# Electronics and Communication Engineering - Microwave Communication

Exercise : Microwave Communication - Section 3
36.
Which one of the following is not a negative resistance device?
Gunn diode
Tunnel diode
Impatt diode
Varactor diode
Explanation:

Gunn diode, Tunnel diode and Impatt diodes are negative resistance devices.

The limitation of solid state devices at high frequencies include those associated with transit time and junction capacitances.

The devices used are : Transferred electron oscillators (Gunn diode), Avalanche diode oscillators (Impatt diode, Trapatt diode, Masters, Lasers, Tunnel diode, Varactor etc).

37.
Microwave links are typically 50 km apart
because of atmospheric attenuation
because of output tube power limitation
because of earth's curvature
to ensure that applied dc voltage is not excessive
Explanation:

Earth's curvature limits the distance between microwave links.

38.
For matching over a range of frequencies in a transmission line it is best to use
a balun
double stub
a single stub of adjustable position
Explanation:

Double stub is useful for matching over a range of frequencies.

39.

Assertion (A): In the interaction region of magnetron an electron is subjected to three forces: force due to electric field, force due to magnetic field and centrifugal force.

Reason (R): The mechanism of generation of microwaves, in a magnetron, involves interaction of electromagnetic fields with electrons moving in static electric and magnetic fields oriented at right angles to each other.

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:

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.

40.
Which of the following is wrong for a magic used to tee?
E and H arms are decoupled
coplanar arms are coupled
all ports are perfectly matched
A signal into coplanar arm splits equally between E and H arms