Discussion
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- Question
A line has a phase constant of 29.8 rad/m. At 1000 MHz the wavelength is
Options- A. 29.8 m
- B. 2.98 m
- C. 2.1 m
- D. 0.21 m
- Correct Answer
- 0.21 m
Explanation
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Microwave Communication problems
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- 1. The velocity factor of a transmission line depends on
Options- A. temperature
- B. skin effect
- C. relative permittivity of dielectric
- D. none of the above Discuss
Correct Answer: relative permittivity of dielectric
Explanation:
?r of a dielectric changes with humidity.- 2. Assertion (A): A backward wave oscillator has an internal positive feedback.
Reason (R): A positive feedback is necessary for sustained oscillations.
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 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.
- 3. A resistive microwave load with ZL = 150 ? is connected to 50 ? coaxial line. SWR is
Options- A. more than 3
- B. less than 3
- C. equal to 3
- D. either (a) or (c) Discuss
Correct Answer: equal to 3
Explanation:
- 4. Reflex klystron oscillator is essentially a low power device
Options- A. True
- B. False Discuss
Correct Answer: True
Explanation:
It uses a single cavity resonator for generating microwave oscillations.Its parts are electron gun, resonator, repeller and output coupling.
It operates on the principle of positive feed back.
The repeller electrode is at negative potential and sends the partially bunched electron beam back to resonator cavity.
This positive feedback supports oscillations. Its feature are:
1. Frequency range - 2 to 100 GHz
2. Power output - 10 MW to about 2 W
3. Efficiency - 10 - 20 %
Its applications include radar receivers, local oscillator in microwave devices, oscillator for microwave measurements in laboratories etc.
- 5. A line is excited by a 100 V dc source. If reflection coefficients at both ends are 1 each then
Options- A. there will be no oscillations on line
- B. there will be only 1 or 2 oscillations on line
- C. there will be a finite number of oscillations on line
- D. the oscillations will continue indefinitely Discuss
Correct Answer: the oscillations will continue indefinitely
Explanation:
Because rv = 1 at each end, the line voltage will not reach a steady value and oscillations will continue indefinitely.- 6. The diagram to show distance time history of electrons in klystron amplifier is called
Options- A. apple gate diagram
- B. asynchronous diagram
- C. bunching diagram
- D. velocity modulation diagram Discuss
Correct Answer: apple gate diagram
Explanation:
Applegate diagram is distance time plot.- 7. In a circular waveguide the dominant mode is
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.
- 8. In a travelling wave tube distributed interaction between an electron beam and a travelling wave takes place
Options- A. True
- B. False Discuss
Correct Answer: True
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.
- 9. In a klystron amplifier dc electron velocity is v0 and dc electron charge density is r0 the dc beam current is
Options- A. r0v0
- B. r0/v0
- C. v0/r0
- D. (v0/r0)2 Discuss
Correct Answer: r0v0
Explanation:
= coulomb |sec|m2 = current density.
- 10. Assertion (A): A coaxial line is a non-radiating line.
Reason (R): In a coaxial line the electric and magnetic fields are confined to the region between the concentric conductors.
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 the fields are confined, there is no radiation.
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