Electronics and Communication Engineering - Microwave Communication
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.
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.
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.