Question 1

Consider the following statements Dissipative attenuator has a fixed value of attenuation. Reflective attenuator has a fixed value of attenuation. Both dissipative and reflective attenuators are available only with fixed attenuation. Both dissipativ…

Accepted Answer

Both dissipative and reflective attenuators are available with either fixed or variable attenuation.

Question 2

Consider the following statements Bunching of electrons occurs in two cavity klystron amplifier. Bunching of electrons occurs in multi cavity klystron amplifier. Bunching of electrons occurs in reflex cavity klystron amplifier. Which of the above st…

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

A coaxial RF cable has a characteristic impedance of 50 Ω and C equal to 40 pF/m. The inductance is

Accepted Answer

.

Question 4

The noise figure of multicavity klystron amplifier is very low

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 5

Which one of the following is not a negative resistance device?

Accepted Answer

Gunn diode, Tunnel diode and Impatt diodes are negative resistance devices. The limitation of solid state devices at high frequencies include those associated with transit time and junction capacitances. The devices used are : Transferred electron oscillators (Gunn diode), Avalanche diode oscillators (Impatt diode, Trapatt diode, Masters, Lasers, Tunnel diode, Varactor etc).

Question 6

If a short line is terminated in characteristic impedance it behaves as an infinite line.

Accepted Answer

In a line terminated by Z0, there are no reflections.

Question 7

A wave Em cos (bx - ωt) is a backward wave.

Accepted Answer

It is a forward wave because as t increases, x increases.

Question 8

Which mode has the lowest cut off frequency in circular wave guides?

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 9

The number of TV channels which can be accommodated in a spectrum of 300 MHz is about

Accepted Answer

A TV channel has a bandwidth about 5 MHz. Hence 300 MHz spectrum can accommodate about 50 channels.

Question 10

In a multicavity klystron amplifier the signal to be amplified develops an ac voltage of signal frequency across the gap in buncher cavity

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 11

In the given figure the E and H lines in a coaxial cable

Accepted Answer

H lines are concentric circles around conductor and E lines are radial.

Question 12

Which device has internal positive feedback?

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 13

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

Accepted Answer

Question 14

Assertion (A): The velocity of electromagnetic waves on overhead lines and coaxial cables is the same. Reason (R): Free space has an intrinsic impedance of 377 ohms.

Accepted Answer

Velocity of em waves on caoxial cables is less than that on overhead lines.

Question 15

Assertion (A): Impedance matching can be done by using stubs. Reason (R): A double stub is used for impedance matching when frequency of signal varies.

Accepted Answer

Hollow metallic enclosures exhibit resonance behaviour when excited by electromagnetic field. These enclosures are called cavity resonators.

Question 16

Atomic and molecular resonance is observed in many substances

Accepted Answer

Atomic and molecular distances are very small. Therefore resonance can occur only at microwave frequencies.

Question 17

The expression HZ = C cos(Bx) cos(Ay)yz is for

Accepted Answer

In TM mode Hz = 0.

Question 18

If the minimum range of a radar is to be doubled, the peak power has to be increased by a factor of

Accepted Answer

Range = (power)025.

Question 19

Assertion (A): Klystron amplifiers use one or more intermediate cavities in addition to buncher and catcher cavity. Reason (R): When one or more intermediate cavities are used the bandwidth can be increased.

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 20

The semiconductor diode which can be used in switching circuit in microwave range is

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.

Microwave Communication Questions