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- Question
In a circular waveguide TE21 mode has lowest cutoff frequency
Options- A. True
- B. False
- Correct Answer
- False
ExplanationSome 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.
Microwave Communication problems
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- 1. Impedance inversion may be obtained with
Options- A. a short circuited stub
- B. a quarter wave line
- C. an open circuited stub
- D. a half wave line Discuss
Correct Answer: a quarter wave line
Explanation:
Impedance inversion occurs when load impedance is not matched with characteristic impedance of line.- 2. If VSWR is infinite, the transmission line is terminated in
Options- A. short circuit
- B. complex impedance
- C. open circuit
- D. either (a) or (c) Discuss
Correct Answer: either (a) or (c)
Explanation:
and 
. Since VSWR = ?, |rv| = 1. This can happen if ZL = 0 or ZL = ?, i.e., line is o.c or s.c.
- 3. Assertion (A): In the interaction region of magnetron an electron is subjected to three forces: force due to electric field, force due to magnetic field and centrifugal force.
Reason (R): The mechanism of generation of microwaves, in a magnetron, involves interaction of electromagnetic fields with electrons moving in static electric and magnetic fields oriented at right angles to each other.
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.
- 4. If antenna diameter is increased four times, the maximum range is increased by a factor of
Options- A. 2
- B. 2
- C. 4
- D. 0.2 Discuss
Correct Answer: 4
Explanation:
Range of radar is proportional to antenna diameter.- 5. Microwave links are typically 50 km apart
Options- A. because of atmospheric attenuation
- B. because of output tube power limitation
- C. because of earth's curvature
- D. to ensure that applied dc voltage is not excessive Discuss
Correct Answer: because of earth's curvature
Explanation:
Earth's curvature limits the distance between microwave links.- 6. Assertion (A): For high frequency lines inductive reactance is very high as compared to ac resistance.
Reason (R): Due to skin effect ac resistance of line is higher than dc resistance.
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:
XL = ?L. As frequency increases line resistance increases slightly but XL increases directly as per frequency.- 7. For matching over a range of frequencies in a transmission line it is best to use
Options- A. a balun
- B. a broad band directional coupler
- C. double stub
- D. a single stub of adjustable position Discuss
Correct Answer: double stub
Explanation:
Double stub is useful for matching over a range of frequencies.- 8. Read the following statements about TWT
- It uses thermionic emission.
- It uses an attenuater.
- It is inherently a resonant device.
- It has broad bandwidth.
Options- A. 1, 2, 3 and 4
- B. 1, 2 and 3
- C. 1, 3 and 4
- D. 1, 2 and 4 Discuss
Correct Answer: 1, 2 and 4
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. Assertion (A): Gunn diode is a transferred electron device.
Reason (R): A Gunn oscillator uses the phenomenon of 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.
- 10. A reflex klystron oscillator is a
Options- A. low power device
- B. high power device
- C. high efficiency device
- D. both (a) and (b) Discuss
Correct Answer: low power device
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.
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