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The Cassegrain Antenna

Figure 1: Principle of a Cassegrain antenna

Figure 1: Principle of a Cassegrain antenna

The Cassegrain Antenna

Cassegrain antennas (pronunciation: kasgʀɛn) are antennas, which were constructed in the style of a mirror telescope of the same name.

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 reflects from the primary mirror up to the eye-piece and out the side of 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 up to the secondary mirror. The viewer then peers through the hole in the primary reflecting mirror to see the image.

Figure 2: A Cassegrain antenna used in a fire-control radar.

A Cassegrain antenna used in a fire-control radar.

Figure 2: A Cassegrain antenna used in a fire-control radar.

In telecommunication and radar use, a Cassegrain antenna is an antenna in which the feed radiator is at or near the surface of a concave paraboloidal main reflector and directs to a convex hyperboloidal sub-reflector. Both reflectors have a common focal point. Energy from the feed (a horn mostly) illuminates the secondary reflector, which reflects it to the main reflector, which then forms the desired forward beam.

Advantages
  • The feed radiator is more easily supported, and the antenna is geometrically compact.
  • It provides minimum losses as the receiver can be mounted directly near the horn.
Disadvantage:
  • The sub-reflectors of a Cassegrain type antenna are fixed by bars. These bars and the secondary reflector constitute an obstacle for the rays coming from the primary reflector in the most effective direction.

left-hand circular
right-hand circular
linear
horizontal
linear vertical

Figure 3: Principle of a polarization changing plate

left-hand circular
right-hand circular
linear
horizontal
linear vertical

Figure 3: Principle of a polarization changing plate

However, there is a possibility to avoid the disadvantage of the shadow, as it is practiced for example with the antenna of the tracking radar of the SkyGuard of Oerlikon/Contraves AG. The secondary reflector reflects only horizontally polarized waves, the primary reflector with its metallic surface reflects all electromagnetic waves. A special feature, however, is that when a circularly polarized wave is reflected, it has a reversed direction of rotation after reflection.

Land Roll 
(click to enlarge: 410·496px = 49 kByte)

Figure 4: antennas of the tracking and command guidance radar for the SA-8 “Gecko”

A plate is mounted in front of the primary reflector as a secondary reflector. This consists of two different planes: a polarity-changing layer of flat but wide lamellas, which are positioned at a distance of 45° from the polarisation plane at λ/4. This plate turns the left-rotating circularly polarized wave into a linearly horizontally polarized one when it is first penetrated. As a second plane, thin horizontal wires are stretched which act as reflectors for linear-horizontally polarized waves. The second penetration turns the clockwise circularly polarized wave into a linear-vertically polarized wave for which the secondary reflector is no obstacle.

The wave leaves the horn radiator, for example, left-rotating circularly polarized, the first time it penetrates the polarization-changing plate it produces a horizontally linearly polarized wave, which is reflected at the secondary reflector. The same plate is now crossed from the opposite side, the plate is now mirrored for the wave. So the plate for the wave looks as if it was rotated by 90°! In this case, the wave is returned to exactly the polarization with which it arrived, i.e. left-handed circular polarized.

At the primary reflector, the wave changes its direction of rotation during reflection and again penetrates the polarization-changing plate, where it now becomes a linear-vertically polarized wave. This wave can penetrate the secondary radiator from the thin wires and is thus radiated vertically polarized.

In reception mode, the inverse path is followed.

The polarization changing plates you can see by the antennas of SA-8 “Gecko” (NATO reporting name “Land Roll”). This is a tracking and command guidance radar system for surface-to-air missiles.


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