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- Question
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)
- Correct Answer
- equal to 3
Explanation
Microwave Communication problems
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- 1. 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.
- 2. 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.- 3. Assertion (A): The phenomenon of differential mobility is called transferred electron effect.
Reason (R): GaAs exhibits transferred electron effect.
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:
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.
- 4. VSB modulation is preferred in TV because
Options- A. it reduces the bandwidth requirement to half
- B. it avoids phase distortion at low frequencies
- C. it results in better reception
- D. none of the above Discuss
Correct Answer: it reduces the bandwidth requirement to half
Explanation:
VSB (vestigial side band) transmission transmits one side band fully and the other side band partially thus, reducing the bandwidth requirement.- 5. The range of a cordless telephone is about
Options- A. 1000 m
- B. 500 m
- C. 100 m
- D. 10 m Discuss
Correct Answer: 100 m
Explanation:
The range depends on quality of instrument and is about 100 m.- 6. 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.
- 7. 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.- 8. 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 Discuss
Correct Answer: 0.21 m
Explanation:
.
- 9. 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.- 10. 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.
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