This set of Reflector Antenna online quiz focuses on “Aperture Blockage”.
1. Which of the following is used to avoid the aperture blockage due to secondary reflector?
a) Offset feed system
c) Plane reflector
d) Parabolic antenna
Explanation: Offset feed system is used to avoid the aperture blocking effect due to the dependence of secondary reflector. In this a feed radiator is placed at the focus in an inclined position such that all rays are collimated without formation of blockage region. In Cassegrain, plane reflector there is a blocking effect.
2. Which of the following causes aperture blockage in a Cassegrain antenna?
a) Main reflector
c) Feed radiator
d) Noise from surroundings
Explanation: The sub-reflector causes the aperture blockage in a Cassegrain antenna. It can be reduced by reducing the size of the sub-reflector.
3. In which of the following the secondary reflector faces the primary reflector?
c) Corner reflector
d) Plane reflector
Explanation: In Gregorian antenna, sub-reflector is ellipsoidal and has its concave side faces the primary reflector. In Cassegrain, sub-reflector is hyperboloid with its convex side facing the feed. Corner and plane reflectors are normal reflector antennas while Cassegrain and Gregorian are dual reflector antennas.
4. Which of the following statement is true regarding a Gregorian antenna?
a) Sub-reflector ellipsoidal lies closer to the focus of the paraboloid
b) Sub-reflector is a hyperboloid with its convex facing feed
c) Sub-reflector is an ellipsoidal with its concave facing reflector
d) Sub-reflector is an ellipsoidal with its concave facing feed
Explanation: Gregorian antenna is a dual reflector antenna having a concave secondary reflector. The secondary reflector also known as sub-reflector is a ellipsoidal. It lies beyond the focus of the paraboloid.
5. In which of the following the aperture blocking is reduced by permitting antenna to operate in a single polarization?
a) Truncated reflector
c) Polarization-twist reflector
d) Plane reflector
Explanation: Aperture blocking is reduced by permitting the antenna to operate with a single polarization in Polarization-twist reflector. Truncated reflector is a type of parabolic reflector. Cassegrain is a dual reflector.
6. In which of the following a sub-reflector called trans-reflector is present?
a) Polarization-twist reflector
b) Plane reflector
Explanation: It is present in Polarization-twist reflector. Sub-reflector consists of a horizontal grating of wires. Cassegrain contains a hyperboloid sub-reflector. Gregorian contains an ellipsoidal sub-reflector.
7. Which of the following is not used for aperture blocking?
a) Reducing size of sub-reflector
b) Offset feed system
c) High directive feed
d) Reducing spillover
Explanation: A high directive feed means a large feed. It partially shadows the primary reflector and becomes an obstacle for blockage. Aperture blocking can be avoided by reducing size of sub-reflector, thereby spillover and by using Offset feed system.
8. Which of the following condition holds good for minimum total aperture blocking in Cassegrain?
a) Area of sub-reflector and projected area of feed are equal
b) Area of primary reflector and projected area of feed are equal
c) Area of sub-reflector greater than area of primary reflector
d) Area of sub-reflector is less than projected area of feed
Explanation: Minimum total aperture blocking occurs when the area of sub-reflector and projected area of feed are approximately equal. In other conditions, the main ray back radiated ray gets added up with collinear rays from the primary reflector.
9. In Polarization-twist reflector, sub-reflector consists of a horizontal grating of wires.
Explanation: In Polarization-twist reflector, sub-reflector consists of a horizontal grating of wires called trans-reflector. In this the antenna is permitted to operate only at a single polarization. The polarized radiation is rotated by 90° by sub-reflector at the primary reflector with the help of twist reflector.
Sanfoundry Global Education & Learning Series – Antennas.
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