Question 1

When a line short circuited at far end, the minimum voltage occurs at

Accepted Answer

When a line is short-circuited at far end, the voltage at far end is zero.

Question 2

The bunching action which occurs in multicavity klystron amplifier can be represented by Applegate diagram

Accepted Answer

A Klystron is a vacuum tube used for generation/amplification of microwaves. An electron beam is produced by oxide coated indirectly heated cathode and is focussed and accelerated by focussing electrode. This beam is transmitted through a glass tube. The input cavity where the beam enters the glass tube is called buncher. As electrons move ahead they see an accelerating field for half cycle and retarding field for the other half cycle. Therefore, some electrons are accelerated and some are retarded. This process is called velocity modulation. The velocity modulation causes bunching of electrons. This bunching effect converts velocity modulation into density modulation of beam. The input is fed at buncher cavity and output is taken at catcher cavity. In a two cavity klystron only buncher and catcher cavity are used. In multi cavity klystron one or more intermediate cavities are also used. The features of a multicavity klystron are : 1. Frequency range - 0.25 GHz to 100 GHz 2. Power output - 10 kW to several hundred kW 3. Power gain - 60 dB (nominal value) 4. Efficiency - about 40%. A multicavity klystron is used in UHF TV transmitters, Radar transmitter and satellite communication.

Question 3

In a vacuum tube, the transit time of electron between cathode and anode is important at

Accepted Answer

At high frequencies transit time is large as compared to the period of microwave signal.

Question 4

A duplexer is used to

Accepted Answer

Same antenna is used for transmission and reception.

Question 5

Assertion (A): A line of length and short circuited at far end has an input impedance of infinity. Reason (R): A line of length and short circuited at far end behaves as a parallel resonant circuit.

Accepted Answer

A parallel tuned circuit has an infinite impedance if R is zero.

Question 6

A quarter wave line short circuited at load end behaves as

Accepted Answer

A quarter wave line short-circuited at far end behaves as a parallel tuned circuit.

Question 7

In the given figure the reflected voltage wave after first reflection is

Accepted Answer

.

Question 8

If other parameter are constant, transmission loss at 5 GHz as compared to that at 0.5 GHz is

Accepted Answer

As frequency increases, transmission loss increases slightly.

Question 9

In Reflex Klystron oscillator the focussing electrode is at a high potential

Accepted Answer

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.

Question 10

Assertion (A): PIN diode can be used as attenuator and limiter. Reason (R): PIN diode has a thin intrinsic layer.

Accepted Answer

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.

Question 11

For the strip line in the given figure the capacitance per unit length =

Accepted Answer

frads/unit length.

Question 12

A magnetron has a cylindrical cathode surrounded by an anode structure having cavities opening into interaction space by means of slots

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 13

A loss less line is terminated in a circular lines are E lines

Accepted Answer

= finite number and VSWR = finite number.

Question 14

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 de…

Accepted Answer

All the three are correct.

Question 15

The cut off wavelength in circular waveguide

Accepted Answer

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.

Question 16

In a TWT the axial component of electric field advances along the tube

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 17

The directive gain of a transmitting antenna is

Accepted Answer

Question 18

A transmission line has Z0 = 300 Ω and ZL = (300 - j300) ohm. The transmission coefficient is

Accepted Answer

Transmission coefficient = 1.265 ∠- 18.43°.

Question 19

A coaxial line has L = 500 nH/m and C = 50 pF/m. The characteristic impedance is

Accepted Answer

.

Question 20

Assertion (A): A half wavelength line can be used as a 1 : 1 transformer. Reason (R): The input impedance of a half wavelength line is equal to load impedance.

Accepted Answer

Since a half wavelength line has an input impedance equal to load impedance, the impedance transformation ratio is 1:1.

Microwave Communication Questions