# Microwave Engineering Questions and Answers – System Aspects of Antennas

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This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “System Aspects of Antennas”.

1. If an antenna has a directivity of 16 and radiation efficiency of 0.9, then the gain of the antenna is:
a) 16.2
b) 14.8
c) 12.5
d) 19.3

Explanation: Gain of an antenna is given by the product of radiation efficiency of the antenna and the directivity of the antenna. Product of directivity and efficiency thus gives the gain of the antenna to be 16.2.

2. Gain of an antenna is always greater than the directivity of the antenna.
a) True
b) False

Explanation: Gain of an antenna is always smaller than the directivity of an antenna. Gain is given by the product of directivity and radiation efficiency. Radiation efficiency can never be greater than one. So gain is always less than or equal to directivity.

3. A rectangular horn antenna has an aperture area of 3λ × 2λ. Then the maximum directivity that can be achieved by this rectangular horn antenna is:
a) 24 dB
b) 4 dB
c) 19 dB
d) Insufficient data

Explanation: Given the aperture dimensions of an antenna, the maximum directivity that can be achieved is 4π A/λ2, where A is the aperture area and λ is the operating wavelength. Substituting the given values in the above equation, the maximum directivity achieved is 19 dB.
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4. A rectangular horn antenna has an aperture area of 3λ × 2λ. If the aperture efficiency of an antenna is 90%, then the directivity of the antenna is:
a) 19 dB
b) 17.1 dB
c) 13 dB
d) 21.1 dB

Explanation: Given the aperture dimensions of an antenna, the directivity that can be achieved is ap4π A/λ2, where A is the aperture area and λ is the operating wavelength, ap is the aperture efficiency. Substituting the given values in the above equation, the directivity achieved is 17.1 dB.

5. If an antenna has a directivity of 16 and is operating at a wavelength of λ, then the maximum effective aperture efficiency is:
a) 1.27λ2
b) 2.56λ2
c) 0.87λ2
d) None of the mentioned

Explanation: Maximum effective aperture efficiency of an antenna is given by D λ2/4π, D is the directivity of the antenna. Substituting in the equation the given values, the maximum effective aperture is 1.27λ2.

6. A resistor is operated at a temperature of 300 K, with a system bandwidth of 1 MHz then the noise power produced by the resistor is:
a) 3.13×10-23 watts
b) 4.14×10-15 watts
c) 6.14×10-15 watts
d) None of the mentioned

Explanation: For a resistor noise power produced is given by kTB, where T is the system temperature and B is the bandwidth. Substituting in the above expression, the noise power produced is 4.14×10-15 watts.

7. With an increase in operating frequency, the background noise temperature:
a) Increases
b) Decreases
c) Remains constant
d) Remains unaffected

Explanation: The plot of frequency v/s background noise temperature shows that with the increase of the signal frequency, the background noise temperature increases. Also, with the increase of the elevation angle from the horizon, background noise temperature increases.

8. The noise temperature of an antenna is given by the expression:
d) None of the mentioned

Explanation: The noise temperature of an antenna is given by the expression radTb + (1-rad) Tp. here, Tb is the brightness temperature and Tp is the physical temperature of the system. rad is the radiation efficiency. Noise temperature of a system depends on these factors.

9. Low is the G/T ratio of an antenna, higher is its efficiency.
a) True
b) False

Explanation: In the G/T ratio of an antenna, G is the gain of an antenna and T is the antenna noise temperature. Higher the G/T ratio of an antenna better is the performance of the antenna.

10._________ has a constant power spectral density.
a) White noise
b) Gaussian noise
c) Thermal noise
d) Shot noise

Explanation: Thermal noise has a power spectral density for a wide range of frequencies. Its plot of frequency v/s noise power is a straight line parallel to Y axis.

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