InterviewSolution

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51.	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 CassegrainThe question was posed to me by my school teacher while I was bunking the class.My doubt is from Feeding Systems topic in portion Reflector Antenna of Antennas
Answer» Right option is (b) GREGORIAN Easy EXPLANATION: 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.

52.	In a Cassegrain feed system; the feed is placed at _____(a) Focus(b) Vertex(c) Directrix(d) Anywhere between vertex and focusThis question was addressed to me in an interview for internship.Asked question is from Feeding Systems in division Reflector Antenna of Antennas
Answer» Correct ANSWER is (b) VERTEX The best explanation: 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.

53.	Which of the following is formed by moving parabolic counter parallel to itself?(a) Parabolic cylinder(b) Pill box(c) Cheese antenna(d) Truncated antennaThis question was addressed to me in a job interview.My question comes from Application of Parabolic Reflector in division Reflector Antenna of Antennas
Answer» Right answer is (a) Parabolic CYLINDER The 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.

54.	Cassegrain antennas cannot be used in radio telescopes.(a) True(b) FalseThe question was asked in final exam.Query is from Application of Parabolic Reflector in division Reflector Antenna of Antennas
Answer» The CORRECT option is (b) False The 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.

55.	In which of the following a sub-reflector called trans-reflector is present?(a) Polarization-twist reflector(b) Plane reflector(c) Cassegrain(d) GregorianThis question was posed to me in an online interview.My doubt is from Aperture Blockage topic in portion Reflector Antenna of Antennas
Answer» The CORRECT OPTION is (a) Polarization-twist reflector Best 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.

56.	The beam gets deteriorated when it is moved along a line perpendicular to the main axis passing through focus?(a) True(b) FalseThe question was asked in unit test.Enquiry is from Feeding Systems in section Reflector Antenna of Antennas
Answer» RIGHT option is (a) True Explanation: 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.

57.	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 anywhereThe question was posed to me during an online interview.I'd like to ask this question from Feeding Systems in portion Reflector Antenna of Antennas
Answer» The CORRECT option is (c) at the focus point Explanation: 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.

58.	In a paraboloid antenna, all rays leaving the focal point are collimated along the reflector’s axis after reflection.(a) True(b) FalseThis question was posed to me in final exam.The question is from Parabolic Reflector Antenna topic in section Reflector Antenna of Antennas
Answer» Correct option is (a) True Best explanation: According to the geometry of the paraboloid reflector, ⇨ All rays leaving the focal point are collimated ALONG the reflector’s axis after reflection. ⇨ All OVERALL ray path LENGTHS (from the focal point to the reflector and on to the aperture PLANE) are the same and equal to 2F.

59.	In a corner reflector antenna, if the spacing between vertex of reflector and feed is 2m then side length of reflector sheet is _____cm.(a) 4(b) 2(c) 400(d) 200This question was addressed to me by my school teacher while I was bunking the class.Enquiry is from Corner Reflector in chapter Reflector Antenna of Antennas
Answer» The correct answer is (C) 400 For explanation I would say: In a CORNER REFLECTOR antenna, side LENGTH of reflector sheet and l=2d Where, d is DISTANCE between feed and vertex of the reflector. Given d=2m so l=2*2=4m=400cm.

60.	In a corner reflector, included angle α refers to _______(a) angle at which two plane reflectors are joined(b) angle between vertex and the feed radiator(c) angle between major axis to the main beam(d) angle between vertex and the main beam axisThis question was addressed to me by my college professor while I was bunking the class.The origin of the question is Corner Reflector in division Reflector Antenna of Antennas
Answer» Correct answer is (a) angle at which TWO PLANE reflectors are JOINED To explain I WOULD say: A CORNER reflector consists of two plane reflectors joined at a corner to allow radiation in forward direction only. Included angle is the angle at which two reflectors are joined, measured at the corner.

61.	Reflector antenna operates on the Geometric optics principle?(a) True(b) FalseThis question was posed to me during an online interview.My question comes from Basics in chapter Reflector Antenna of Antennas
Answer» RIGHT ANSWER is (a) True The best I can explain: Reflector antenna is high gain antenna and works on the principle of the Geometric optics. Geometrical optics shows that if a beam of PARALLEL rays is incident on a reflector antenna whose geometrical SHAPE is a parabola, then the array BEAMS will converge at the focal point.

62.	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 antennasThe question was asked in an international level competition.I want to ask this question from Application of Parabolic Reflector topic in division Reflector Antenna of Antennas
Answer» CORRECT choice is (c) It uses only a SINGLE horn feed and a multiple array of passive reflectors The 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.

63.	What is the value of magnification of the Cassegrain antenna if its sub-reflector eccentricity is 2?(a) 3(b) 2(c) 1/3(d) 1/2The question was posed to me during an interview.My question is based upon Basics in division Reflector Antenna of Antennas
Answer» RIGHT CHOICE is (a) 3 Easy EXPLANATION: Given ECCENTRICITY e=2 Magnification \(M=\frac{e+1}{e-1}=3 \)

64.	Which of the following is false regarding a reflector antenna?(a) Reflector antennas are high gain antennas with two antennas(b) Both the primary and secondary antennas are excited(c) The pattern of the reflector in the reflector antenna is the Secondary pattern(d) A dual reflector contains two reflectors and one primary feedI had been asked this question during an online exam.Query is from Basics in portion Reflector Antenna of Antennas
Answer» Correct choice is (b) Both the primary and SECONDARY antennas are excited The explanation: In a reflector antenna the HORN or a dipole acts as a feed and the antenna which is excited is the primary antenna. Reflector is the secondary antenna. Reflector antenna is a HIGH GAIN antenna consisting of primary and secondary antennas.