Question 1

A balun should have

Accepted Answer

Turn ratio is so selected as to give low SWR at both ports.

Question 2

The width of a radio beam from a 1 m diameter parabolic antenna at 1 GHz is about

Accepted Answer

Beam width between nulls of a parabolic antenna . Therefore 50° is the nearest value.

Question 3

In microwave system the function of mode filter is

Accepted Answer

Mode filter suppress modes with lower cutoff frequencies.

Question 4

The main feature of a parametric amplifier is

Accepted Answer

Since reactance does not contribute thermal noise to the circuit, it is a low noise device.

Question 5

If a line having Z0 = 300 ∠ 0 W is open circuited at far end, VSWR is

Accepted Answer

If line is o.c. |rv| = 1 and VSWR = ∞.

Question 6

Which one of the following is transferred locking electron device C antenna?

Accepted Answer

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.

Question 7

The directive gain of a transmitting antenna is proportional to

Accepted Answer

and is thus proportional to cross sectional area.

Question 8

A magnetron requires an external magnetic field with flux lines parallel to axis of cathode

Accepted Answer

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.

Question 9

High speed logic circuits use

Accepted Answer

High speed means time period is small. Therefore pulses should also have small width.

Question 10

The total field developed by an antenna array at a distant point is

Accepted Answer

Since field is a phasor quantity we have to take phasor sum.

Question 11

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

Accepted Answer

A short-circuited stub is a suitable method of matching.

Question 12

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.

Accepted Answer

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.

Question 13

To couple a coaxial line to a parallel wire line it is best to use

Accepted Answer

A balun gives 4 : 1 impedance transformation.

Question 14

Assertion (A): Magnetron is generally used in n mode. Reason (R): Frequency for p mode can be easily separated from adjacent modes.

Accepted Answer

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.

Question 15

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. Which of the above …

Accepted Answer

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.

Question 16

For a circular wave guide

Accepted Answer

When a circular waveguide is rotated by 90°, the configuration remain the same.

Question 17

In a backward wave oscillator the wave

Accepted Answer

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.

Question 18

If f is the frequency of electromagnetic wave, fc is cutoff frequency, then in a rectangular waveguide

Accepted Answer

Wavelength should be less than cutoff wavelengths. Therefore frequency should be higher than cutoff frequency for minimum attenuation.

Question 19

In mode filter of the given figure which of the following modes is unaffected

Accepted Answer

TE01 mode is unaffected because its horizontally oriented electric field is every where perpendicular to the conducting strips.

Question 20

A branched duplexer requires

Accepted Answer

A branched duplexer funds use in narrow bandwidth applications. TR tube stands for transmit-receive tube and ATR tube stands for antitransmit receive tube. Both these tubes are used in branched duplexer.

Microwave Communication Questions