Electronics and Communication Engineering - Microwave Communication

1.
The velocity factor of a transmission line depends on
temperature
skin effect
relative permittivity of dielectric
none of the above
Answer: Option
Explanation:

r of a dielectric changes with humidity.


2.
A loss less line of characteristic impedance Z0 is terminated in pure reactance of -jZ0 value. VSWR is
10
2
1
infinity
Answer: Option
Explanation:

A pure reactance does not absorb any power.

Therefore VSWR is infinite because |rv| = 1.


3.
In a klystron amplifier the input cavity is called
buncher
catcher
Pierce gun
collector
Answer: Option
Explanation:

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.


4.
In a circular waveguide the dominant mode is
TE01
TE11
TE20
TE21
Answer: Option
Explanation:

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.


5.
The reflection coefficient on a line is 0.2 ∠45°. The SWR is
0.8
1.1
1.2
1.5
Answer: Option
Explanation: