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A Cassegrain reflector antenna has a sub-reflector whose geometrical shape is:

A Cassegrain reflector antenna has a sub-reflector whose geometrical shape is: Correct Answer Hyperboloid

Explanation:

Cassegrain reflector antenna

The Cassegrain feed, which is shown in Figure, has a dual reflector.

  • The main reflector is a parabola
  • The secondary reflector is a hyperbola, and the feed is placed along the axis of the parabola at or near the vertex.


The rays that emanate from the feed illuminate the secondary reflector, which is located at the focal point of the paraboloid.

The rays are then reflected by the primary reflector and are converted to parallel rays.

The Cassegrain feed arrangement is much easier for servicing and adjustment since both the transmitting and receiving equipment can be located behind the primary reflector.

The dual reflector Cassegrain antenna

Conclusion:

A Cassegrain reflector antenna has a sub-reflector whose geometrical shape is Hyperboloid.

Extra concept:

Parabolic Reflectors

  • Geometrical optics shows that if a beam of parallel rays are incident on a reflector antenna whose geometrical shape is a parabola, the array beams will converge at a spot known as the focal point.
  • In the same manner, a radiating feed point at the focus of the parabola will produce rays that, when bounced off from the parabola, will travel in a parallel beam.


Two examples of parabolic reflectors are shown in Figure.

[ alt="F1 Tapesh 14.12.20 Pallavi D6" src="//storage.googleapis.com/tb-img/production/20/12/F1_Tapesh_14.12.20_Pallavi_D6.png" style="width: 290px; height: 155px;">

The feed point (a dipole and a horn) is located at the focal point of the parabola; such antennas are known as front-fed.

This arrangement, though typical, can use long transmission lines, with the accompanying losses. 

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