Question 1

A line has an attenuation of 0.054 Np/m. The attenuation in decibels is

Accepted Answer

0.054 x 8.68 = 0.47 dB/m.

Question 2

Power gain of a multicavity klystron amplifier is

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 cavity resonator is

Accepted Answer

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

Question 4

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 5

In a microwave coaxial line, the maximum operating frequency is f max and breakdown strength of dielectric is Ed' Then maximum power handling capacity is proportional to

Accepted Answer

Power handling capacity μ (voltage)2 and is inversely proportional to f2 max.

Question 6

If a line is open circuited Zin = Z0 tanh (gl).

Accepted Answer

The quantities required to be measured in microwave circuits are frequency, power and impedance. Microwave frequency measurement is done by slotted line, resonant cavities and transfer oscillator. Microwave power measurement uses bolometers and micromave power meters. A self balancing bridge working on the principle of power substitution is commomly used. Microwave impedance measurement is done by measurement of reflection coefficient and VSWR. A slotted line and probe is a basic tool for these measurements.' Network analyser enables rapid impedance measurement over a broad frequency range.

Question 7

Assertion (A): PIN diode is used as a fast switch. Reason (R): PIN diode has very high resistance when reverse biased and very low resistance when forward biased.

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 8

Consider the following applications Switch Attenuator Phase shifter Oscillator In which of the above can a PIN diode be used?

Accepted Answer

PIN diode is not used in oscillators.

Question 9

Roughly the time required for microwave cooking as compared to conventional cooking is

Accepted Answer

Time required for microwave cooking is much less than the time required for conventional cooking.

Question 10

Which of the following is wrong for a magic used to tee?

Accepted Answer

Coplanar arms are decoupled.

Question 11

The component in the given figure is

Accepted Answer

Z01 provides series inductances and teflon filled low impedance line Z02 provides shunt capacitance.

Question 12

A pulsed radar produces 1 μ sec pulses at a rate of 1000 per second. The duty cycle is

Accepted Answer

1 x 10-6 x 1000 = 0.001.

Question 13

Assertion (A): Impedance measurement at microwave frequencies is done by finding SWR. Reason (R): SWR and reflection coefficient depend on the characteristic impedance and load impedance.

Accepted Answer

Question 14

A klystron amplifier generally uses Pierce gun

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 15

Consider the following applications TV tuning Active filter Microwave frequency multiplication In which of above can a varactor diode be used?

Accepted Answer

Varactor diode is used in all the three applications.

Question 16

When a line is loaded the characteristic impedance is equal to

Accepted Answer

or .

Question 17

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

Accepted Answer

If line is s.c |rv| = 1 and VSWR = ∞.

Question 18

A reflex klystron oscillator uses

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 19

Ga As exhibits negative differential mobility

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 20

Microwave links are typically 50 km apart

Accepted Answer

Earth's curvature limits the distance between microwave links.

Microwave Communication Questions