Discussion
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- Question
A line has Z0 = 300 ? 0° ?. If ZL = 150 ? 0° ?, reflection coefficient is
Options- A. 0.5
- B. 0.3333
- C. -0.3333
- D. -0.5
- Correct Answer
- -0.3333
ExplanationReflection coefficient
Microwave Communication problems
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- 1. As the frequency is increased, the charging MVAR in a cable
Options- A. decreases
- B. increases
- C. remain the same
- D. decreases or remains the scheme Discuss
Correct Answer: increases
Explanation:
Charging current and MVAR are proportional to frequency.- 2. Which of the following lines is non-radiating?
Options- A. Open two wire
- B. Coaxial
- C. Both
- D. None of the above Discuss
Correct Answer: Coaxial
Explanation:
Since fields are confirmed within a coaxial cable it is non-radiating.- 3. In a three cavity klystron amplifier, the oscillations are excited in
Options- A. input cavity
- B. output cavity
- C. intermediate cavity
- D. both (a) and (b) Discuss
Correct Answer: intermediate cavity
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.
- 4. Impedance level of Impatt diodes is generally lower than that of Gunn diodes
Options- A. True
- B. False Discuss
Correct Answer: True
Explanation:
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.
- 5. Which TM mode in rectangular waveguide has lowest cutoff frequency?
Options- A. TM11
- B. TM01
- C. TM10
- D. TM21 Discuss
Correct Answer: TM11
Explanation:
Hz = 0
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.
- 6. A 10 km long line has a characteristic impedance of 400 ohms. If line length is 100 km, the characteristic impedance is
Options- A. 4000 ?
- B. 400 ?
- C. 40 ?
- D. 4 ? Discuss
Correct Answer: 400 ?
Explanation:
and is independent of line length.
- 7. Microwave resonators are used in
Options- A. microwave oscillators
- B. microwave narrow band amplifier
- C. microwave frequency metres
- D. all of the above Discuss
Correct Answer: all of the above
Explanation:
They are used in all the devices.- 8. In a klystron amplifier the input cavity is called
Options- A. buncher
- B. catcher
- C. Pierce gun
- D. collector Discuss
Correct Answer: buncher
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.
- 9. Assertion (A): TWT uses a focussing mechanism to prevent the electron beam from spreading.
Reason (R): In a TWT the electron beam has to travel a much longer distance than in klystron.
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:
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.
- 10. For a 50 ? resistor for 3 GHz application, the stray capacitance should be less than
Options- A. 1 ?F
- B. 1 nF
- C. 1 pF
- D. 0.1 pF Discuss
Correct Answer: 0.1 pF
Explanation:
XC should be at least ten times greater than R. Therefore for 50 ? resistor at 3 GHz, C should be less than 0.1 pF.
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