wave
Options- A. travelling along the line winds itself back and forth
- B. progresses only in forward direction
- C. progresses only in backward direction
- D. either (a) or (c) Discuss
Correct Answer: travelling along the line winds itself back and forth
Explanation:
It is somewhat similar to TWT and can deliver microwave power over a wide frequency band.It has an electron gun and a helix structure. However the interaction between electron beam and RF wave is different than in TWT.
The growing RF wave travels in opposite direction to the electron beam.
The frequency of wave can be changed by changing the voltage which controls the beam velocity.
Moreover the amplitude of oscillations can be decreased continuously to zero by changing the beam current.
It features are:
1. Frequency range - 1 GHz to 1000 GHz.
2. Power output - 10 mV to 150 mW (continuous wave) 250kW (pulsed).
It is used as signal source in transmitters and instruments.
- 2. For a circular wave guide
Options- A. cutoff frequency for TE10 and TE01 modes are same
- B. cutoff frequency for TE10 and TE01 modes are different
- C. cutoff frequency for TE10 mode = twice the cutoff frequency for TE20 mode
- D. cutoff frequency for TE10 mode = haf the cutoff frequency for TE20 mode Discuss
Correct Answer: cutoff frequency for TE10 and TE01 modes are same
Explanation:
When a circular waveguide is rotated by 90°, the configuration remain the same.- 3. Consider the following statements
- Impedance of Gunn diode is about tens of ohms.
- Impedance of Impatt diode is a few ohms.
- Impedance of Impatt diode are of the same order.
- Impedance of Impatt diode is more than that of Gunn diode.
Options- A. all
- B. 1 and 2 only
- C. 1, 2 and 3
- D. 1, 2 and 4 Discuss
Correct Answer: 1 and 2 only
Explanation:
A Gunn diode uses GaAs which has a negative differential mobility, i.e., a decrease in carrier velocity with increase in electric field.This effects is called transferred electron effect. The impedance of a Gunn diode is tens of ohms.
A Gunn diode oscillator has a resonant cavity, an arrangement to couple Gunn diode to cavity, biasing arrangement for Gunn diode and arrangement to couple RF power to load.
Applications of Gunn diode oscillator include continuous wave radar, pulsed radar and microwave receivers.
An Impatt diode has n+ - p - i - p + structure and is used with reverse bias.
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.
- 4. Assertion (A): Magnetron is generally used in n mode.
Reason (R): Frequency for p mode can be easily separated from adjacent modes.
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:
It is somewhat similar to TWT and can deliver microwave power over a wide frequency band.It has an electron gun and a helix structure. However the interaction between electron beam and RF wave is different than in TWT.
The growing RF wave travels in opposite direction to the electron beam.
The frequency of wave can be changed by changing the voltage which controls the beam velocity.
Moreover the amplitude of oscillations can be decreased continuously to zero by changing the beam current.
It features are:
1. Frequency range - 1 GHz to 1000 GHz.
2. Power output - 10 mV to 150 mW (continuous wave) 250kW (pulsed).
It is used as signal source in transmitters and instruments.
- 5. 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.- 6. In mode filter of the given figure which of the following modes is unaffected
Options- A. TE01
- B. TE10
- C. TE11
- D. all TM Discuss
Correct Answer: TE01
Explanation:
TE01 mode is unaffected because its horizontally oriented electric field is every where perpendicular to the conducting strips.- 7. A branched duplexer requires
Options- A. TR tubes
- B. ATR tube
- C. both TR and ATR tube
- D. none of the above Discuss
Correct Answer: both TR and ATR tube
Explanation:
A branched duplexer funds use in narrow bandwidth applications.TR tube stands for transmit-receive tube and ATR tube stands for antitransmit receive tube.
Both these tubes are used in branched duplexer.
- 8. When a line short circuited at far end, the minimum voltage occurs at
Options- A. far end
- B. source end
- C. midway between source and far end
- D. none of the above Discuss
Correct Answer: far end
Explanation:
When a line is short-circuited at far end, the voltage at far end is zero.- 9. The bunching action which occurs in multicavity klystron amplifier can be represented by Applegate diagram
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.
- 10. In a vacuum tube, the transit time of electron between cathode and anode is important at
Options- A. low frequencies
- B. high frequencies
- C. both (a) and (b)
- D. frequencies which are neither very low nor very high Discuss
Correct Answer: high frequencies
Explanation:
At high frequencies transit time is large as compared to the period of microwave signal.
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