Exercise: Microwave Communication

Questions for: Microwave Communication

The directive gain of a transmitting antenna is proportional to
A:
its cross-sectional area
B:
square of cross-sectional area
C:
square root of cross-sectional area
D:
cube root of cross-sectional area
Answer: A

and is thus proportional to cross sectional area.

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In high frequency circuits impedance matching can be done by
  1. open circuited stubs
  2. short circuited stubs
  3. transformer
Which of the above correct?
A:
1, 2 and 3
B:
1 and 2 only
C:
1 and 3 only
D:
2 and 3 only
Answer: A

All the three can be used for impedance matching in high frequency circuits.

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Consider the following statements
  1. Impedance of Gunn diode is about tens of ohms.
  2. Impedance of Impatt diode is a few ohms.
  3. Impedance of Impatt diode are of the same order.
  4. Impedance of Impatt diode is more than that of Gunn diode.
Which of the above statement are correct?
A:
all
B:
1 and 2 only
C:
1, 2 and 3
D:
1, 2 and 4
Answer: B

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.

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In Reflex Klystron oscillator the focussing electrode is at a high potential
A:
True
B:
False
C:
D:
Answer: B

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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The cut off wavelength in circular waveguide
A:
guide diameter
B:
square of guide diameter
C:
cube of guide diameter
D:
square root of guide diameter
Answer: A

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

λc = 1.706 D for TE11 mode

λc = 1.029 D for TE21 mode

λc = 0.82 D for TE01 mode

λc = 1.306 D for TM01 mode.

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