I like all antennas, but if I had to pick a special favourite it would definitely be the reflector antenna. I think it is so amazing how they can just move the radio frequency waves (RF) around in space using a shaped lump of metal and manipulate it into something useful. They are also the antennas I have spent the most amount of time designing, getting manufactured, testing and interpreting the results of. However, they are probably not the first antenna that comes to mind for most people, so read on and learn more about them.
A reflector antenna is an antenna which has a feed that transmits and receives RF and RF surfaces that reflect electromagnetic radiation. Reflector antennas are reciprocal, so they work in either transmit or receive. To collect enough signal the size of the collecting surface, the primary reflector, should be much larger than the wavelength you are trying to detect. Because they work well over a large range of the electromagnetic spectrum reflector antennas come in all shapes and sizes. Reflector antennas give you high gains, and therefore narrow beamwidths so are often used for communication. There are many times of reflector antennas: Front fed, Offset feed, Gregorian, Cassegrain and Twist Reflector to name the most common. It is also interesting that you can design the feed and the reflective surfaces separately, as long as the relative positions of the construction is known.
These types of antennas lend themselves to activities that require beam scanning. This is, as it sounds, the ability to move the beam of the antenna around in space. This can be achieved by either moving the antenna as a whole, or moving the primary reflector in the antenna itself.
Reflector antennas are very versatile and used in a huge range of different applications: radio astronomy, radar, airport security, telecommunications, space etc.
FRONT FED ANTENNA
A front fed antenna has its feed located at the focus of the primary parabolic dish. This is a common type of antenna and can be seen in most radio telescopes, radar, satellite communication dishes and older home satellite dishes. They are high gain antennas and used for point to point communications. Depending on the frequencies used the dish can be made from sheet metal or wire grids.
Putting the feed at the focus causes a blockage in the signal from the feed and its support struts, so this kind of antenna can only be used if this additional loss can be tolerated. Beam scanning is achieved by moving the entire antenna keeping feed and dish in registration.
OFFSET FED ANTENNA
This antenna, as the name suggests, is very similar to a front fed antenna but it has the feed moved away from the centre of the dish. The reflector dish is an asymmetrical segment of a parabola which sends the focus away from the centre of the dish, to where the feed is located.
This helps to remove the blockage problem found in front fed antennas and can be commonly seen on the side of your house in new home satellite dishes and in airport surveillance radar. There is more complexity to the design of an offset fed antenna than a front fed. Both antennas have issues with trailing wires to get power to the receiver and transmitter, the design of the support mechanism for the feed and long RF paths from the feed to the receiver or transmitter. Beam scanning is achieved by moving the entire antenna keeping feed and dish in registration.
CASSEGRAIN ANTENNA
A true Cassegrain antenna has a feed horn located at the vertex of the parabola and a hyperbolic convex sub-reflector at the focal point. It is based on the Cassegrain telescope system. A Cassegrain antenna is a popular choice for airborne and ground based antenna systems and commonly found in radio telescopes, and telecommunication antennas.
Cassegrain antennas have many advantages over other reflector antenna systems. The placement of the feed behind the primary reflector means the required hardware and electronics can be hidden away from the RF path. This reduces the amount of blockage caused by the hardware to the size of the sub reflector, the hardware can be larger and is more accessible. The sub-reflector shaping gives amplitude and phase control of the aperture illumination, spill over from the sub-reflector is directed out into space so it does not degrade the signal, and long transmission lines to the feed are eliminated.
It is possible to get some beam scanning ability from a Cassegrain antenna before incurring significant beam deformation. Alternatively, it is possible to scan the whole Cassegrain keeping the dishes in registration, but mechanically this can be challenging and reduces the aperture size of the antenna.
GREGORIAN ANTENNA
A Gregorian antenna is very similar to a Cassegrain, but the sub-reflector is a concave ellipsoid. It is based on the Gregorian telescope design. 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.
TWIST REFLECTOR ANTENNA
A polarisation selective scanning twist reflector antenna is a way of reducing aperture blockage in systems that operate only in one linear polarisation and need to perform beam scanning without significant beam deformation.
This type of antenna has its basis in a Cassegrain antenna. The configuration and shape of the two reflecting surfaces have been changed from a Cassegrain, which results in improved gain, efficiency and beam. The parabola is replaced by a scanning flat plate, which increases the angular width of radiation that can be accepted by the antenna. The sub-reflector is changed to a polarisation selective grid which allows the transmission or reflection of RF radiation depending on the polarisation.
Twist reflector antennas have several advantages over traditional Cassegrain systems. They have good beam scanning capabilities with limited impact on beam patterns, are compact designs which maximises aperture usage. They can be found in a wide range of applications, for example in ground based and airborne scanning antennas. Twist reflectors can have either a single or a multiple channel feed. However, they can only work in one polarisation and the complexity of design and manufacture is higher.
