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Right answer is (d) Decreases the gain in FORWARD direction

The BEST EXPLANATION: A plane or a flat reflector is used to radiate energy in desired direction by placing a feed in front of it. It REDUCES the backward radiation and increases the gain in the forward direction. A reflector ANTENNA is a high gain antenna.

Correct answer is (a) Cassegrain ANTENNA

For explanation: A dual reflector antenna CONSISTS of two reflectors and ONE feed antenna. Cassegrain antenna is the best example of dual reflector antenna SINCE it contains main reflector as PARABOLIC and sub-reflector as hyperbola.

Right option is (a) INCREASED

For explanation I would say: The SYSTEM EFFICIENCY in a corner reflector depends on the spacing between VERTEX of the corner and the feed.It is adjusted depending on the included angle. As the included decreases, spacing must be increased.

Right ANSWER is (a) Orange peel antenna

To elaborate: 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.

Right choice is (c) FOCUS of HYPERBOLOID reflector

The explanation: 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.

The CORRECT CHOICE is (a) 5

Easy EXPLANATION: Aperture RATIO \(= \frac{d}{λ}\)

\(λ=\frac{c}{f}=\frac{3×10^8}{1.5×10^9}=0.2m\)

Aperture ratio\( = \frac{1}{0.2}=5.\)

Correct option is (c) Both included ANGLE & Polarization of feed element

The best EXPLANATION: The number of images, polarity and POSITION in the analysis of the radiation field of corner reflector DEPENDS on both included angle & Polarization of feed element with perpendicular polarization. Angle between the reflecting plates is called included angle.

Correct choice is (a) 90°

For EXPLANATION: For a square reflector angle N=2 and corner angle is GIVEN by \(α=\FRAC{π}{N}=\frac{π}{2}=90°.\) A corner reflector consists of two plane reflectors joined at a corner to allow RADIATION in forward direction only. This is one of the most practically used antennas.

Correct answer is (a) True

Best explanation: The PLANE REFLECTOR allows back and side radiation. In ORDER to avoid this it is modifies to a corner reflector which consists of two plane reflectors joined at a corner to allow radiation in forward direction only.

The correct answer is (a) Gregorian

For explanation I would say: 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.

The correct answer is (a) PARABOLIC reflector

The best 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.

Right option is (c) Sub-REFLECTOR is an ellipsoidal with its concave facing reflector

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.

Right option is (a) True

The explanation is: 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.

Right answer is (B) 180°

The BEST explanation: The CORNER angle DEPENDS on value of N (CURRENT element).For a flat reflector N=1. So the corner angle is \(α=\frac{π}{N}=π=180°.\)

Correct option is (B) Back radiation is not reduced compare to the PLANE reflector

Explanation: Compared to the plane reflector antenna, back and side radiations are reduced in the corner reflector. A corner reflector consists of two plane reflectors joined at a corner to allow radiation in forward direction only. As the included angle DECREASES, spacing will INCREASES.

The CORRECT answer is (a) Spillover

The best explanation: When the feed PATTERN exceeds beyond reflectors rim, not all the energy is redirected by the reflector. This power loss is MEASURE results in spillover efficiency. Illumination efficiency is the combination of both taper and spillover efficiency. APERTURE efficiency is the ratio of effective aperture to physical aperture. Taper efficiency gives the directivity measure of the ANTENNA.

Right OPTION is (a) The main reflector is PARABOLIC and sub-reflector is hyperbolic

Explanation: A Cassegrain antenna is a dual reflector antenna and is axis symmetry. It consists of two REFLECTORS and one PRIMARY feed. The main reflector is parabolic and sub-reflector is hyperbolic (convex).

Right option is (a) OFFSET feed

The best I can EXPLAIN: 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.

Correct choice is (a) Area of sub-reflector and projected area of feed are equal

The EXPLANATION is: 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.

The correct choice is (c) High directive feed

To EXPLAIN: 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.

Correct option is (b) Gregorian

Easiest 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.

Correct answer is (a) Feed RADIATOR

Explanation: 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.

Right option is (a) \(\frac{140λ}{d} \)

Easy EXPLANATION: A paraboloid antenna with a circular APERTURE has

BWFN (Beam width between two FIRST NULLS) =\(\frac{140λ}{d}, \) (d is DIAMETER of circular aperture)

For a rectangular aperture \(BWFN =\frac{115λ}{L}, \) (L is length of rectangular aperture).

Correct answer is (a) Pill BOX

To explain: Pill box is SHORT parabolic right cylinder ENCLOSED by parallel PLATES. It produces a wide beam in one plane and narrow beam in other plane. Cassegrain is a dual REFLECTOR. Truncated is a type of paraboloid.

Right choice is (a) Cheese 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.

The CORRECT choice is (a) Spill over DECREASES DUE to back lobe of primary RADIATOR

Easiest explanation: Spill over occurs due to the non-captured radiation by the reflector. From the primary radiators also some of forward radiation gets added up with the desired PARALLEL beams. This is called back lobe radiation. It increases due to the back lobe of the primary radiator.

The correct option is (a) Convex-convex

For explanation I would SAY: Corner, Cassegrain and GREGORIAN belong to reflector antenna. Convex-convex is a type of LENS antenna. Gregorian is a DUAL reflector antenna with a concave sub-reflector.

The correct answer is (a) 1.414

To ELABORATE: From the design equations of a corner reflector,

Aperture dimension DA= 1.414l

Where l is side length of reflector sheet and l=2d where d is distance between FEED and VERTEX of the reflector.

The correct option is (a) 0.125λ

Easy EXPLANATION: 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.

Correct option is (c) CORNER

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.

The correct answer is (a) It broadens

To explain: 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.

Right choice is (a) 14 DEGREES

For EXPLANATION: A paraboloid antenna with a circular APERTURE has

BWFN (Beam WIDTH between two FIRST nulls) = \(\frac{140λ}{d}, \) (d is diameter of circular aperture)

⇨ \(BWFN =\frac{140λ}{10λ}=14 degrees.\)

Right choice is (a) DUAL reflector antenna

For explanation: Usually a FEED FORWARD is used in reflector antenna and feed antenna is PLACED at focus of the reflector. When a sub-reflector is placed at the focus along with the feed antenna it is called dual reflector antenna. Example of dual reflector antenna: Cassegrain antenna.

Convex-Convex is a lens antenna.

The correct OPTION is (a) Truncated paraboloid

To elaborate: When a portion of the paraboloid reflector is cut off or truncated it is called as truncated paraboloid. PILL box provides WIDE BEAM in one plane and narrow beam in other plane. Cassegrain is a dual reflector ANTENNA and Corner reflector is also a type of reflector antenna.

The correct answer is (a) 0.25 to 0.5

Best EXPLANATION: The ratio of FOCAL length (f) to diameter (d) f/d < \(\frac{1}{4}\) indicates radiation away from the parabolic surface of the reflector.So for practical applications it LIES between 0.125 and 0.5.

Right option is (a) Axial feed

For explanation I would say: 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%.

The correct choice is (c) POLARIZATION-twist 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.

Correct answer is (b) Spherical to plane

To EXPLAIN I WOULD say: In a parabolic REFLECTOR antenna, the wave conversion mechanism used is spherical to plane. It works on the principle of geometric optics. The reflected plane waves TRAVEL parallel o the major AXIS of the reflector.

The CORRECT OPTION is (b) SECONDARY PATTERN

Easy explanation: In a REFLECTOR antenna, primary pattern is the feed pattern and secondary pattern is the pattern of reflector. Reflector antennas are high gain antenna and are used in RADARs and for some communication purpose.