Assertion (A): Magnetron is generally used in n mode.

Reason (R): Frequency for p mode can be easily separated from adjacent modes.

A balun gives 4 : 1 impedance transformation.

Oscillator circuits using vacuum tubes (triodes, pentodes) have the following limitations at very high frequencies (microwave region).

The stray capacitances and inductances become important and affect the operation of the circuit.

At low frequencies the transit time between cathode and anode is a small fraction of period of oscillation.

However, at microwave frequencies this transit time becomes comparable to time period of oscillations.

A short-circuited stub is a suitable method of matching.

Since field is a phasor quantity we have to take phasor sum.

High speed means time period is small. Therefore pulses should also have small width.

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.

An Impatt diode has n⁺ - p - i - p ⁺ structure and is used with reverse bias.

It exhibits negative resistance and operates on the principle of avalanche breakdown. Impatt diode circuits are classified as broadly tunable circuit, low Q circuit and high Q circuit.

The impedance of Impatt diode is a few ohms. The word Impatt stands for Impact Avalanche Transit Time diode.

The features of Impatt diode oscillator are : Frequency 1 to 300 GHz, Power output (0.5 W to 5 W for single diode circuit and upto 40 W for combination of several diodes), efficiency about 20%.

Its applications include police radar systems, low power microwave transmitter etc.

When a circular waveguide is rotated by 90°, the configuration remain the same.

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.

Wavelength should be less than cutoff wavelengths.

Therefore frequency should be higher than cutoff frequency for minimum attenuation.

TE₀₁ mode is unaffected because its horizontally oriented electric field is every where perpendicular to the conducting strips.