Antennas Multiple Choice Questions on “Feeding Systems”.
1. Which of the following is called as the source placed at the focus?
a) Feed radiator
b) Reflector
c) Secondary radiator
d) Primary / reflector
Answer: a
Clarification: The source placed at the focus is called as feed radiator. It is also known as primary radiator. The reflector is called as the secondary radiator. A reflector antenna contains primary and secondary radiators.
2. In which of the following cases, a parabolic reflector primary radiator is said to be ideal feed?
a) The entire reflector is illuminated with no radiation in unwanted direction when feed radiated entire energy towards it
b) Some part of reflector is illuminated with no radiation in unwanted direction when feed radiated entire energy towards it
c) Some part of reflector is illuminated with radiation in unwanted direction when feed radiated entire energy towards it
d) The entire reflector is illuminated with small radiation in unwanted direction when feed radiated entire energy towards it
Answer: a
Clarification: A parabolic reflector primary radiator is said to be ideal feed, when the feed radiates entire energy, the reflector is fully illuminated and there is no energy radiation in unwanted direction.
3. To obtain maximum beam pattern, the primary radiator is placed ______ in a reflector antenna.
a) between Focus and Directrix
b) after the focus
c) at the focus point
d) can be placed anywhere
Answer: c
Clarification: Maximum beam pattern is obtained only when the feed is placed at the focus of the parabola. Reflector antenna is a high gain antenna. So this is one of the important points to obtain the maximized gain.
4. When the feed is moved along the main axis in a reflector antenna what happens to the beam pattern?
a) It broadens
b) It deteriorates
c) Beam remains unchanged
d) Side lobes are increased
Answer: a
Clarification: Maximum beam pattern is obtained only when the feed is placed at the focus of the parabola. When the feed is moved along the main axis, beam gets broadened.
5. The beam gets deteriorated when it is moved along a line perpendicular to the main axis passing through focus?
a) True
b) False
Answer: a
Clarification: Beam pattern is maximum only when the feed is placed at the focus of the parabola. When the feed is moved along the main axis, beam gets broadened. When it is moved along perpendicular line beam gets deteriorated.
6. In a Cassegrain feed system; the feed is placed at _____
a) Focus
b) Vertex
c) Directrix
d) Anywhere between vertex and focus
Answer: b
Clarification: Cassegrain is a dual reflector antenna. In this the feed is placed at the vertex of the parabolic reflector. The focus of the sub-reflector coincides with the focus of parabolic reflector and it illuminates the reflector.
7. Which of the following coincides with the focus of the parabolic reflector in a Cassegrain antenna?
a) Feed
b) Parabolic reflector
c) Focus of hyperboloid reflector
d) Focus of primary radiator
Answer: c
Clarification: The focus of the sub-reflector coincides with the focus of parabolic reflector and it illuminates the reflector. Since it is a Cassegrain, the sub-reflector is a hyperboloid reflector.
8. Which of the following regarding the Cassegrain feed system is false?
a) Spill over is reduced
b) It is a dual reflector antenna
c) Minor lobe radiation increases
d) A convex sub-reflector is used
Answer: c
Clarification: Spill over is the power loss when the reflector fails to redirect the energy. With the use of sub-reflector, this is reduced. Thus the minor lobe radiation also decreased. A convex hyperboloid is used as a sub-reflector. It is a dual reflector antenna with primary and two secondary radiators.
9. When a dipole with a parasitic reflector is used as a feed system, the distance between them is _____
a) 0.125λ
b) 0.4λ
c) 1λ
d) 0.625λ
Answer: a
Clarification: In a parabolic reflector antenna, a dipole along a parasitic reflector serves well as a feed radiator. The distance between parasitic reflector and the dipole is 0.125λ. The distance between dipole to dipole is approximately 0.4λ.
10. Which of the following uses a concave sub-reflector in a dual reflector antenna?
a) Cassegrain
b) Gregorian
c) Both Gregorian &Cassegrain
d) Neither Gregorian nor Cassegrain
Answer: b
Clarification: Cassegrain and Gregorian are dual reflector antennas. Gregorian antenna has a concave sub-reflector which is elliptic. Cassegrain uses a hyperboloid reflector which is convex towards the feed. The sub-reflectors are used to reduce the spillover efficiency.