# Electronics and Communication Engineering - Microwave Communication

Exercise : Microwave Communication - Section 1
6.
Microwave resonators are used in
microwave oscillators
microwave narrow band amplifier
microwave frequency metres
all of the above
Explanation:

They are used in all the devices.

7.

Assertion (A): TWT uses a focussing mechanism to prevent the electron beam from spreading.

Reason (R): In a TWT the electron beam has to travel a much longer distance than in klystron.

Both A and R are correct and R is correct explanation of A
Both A and R are correct but R is not correct explanation of A
A is correct but R is wrong
A is wrong but R is correct
Explanation:

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.

8.
In a TWT the amplitude of resultant wave travelling down the helix
increases exponentially
increases linearly
decreases exponentially
is almost constant
Explanation:

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.

9.
Which of the following is not a travelling wave?
e = Em sin (βx - ωt)
e = Em cos (βx - ωt)
e = Em sin (ωt - βx)
e = Em sin (βx)
Explanation:

In a travelling wave both x and t increase simultaneously so that a constant phase point moves in the direction of positive (or negative) x.

10.
Both Impatt and Trapatt devices use avalanche effect
True
False
Explanation:

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.

Avalanche diode can also be operated in large signal high efficiency mode called Trapped Avalanche Transit Time mode.

The Trapatt oscillations depend on the delay in the current caused by avalanche process.

The avalanche delay makes it possible to increase the diode voltage well above the breakdown voltage.

Therefore a very rapid multiplication of charge carriers occurs. A Trapatt diode is also a negative resistance device.

The features of Trapatt diode oscillator are : Frequency 3 to 50 GHz, Power output 1-3 W, efficiency about 25%.

Its applications are low power doppler radar, microwave beacon landing system etc.