This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “Application of Parabolic Reflector”.
1. Which one of the following antennas is mostly used in TV Dish?
a) Parabolic reflector
b) Lens antenna
c) Log periodic
d) Rhombus antenna
Explanation: Parabolic reflector antenna also known as Dish antenna is widely used in the TV dishes. It is a high gain antenna used in VHF. Due to its narrow beamwidth it is widely used as dish antenna.
2. Which of the following antenna has a shape of letter ‘C’?
a) Orange peel antenna
b) Pill box
c) Yagi-Uda antenna
d) Wire dipole
Explanation: Orange peel antenna is one of the types of parabolic antenna with its shape in form of letter C. It is mainly used in the search radars. Pill box antenna gives wide and narrow beams in different planes. Yagi-Uda antenna is formed with the help of dipoles and reflectors placed at some distances to get a unidirectional beam with good directivity.
3. Which of the following statements is false about Array-fed antenna?
a) It is widely used to create a downlink radiation in satellite communication
b) An array of feed horns are clustered at focal point to produce arbitrary shaped beam
c) It uses only a single horn feed and a multiple array of passive reflectors
d) It can also be used with secondary reflector antennas
Explanation: Array-fed antenna uses a cluster of horn feeds instead of a single horn to produce an arbitrary beam. It is mainly used in direct satellite communication for downlink radiation. It is also used with secondary reflectors like Cassegrain.
4. Which type of feed in parabolic antenna blocks the beam limiting aperture efficiency to 55%?
a) Axial feed
b) Offset feed
Explanation: In Axial feed, the feed is located at the focal point along the beam axis infront of dish and is pointed back and blocks some part of beam. Offset is used to avoid the aperture blocking. In Cassegrain and Gregorian the aperture efficiency is approximately 70%.
5. Which of the following reflector antenna doesn’t require a directive feed?
Explanation: Corner reflector doesn’t require a directional feed because the direct and reflected waves are combined properly and also it doesn’t require any specific focal point. In Parabolic, hyperbolic the feed is directed towards the focal point.
6. In which of the following secondary reflector is concave?
Explanation: Cassegrain and Gregorian are dual reflector antennas having primary and secondary reflectors. In Gregorian concave ellipsoidal is used as secondary reflector whereas in Cassegrain convex hyperboloid. Corner and parabolic are reflector antennas and secondary reflectors are not used widely.
7. In which of the following secondary reflector is convex?
Explanation: Dual reflector antennas have both primary and secondary reflectors. Cassegrain and Gregorian antennas are dual reflector antennas. Secondary reflector is convex for Cassegrain antenna. In Gregorian concave ellipsoidal is used as secondary reflector. Parabolic and corner antennas are normal reflector antennas.
8. Cassegrain antennas cannot be used in radio telescopes.
Explanation: One of the advantages of Cassegrain antenna is that its feed is not placed in front of the dish or at the focal point. So, it can be used with bulky feeds which requires in radio telescopes and large satellite communications.
9. Which of the following antenna is a combination of pillbox and parabolic cylinder?
a) Cheese antenna
b) Orange peel antenna
c) Corner antenna
Explanation: A pillbox alone sometimes is called as a Cheese antenna as it is made from the parabolic cylinder. It is a thin slice of the parabolic cylinder cross-section covered with plates. Orange peel antenna consists of array of feeds and is C shaped. Cassegrain is a dual reflector antenna. Corner antenna is made from joining two plane sheet reflectors.
10. Which of the following is formed by moving parabolic counter parallel to itself?
a) Parabolic cylinder
b) Pill box
c) Cheese antenna
d) Truncated antenna
Explanation: A parabolic cylinder is formed by moving the parabolic counter parallel to itself. It consists of a line of focus unlike the focal point in basic parabolic antenna. Pillbox or cheese antenna is obtained from the parabolic cylinder reflector. Truncated is obtained by cutting some part of parabolic antenna.
11. Large f/D ratio is used for deep-dish reflectors.
Explanation: The ratio of focal length to the aperture size is known as the f/D ratio. Large f/D ratio is used for shallow-dish reflectors. Small f/D ratio is used for deep-dish reflectors. As the f/D ratio increases, the spillover efficiency of antenna reduces.
12. In which of the following feed there is no impedance mismatch?
a) Offset feed
b) Cassegrain feed
c) Front feed
d) Gregorian feed
Explanation: In Front feed, due to the reflections from the dishes impedance mismatch occurs. Cassegrain and Gregorian are front feed dual reflectors. Offset feed there is no aperture blocking and no impedance mismatch.
Sanfoundry Global Education & Learning Series – Antennas.
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