Microwave Communication problems
- 1. Assertion (A): Artificial transmission lines are frequently used in laboratories.
Reason (R): An artificial transmission line can be used to represent an actual line and can also be used as a delay circuit, as attenuator, as filter network etc.
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:
- 2. In a TWT the amplitude of resultant wave travelling down the helix
Options- A. increases exponentially
- B. increases linearly
- C. decreases exponentially
- D. is almost constant Discuss
Correct Answer: increases exponentially
Explanation:
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.
- 3. In a turnstile antenna two-half wave resonant wire radiators are placed at 90° to each other in the same plane and are excited
Options- A. in phase
- B. in phase opposition
- C. 90° out of phase with each other
- D. 45° out of phase with each other Discuss
Correct Answer: 90° out of phase with each other
Explanation:
- 4. The frequency of oscillation in a backward wave oscillator can be changed by
Options- A. varying the voltage which controls beam velocity
- B. varying the beam current
- C. both by varying the beam current and by light varying the voltage which controls beam velocity
- D. changing the rate of thermionic emission Discuss
Correct Answer: varying the voltage which controls beam velocity
Explanation:
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. Assertion (A): The impedance of a matched load is equal to characteristic impedance of line.
Reason (R): A matched termination absorbs all the power incident on it.
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:
- 6. Which of the following is not a travelling wave?
Options- A. e = Em sin (?x - ?t)
- B. e = Em cos (?x - ?t)
- C. e = Em sin (?t - ?x)
- D. e = Em sin (?x) Discuss
Correct Answer: e = Em sin (?x)
Explanation:
- 7. Which of the following parameters is negligible in transmission lines?
Options- A. R
- B. L
- C. C
- D. G Discuss
Correct Answer: G
Explanation:
- 8. Which of the following devices uses a helix?
Options- A. Klystron amplifier
- B. Klystron oscillator
- C. TWT
- D. Both (a) and (b) Discuss
Correct Answer: TWT
Explanation:
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. Which TM mode in rectangular waveguide has lowest cutoff frequency?
Options- A. TM11
- B. TM01
- C. TM10
- D. TM21 Discuss
Correct Answer: TM11
Explanation:
Ex = ± ZTM Hy
Ey = ± ZTM Hx
where E0 is the amplitude of the wave.
The expressions for ?, ?c, fc, ?g, a are the same as for TE waves.
For TM wave the lowest cut off frequency in rectangular wave guide is for TM11 mode.
If 
, the cutoff frequency for TM11 mode is about 12% more than that for TE20 mode.
- 10. Impedance level of Impatt diodes is generally lower than that of Gunn diodes
Options- A. True
- B. False Discuss
Correct Answer: True
Explanation:
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.
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