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- Question
For the strip line in the given figure the capacitance per unit length =
Options- A.
- B.
- C.
- D.
- Correct Answer
-
Explanation
frads/unit length.
Microwave Communication problems
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- 1. Assertion (A): PIN diode can be used as attenuator and limiter.
Reason (R): PIN diode has a thin intrinsic layer.
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 PIN diode has an intrinsic (i) layer between p and n layers. When reverse bias is applied depletion layers are formed at p-i and i-n junctions.The effective/width of depletion layer increases by the width of i layer. It can be used as a voltage controlled attenuator.
At high frequencies the rectification effect ceases and impedance of diode is effectively that of i layer.
This impedance varies with the applied bias. It is used in high frequency switching circuits, limiters, modulators etc.
- 2. In Reflex Klystron oscillator the focussing electrode is at a high potential
Options- A. True
- B. False Discuss
Correct Answer: False
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.
- 3. If other parameter are constant, transmission loss at 5 GHz as compared to that at 0.5 GHz is
Options- A. 20 dB more
- B. 20 dB less
- C. 700 dB more
- D. 100 dB less Discuss
Correct Answer: 20 dB more
Explanation:
As frequency increases, transmission loss increases slightly.- 4. In the given figure the reflected voltage wave after first reflection is
Options- A. 18 V
- B. -18 V
- C. 30 V
- D. -30 V Discuss
Correct Answer: -18 V
Explanation:
.
- 5. A quarter wave line short circuited at load end behaves as
Options- A. an inductance
- B. an inductance and capacitance in series
- C. a capacitance
- D. an inductance and capacitance in parallel Discuss
Correct Answer: an inductance and capacitance in parallel
Explanation:
A quarter wave line short-circuited at far end behaves as a parallel tuned circuit.- 6. A magnetron has a cylindrical cathode surrounded by an anode structure having cavities opening into interaction space by means of slots
Options- A. True
- B. False Discuss
Correct Answer: True
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. A loss less line is terminated in a circular lines are E lines
Options- A. SWR = 0
- B. SWE = 0
- C. SWR is finite
- D. SWR = ? Discuss
Correct Answer: SWR is finite
Explanation:
= finite number and VSWR 
= finite number.
- 8. The directivity in a receiving antenna
- increases the intercept area in forward direction
- reduces the noise picked up from other sources
- provides a means of discriminating against undesired signals originating in directions other than in which the desired transmitter lies.
Options- A. 1 only
- B. 1 and 2 only
- C. 1, 2 and 3
- D. 2 and 3 only Discuss
Correct Answer: 1, 2 and 3
Explanation:
All the three are correct.- 9. The cut off wavelength in circular waveguide
Options- A. guide diameter
- B. square of guide diameter
- C. cube of guide diameter
- D. square root of guide diameter Discuss
Correct Answer: guide diameter
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.
- 10. In a TWT the axial component of electric field advances along the tube
Options- A. at a velocity that is almost equal to speed of light
- B. at a velocity that is a small fraction of speed of light
- C. at a velocity that is about 50% of speed of light
- D. at a velocity that may be even more than speed of light Discuss
Correct Answer: at a velocity that is a small fraction of speed of light
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.
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- 1. Assertion (A): PIN diode can be used as attenuator and limiter.