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.

Because r_v = 1 at each end, the line voltage will not reach a steady value and oscillations will continue indefinitely.

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.

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.

?_r of a dielectric changes with humidity.

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Applegate diagram is distance time plot.

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

?_c = 1.706 D for TE₁₁ mode

?_c = 1.029 D for TE₂₁ mode

?_c = 0.82 D for TE₀₁ mode

?_c = 1.306 D for TM₀₁ mode.

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.