Background
The idea of using parabolic reflectors for radio antennas was taken from optics, where the power of a parabolic mirror to focus light into a beam has been known between the 8th century BC and 6th century AD. The designs of some specific types of parabolic antenna, such as the Cassegrain and Gregorian, come from similarly named analog types of reflecting telescope, which was invented by astronomers in the 15th century.
Heinrich Hertz constructed the world’s first parabolic reflector antenna in 1888. The antenna had a diameter of 1.2 meters, a focal distance of 0.12 meters, and was used at an operating frequency of approximately 450 MHz. The reflector was made of a zinc metal sheet supported by a wooden frame and had a spark-gap excited dipole along the focal line. With two such antennas, one used for transmitting the signal and the other for receiving, Hertz successfully demonstrated the existence of electromagnetic waves which had been predicted by James Clerk Maxwell some 22 years earlier.
The first parabolic antenna used for satellite communications was established in 1962 at Goonhilly in Cornwall, England, UK, and was intended to communicate with Telstar.
Cassegrain Antennas
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.
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.
Gregorian Antennas
A Gregorian antenna is an antenna with a secondary sub-reflector in front of a concave primary mirror, which is usually a paraboloid. The Gregory antenna is designed like the Cassegrain antenna. In the Gregorian antenna, the radiator is located behind the primary focus. Because the sub-reflector is an ellipsoidal concave, it has two focal points between the two reflectors. One focal point of this ellipsoid coincides with the primary focus of the paraboloid. The best position of the primary radiator is the second focal point of the ellipsoid.
The Gregorian has very similar uses, advantages, and disadvantages to the Cassegrain, but it is less compact so less often used when volume considerations are high.
Axial or Front Feed Antennas
The parabolic reflector or dish antenna consists of a radiating element which may be a simple dipole or a waveguide horn antenna. This is placed at the focal point of the parabolic reflecting surface. The energy from the radiating element is arranged so that it illuminates the reflecting surface. Once the energy is reflected it leaves the antenna system in a narrow beam. As a result, considerable levels of gain can be achieved.
Achieving this is not always easy because it is dependent upon the radiator that is used. For lower frequencies, a dipole element is often employed whereas at higher frequencies a circular waveguide may be used. Circular waveguides provide one of the optimum sources of illumination.
The focal feed system is one of the most widely used feed systems for larger parabolic reflector antennas as it is straightforward. The major disadvantage is that the feed and its supports block some of the beams, and this typically limits the aperture efficiency to only about 55 to 60%.
Off Set Feed Antennas
An offset dish antenna or off-axis dish antenna is a type of parabolic antenna. It is so-called because the antenna feed is offset to the side of the reflector, in contrast to the common “front-feed” parabolic antenna where the feed antenna is suspended in front of the dish, on its axis. As in a front-fed parabolic dish, the feed is located at the focal point of the reflector, but the reflector is an asymmetric segment of a paraboloid, so the focus is located to the side.
The design is most widely used for small parabolic antennas or “mini-dishes”, where the feed structure is large enough in relation to the dish to block a significant proportion of the signal. Another application is on satellites, particularly the direct broadcast satellites which use parabolic dishes to beam television signals to homes on Earth. Because of the limited transmitter power provided by their solar cells, satellite antennas must function as efficiently as possible. The offset design is also widely used in radar antennas.
PALS Product Focus
PALS and DNI Ltd. have forged a close partnership to bring their products to a global market. With their shared values, integrity, and flexibility, PALS and DNI provide a Professional and cost-effective approach to the global satellite communications market. All PALS OEM products are designed and manufactured in Turkey
PFA-370 Transportable Antenna
The PFA-370 is designed as a portable 3.7m antenna system. It can operate on C, X, Ku, Ka bands and in different configurations. The reflector structure consists of lightweight carbon fiber panels. Feed options include RxO, Tx/Rx, 2-port, or multiple ports with either circular or linear polarisation according to feed options. Customized design and different feed arm options are possible to provide interchangeable feeds. The reflector is assembled with 19 carbon fiber panels. The antenna is designed to be lightweight and modular with robust components.
The PFA-370 supports manual, auto, and one-button capture satellites within 5 minutes. Its feed structure assures high gain, low sidelobe, and high RF performance.
The PFA-370’s motorized version can be used with PALS PAC550 military-type antenna controller. It is provided with a GPS system, position detection system, polarisation adjustment system, servo drive system, and high-performance satellite beacon receiving system.
