Microwave Engineering Multiple Choice Questions on “Antenna Radiation”.
1. An antenna source that radiates energy uniformly in all the directions is called:
A. Isotropic source
B. Anisotropic source
C. Point source
D. None of the mentioned
Answer: A
Clarification: Isotropic source radiates energy in all the direction uniformly. For such a source, the radial component Sr of the pointing vector is independent of θ and φ. The three dimensional power pattern of n isotropic source is a sphere.
2. Antennas that radiate energy only in a specified are called anisotropic antennas.
A. True
B. False
Answer: A
Clarification: All physically realizable, simplest antennas also have directional properties. That is, they radiate energy in one direction than in any other direction. Such sources are called anisotropic point sources.
3. The expression for pointing vector of an isotropic point source at a distance ‘r’ from the source is given by:
A. P/ 4πR2
B. P/4π
C. P/ 4πR
D. P×4πR2
Answer: A
Clarification: The pointing field vector for an isotropic source is given by the expression P/ 4πR2.P is the total power radiated y the source. As the distance of the point from the source increases, the magnitude of pointing vector decreases.
4. A source has a cosine radiation-intensity pattern given by U=UM cos (θ). The directivity of this source is:
A. 2
B. 4
C. 6
D. 8
Answer: B
Clarification: To find the directivity of the given source, the power radiated by the given source is found out by the method of integration. Taking the ratio of the power radiated by the given source to the power radiated by an isotropic source gives the directivity. Following the above steps, the directivity of the given source is 4.
5. A source has a cosine power pattern that is bidirectional. Given that the directivity of a unidirectional source with cosine power pattern has a directivity of 4, then the directivity of the unidirectional source is:
A. 1
B. 2
C. 4
D. 8
Answer: B
Clarification: Given the directivity of unidirectional power pattern, the directivity of bidirectional power pattern is half of it. Hence the directivity of the source is 2.
6. A source has a radiation intensity pattern given by U=UM sin θ. The directivity of the source with this power pattern is:
A. 1
B. 1.27
C. 2.4
D. 3.4
Answer: B
Clarification: To find the directivity of the given source, the power radiated by the given source is found out by the method of integration. Taking the ratio of the power radiated by the given source to the power radiated by an isotropic source gives the directivity. Following the above steps, the directivity of the given source is 1.27.
7. A source has a sine squared radiation intensity power pattern. The directivity of the given source is:
A. 1.5
B. 3
C. 2.5
D. 3.5
Answer: A
Clarification: To find the directivity of the given source, the power radiated by the given source is found out by the method of integration. Taking the ratio of the power radiated by the given source to the power radiated by an isotropic source gives the directivity. Following the above steps, the directivity of the given source is 1.5.
8. A source with a unidirectional cosine squared radiation intensity pattern is given by UMcos2 (θ). The directivity of the given source is:
A. 6
B. 8
C. 2
D. 7
Answer: A
Clarification: To find the directivity of the given source, the power radiated by the given source is found out by the method of integration. Taking the ratio of the power radiated by the given source to the power radiated by anisotropic source gives the directivity. Following the above steps, the directivity of the given source is 6.
9. Considering distance as a parameter, two types of field zones can be defined around an antenna.
A. True
B. False
Answer: A
Clarification: Considering distance as a parameter, two types of field zones can be defined around an antennA. .The field near the antenna is called near field or Fresnel region and the other region is the far field that is also called as Fraunhofer region.
10. If the field strength at receiving antenna is 1 µV/m, and the effective aperture area is 0.4 m2 and the intrinsic impedance of the medium is 377 Ω, then the power received by the antenna is:
A. 1.06 pW
B. 1.06 fW
C. 2 µW
D. None of the mentioned
Answer: B
Clarification: The received power by the antenna is given by E2Ae/Zₒ. Substituting the known values in the above equation, the power received is 1.06×10-15 watts.
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