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The Cassegrain Antenna
Figure 1: Principle of a Cassegrain telescope
Sieur Guillaume Cassegrain was a French sculptor who invented a form of reflecting telescope. A Cassegrain telescope consists of primary and secondary reflecting mirrors. In a traditional reflecting telescope, light is reflected from the primary mirror up to the eye-piece and out the side the telescope body. In a Cassegrain telescope, there is a hole in the primary mirror. Light enters through the aperture to the primary mirror and is reflected back up to the secondary mirror. The viewer then peers through the hole in the primary reflecting mirror to see the image.
Figure 2: A Cassegray antenna used in a fire-control radar.
In telecommunication and radar use, a Cassegrain antenna is an antenna in which the feed radiator is mounted at or near the surface of a concave main reflector and is aimed at a convex subreflector. Both reflectors have a common focal point. Energy from the feed unit (a feed horn mostly) illuminates the secondary reflector, which reflects it back to the main reflector, which then forms the desired forward beam.
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Figure 3: Principle of a polarization changing plate
There is a possibility to avoid the disadvantage of the given shadow, however. E.G. it is practiced at the antenna of the tracking radar of the SkyGuard, manufactured in the company Oerlikon/Contraves AG. The subreflector reflects only horizontally polarised waves. The primary reflector reflects all waves. A plate is appropriate in front of the primary reflector. This plate consists of in an angle of 45° to the polarization level spanned wires each in a distance of λ /4. When the horizontally polarized wave penetrate this plate for the first time, it then becomes a circular polarized wave. On the way back this circular polarized wave then becomes a linearly polarized wave back, but this wave is vertically polarized now!
Figure 4: antennae of the tracking and command guidance radar for the SA-8 „Gecko”
The wave leaves the horn radiator therefore horizontally polarized. It is reflect at the subreflector therefore, reaches than the polarization changing plate, becomes a circular polarized wave and is reflected at the primary reflector. After passing the polarization changing plate secondly wave becomes from the circular polarized electric magnetic wave vertically polarized one now. This one can penetrate the subreflector and is emitted therefore vertically polarizedly.
The polarization changing plates you can see by the antennae of the tracking and command guidance radar system for the SA-8 „Gecko” surface-to-air missile. (NATO reporting name Land Roll.)
