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- Question
To couple a coaxial line to a parallel wire line it is best to use
Options- A. slotted line
- B. balun
- C. directional coupler
- D. ?/4 transformer
- Correct Answer
- balun
ExplanationA balun gives 4 : 1 impedance transformation.
Microwave Communication problems
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- 1. Assertion (A): The condition of minimum distortion in a transmission line is L = RC/G.
Reason (R): Line loading means addition of inductance to satisfy the condition L = RC/G.
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:
Oscillator circuits using vacuum tubes (triodes, pentodes) have the following limitations at very high frequencies (microwave region).The stray capacitances and inductances become important and affect the operation of the circuit.
At low frequencies the transit time between cathode and anode is a small fraction of period of oscillation.
However, at microwave frequencies this transit time becomes comparable to time period of oscillations.
- 2. A (75 - J50) ohm load is connected to a coaxial of Z0 = 75 ohm at 10 GHz. The best method of matching is to connect
Options- A. an inductance at load
- B. a short circuited stub at load
- C. a short circuited stub at specific distance from load
- D. a capacitance at specific distance from load Discuss
Correct Answer: a short circuited stub at specific distance from load
Explanation:
A short-circuited stub is a suitable method of matching.- 3. The total field developed by an antenna array at a distant point is
Options- A. phasor sum of fields produced by individual antennas of the array
- B. algebraic sum of fields produced by individual antennas of the array
- C. either (a) or (b) depending on type of array
- D. neither (a) nor (b) Discuss
Correct Answer: phasor sum of fields produced by individual antennas of the array
Explanation:
Since field is a phasor quantity we have to take phasor sum.- 4. High speed logic circuits use
Options- A. pulses with very small width
- B. pulses with very large width
- C. pulses whose width is neither small nor large
- D. either (b) or (c) Discuss
Correct Answer: pulses with very small width
Explanation:
High speed means time period is small. Therefore pulses should also have small width.- 5. A magnetron requires an external magnetic field with flux lines parallel to axis of cathode
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.
- 6. Assertion (A): Magnetron is generally used in n mode.
Reason (R): Frequency for p mode can be easily separated from adjacent modes.
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.
- 7. Consider the following statements
- Impedance of Gunn diode is about tens of ohms.
- Impedance of Impatt diode is a few ohms.
- Impedance of Impatt diode are of the same order.
- Impedance of Impatt diode is more than that of Gunn diode.
Options- A. all
- B. 1 and 2 only
- C. 1, 2 and 3
- D. 1, 2 and 4 Discuss
Correct Answer: 1 and 2 only
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.
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.
- 8. For a circular wave guide
Options- A. cutoff frequency for TE10 and TE01 modes are same
- B. cutoff frequency for TE10 and TE01 modes are different
- C. cutoff frequency for TE10 mode = twice the cutoff frequency for TE20 mode
- D. cutoff frequency for TE10 mode = haf the cutoff frequency for TE20 mode Discuss
Correct Answer: cutoff frequency for TE10 and TE01 modes are same
Explanation:
When a circular waveguide is rotated by 90°, the configuration remain the same.- 9. In a backward wave oscillator the wave
Options- A. travelling along the line winds itself back and forth
- B. progresses only in forward direction
- C. progresses only in backward direction
- D. either (a) or (c) Discuss
Correct Answer: travelling along the line winds itself back and forth
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.
- 10. If f is the frequency of electromagnetic wave, fc is cutoff frequency, then in a rectangular waveguide
Options- A. attenuation is low when f > fc
- B. attenuation is low when f < fc
- C. attenuation is high when f < fc
- D. either (b) or (c) Discuss
Correct Answer: attenuation is low when f > fc
Explanation:
Wavelength should be less than cutoff wavelengths.Therefore frequency should be higher than cutoff frequency for minimum attenuation.
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- 1. Assertion (A): The condition of minimum distortion in a transmission line is L = RC/G.