# Microwave Engineering Questions and Answers – Antenna Gain and Efficiency

This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Antenna Gain and Efficiency”.

1. A __________ is a device that converts a guided electromagnetic wave on a transmission line into a plane wave propagating in free space.
a) Transmitting antenna
b) Receiving antenna
d) Mixer

Explanation: A transmitting antenna is a device that converts a guided electromagnetic wave on a transmission line into a plane wave propagating in free space. It appears as an electrical circuit on one side, provides an interface with a propagating plane wave.

2. Antennas are bidirectional devices.
a) True
b) False

Explanation: Antennas can be used both as transmitters and receivers. As transmitters they radiate energy to free space and as receivers they receive signal from free space. Hence, they are called bidirectional devices as they are used at both transmitting end and receiving end.

3. Dipole antennas are an example for:
a) Wire antennas
b) Aperture antennas
c) Array antennas
d) None of the mentioned

Explanation: Dipoles, monopoles, oops, Yagi-Uda arrays are all examples for wire antennas. These antennas have low gains, and are mostly used at lower frequencies.

4. _________ antennas consist of a regular arrangement of antenna elements with a feed network
a) Aperture antennas
b) Array antennas
c) Printed antennas
d) Wire antennas

Explanation: Array antennas consist of a regular arrangement of antenna elements with a feed network. Pattern characteristics such as beam pointing angle and side lobe levels can be controlled by adjusting the amplitude and phase excitation of array elements.

5. A parabolic reflector used for reception with the direct broadcast system is 18 inches in diameter and operates at 12.4 GHz. The far-field distance for this antenna is:
a) 18 m
b) 13 m
c) 16.4 m
d) 17.3 m

Explanation: Far field distance for a reflector antenna is given by 2D2/λ. D is the diameter and λ is the operating signal wavelength. Substituting in the above expression, far field distance is 17.3 m.
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6._________ of an antenna is a plot of the magnitude of the far field strength versus position around the antenna.
b) Directivity
c) Beam width
d) None of the mentioned

Explanation: Radiation pattern of an antenna is a plot of the magnitude of the far field strength versus position around the antenna. This plot gives the detail regarding the region where most of the energy of antenna is radiated, side lobes and beam width of an antenna.

7. Antennas having a constant pattern in the azimuthal plane are called _____________
a) High gain antenna
b) Omni directional antenna
c) Unidirectional antenna
d) Low gain antenna

Explanation: Omni directional antennas radiate EM waves in all direction. If the radiation pattern for this type of antenna is plotted, the pattern is a constant signifying that the radiated power is constant measured at any point around the antenna.

8. Beamwidth and directivity are both measures of the focusing ability of an antenna.
a) True
b) False

Explanation: Beamwidth and directivity are both measures of the focusing ability of an antenna. An antenna with a narrow main beam will have high directivity, while a pattern with low beam will have low directivity.

9. If the beam width of an antenna in two orthogonal planes are 300 and 600. Then the directivity of the antenna is:
a) 24
b) 18
c) 36
d) 12

Explanation: Given the beam width of the antenna in 2 planes, the directivity is given by 32400/θ*∅, where θ,∅ are the beam widths in the two orthogonal planes. Substituting in the equation, directivity of the antenna is 18.

10. If the power input to an antenna is 100 mW and if the radiated power is measured to be 90 mW, then the efficiency of the antenna is:
a) 75 %
b) 80 %
c) 90 %
d) Insufficient data

Explanation: Antenna efficiency is defined as the ratio of radiated power to the input power to the antenna. Substituting the given data in the efficiency equation, the efficiency of the antenna is 90%.

Sanfoundry Global Education & Learning Series – Microwave Engineering.
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