# Electronics and Communication Engineering - Microwave Communication

### Exercise :: Microwave Communication - Section 4

41.

A circular polarizer converts a linearly polarized wave into a circularly polarised wave.

 A. True B. False

Explanation:

No answer description available for this question. Let us discuss.

42.

If D is diameter of circular waveguide the cutoff wave-length for TE11 mode is equal to

 A. 1.706 D B. 2.11 D C. 0.82 D D. 0.41 D

Explanation:

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.

43.

The external magnetic field in a magnetron is such that lines are

 A. parallel to the axis of cathode B. perpendicular to the axis of cathode C. inclined to the axis of cathode D. either (b) or (c)

Explanation:

The operation of magnetron is based on interaction of electromagnetic fields with electrons moving in static electric and magnetic fields oriented at 90° with respect to each other.

In a magnetron three forces act on electron viz. force due to electric field (equal to - eE), force due to magnetic field [equal to - e (v x B)] and centrifugal force (equal to mv2/r).

The path of the electron can be found by balancing these three forces. Magnetron can be of three types i.e., negative resistance device, cyclotron frequency device and travelling wave or cavity device.

In negative resistance magnetron use is made of the negative resistance between two anode segments.

It has low efficiency and is used at frequencies less than 0.5 GHz.

In cyclotron magnetron a synchronism exists between ac component of electric field and periodic oscillation of electrons in a direction parallel to the field.

It is used for frequencies higher than 100 MHz.

A cavity magnetron has a number of cylindrical cavities in the interaction region.

The cavity magnetron is the most common type of magnetron. Its features are :

1. Frequency range - 500 MHz to 10 GHz

2. Power output - 250 kW (pulses)

It is used in radar systems, industrial heating systems and microwave ovens.

44.

Varactor diodes use

 A. silicon B. GaAs C. either (a) or (b) D. neither (a) nor (b)

Explanation:

No answer description available for this question. Let us discuss.

45.

A circular waveguide carries TE11 mode whole radial electric field is given by
Er = E0j1(r) sin Φ V/m
where r is radial distance in cm from the axis
The cut off wavelength is

 A. 10 cm B. 3 p cm C. 2 p cm D. 8 cm