Cassegrain antenna

In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes, and communication satellites.
The primary reflector is a paraboloid, while the shape of the convex secondary reflector is a hyperboloid. The geometrical condition for radiating a collimated, plane wave beam is that the feed antenna is located at the far focus of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid. Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However, in offset Cassegrain configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam.
This design is an alternative to the most common parabolic antenna design, called "front feed" or "prime focus", in which the feed antenna itself is mounted suspended in front of the dish at the focus, pointed back toward the dish. The Cassegrain is a more complex design, but in certain applications it has advantages over front feed that can justify its increased complexity:
A disadvantage of the Cassegrain is that the feed horn(s) must have a narrower beamwidth (higher gain) to focus its radiation on the smaller secondary reflector, instead of the wider primary reflector as in front-fed dishes. The angular width the secondary reflector subtends at the feed horn is typically 10–15°, as opposed to 120–180° the main reflector subtends in a front-fed dish. Therefore, the feed horn must be longer for a given wavelength.
A beam waveguide antenna is a type of complicated Cassegrain antenna with a long radio wave path to allow the feed electronics to be located at ground level. It is used in very large steerable radio telescopes and satellite ground antennas, where the feed electronics are too complicated and bulky, or requires too much maintenance and alterations, to locate on the dish; for example those using cryogenically cooled amplifiers. The beam of incoming radio waves from the secondary reflector is reflected by additional mirrors in a long twisting path through the axes of the altazimuth mount, so the antenna can be steered without interrupting the beam, and then down through the antenna tower to a feed building at ground level.
The Cassegrain antenna design was adapted from the Cassegrain telescope, a type of reflecting telescope developed around 1672 and attributed to French Province England priest Laurent Cassegrain. The first Cassegrain antenna was invented and patented by Cochrane and Whitehead at Elliot Bros in Borehamwood, England, in 1952. The patent, British Patent Number 700868, was subsequently challenged in court, but prevailed. The Voyager 1 spacecraft launched in 1977 is, as of September 2024, 24.6 billion kilometers from Earth, the furthest manmade object in space, and it's 3.7 meter S and X-band Cassegrain antenna (picture below) is still able to communicate with ground stations.