Question 1

A matched generator (Zg = Z0) with 1 Ω available. If all ports are matched, the power delivered at ports 1, 2, 3 respectively are

Accepted Answer

Since all ports matched, input SWR = 1. No power is reflected and Pin = P4 = 1 W. This power splits between ports 1 and 2. Therefore P1 = P2 = 0.5 Ω, P3 = 0.

Question 2

The fabrication of microstrip line is done by

Accepted Answer

Printed circuit techniques is used for micro strip line.

Question 3

Loss angle of a good quality cable is about

Accepted Answer

For ideal dielectric loss angle is zero. For good quality cables, loss angle is very small, about 1° only.

Question 4

In the given figure reflection coefficient at load is

Accepted Answer

Question 5

In the given figure the time taken by the wave to make one round trip is

Accepted Answer

Question 6

A waveguide section in a microwave circuit acts as

Accepted Answer

It allows high frequencies.

Question 7

The reflection coefficient on a line is 0.2 ∠45°. The SWR is

Accepted Answer

Question 8

Skin effect is more pronounced at high frequencies.

Accepted Answer

Skin effect increases as frequency increases.

Question 9

In a backward wave oscillator the wave travelling along the line winds itself back and forth

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 10

A quarter wave line open circuited at far end behaves as

Accepted Answer

A quarter wave line o.c. at far end behaves as a series tuned circuit.

Question 11

Assertion (A): Artificial transmission lines are frequently used in laboratories. Reason (R): An artificial transmission line can be used to represent an actual line and can also be used as a delay circuit, as attenuator, as filter network etc.

Accepted Answer

Artificial transmission lines have many applications. One of the applications is to simulate an actual line in the laboratory.

Question 12

In a TWT the amplitude of resultant wave travelling down the helix

Accepted Answer

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.

Question 13

In a turnstile antenna two-half wave resonant wire radiators are placed at 90° to each other in the same plane and are excited

Accepted Answer

Since the two wire radiators are at 90° to each other, they have to be excited 90° out of phase with each other.

Question 14

The frequency of oscillation in a backward wave oscillator can be changed by

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

Assertion (A): The impedance of a matched load is equal to characteristic impedance of line. Reason (R): A matched termination absorbs all the power incident on it.

Accepted Answer

Matched load means no reflections.

Question 16

Which of the following is not a travelling wave?

Accepted Answer

In a travelling wave both x and t increase simultaneously so that a constant phase point moves in the direction of positive (or negative) x.

Question 17

Which of the following parameters is negligible in transmission lines?

Accepted Answer

Shunt conductance can be neglected in most of calculations.

Question 18

Which of the following devices uses a helix?

Accepted Answer

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.

Question 19

Which TM mode in rectangular waveguide has lowest cutoff frequency?

Accepted Answer

Hz = 0 Ex = ± ZTM Hy Ey = ± ZTM Hx where E0 is the amplitude of the wave. The expressions for β, λc, fc, λg, 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.

Question 20

Impedance level of Impatt diodes is generally lower than that of Gunn diodes

Accepted Answer

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.

Microwave Communication Questions