Parabolic Reflector Antenna Feed Systems

- an overview or tutorial about parabolic reflector antenna feed systems with calculations, equations and details of the different types of feed system used.

The feed systems for parabolic reflector antennas or dish antennas are of great importance. The actual antenna in a parabolic antenna, that is, the device that interfaces the transmission line or waveguide containing the radio-frequency energy to free space, is the feed element. The reflector surface is entirely passive. This feed element should usually be at the center of the reflector at the focal point of that dish. The focal point is the point where all reflected waves will be concentrated. The focal length f (distance of focal point from the center of the reflector) is calculated with the following equation:

Focal length     f   =

D 2     16 c

where:
f is the focal length of the reflector
D is reflector diameter in same units as wavelength
c is depth of the reflector

The radiation from the feed element induces a current flow in the conductive reflector surface which, in turn, re-radiates in the desired direction, perpendicular to the directrix plane of the paraboloid. The feed element can be any one of a multitude of antenna types. Whichever type is used, it must exhibit a directivity that efficiently illuminates the reflector and must have the correct polarization for the application -- the polarization of the feed determining the polarization of the entire antenna system. The simplest feed is a half-wave dipole which is commonly used at lower frequencies, sometimes in conjunction with a closely coupled parasitic reflector or "splash plate". At higher frequencies a horn-type becomes more feasible and efficient. To adapt the horn to a coaxial antenna cable, a length of waveguide is used to effect the transition.

There are two dimensions for the parabolic antenna that are of particular importance. These are the focal length, f and the diameter, D. Typically one of the parameters used to specific parabolic antennas is the f / D ratio. As the f/D ratio is often specified along with the diameter, the focal length can be obtained very easily by multiplying its f/D ratio by the specified diameter D.

Focal feed system

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. In fact the circular waveguide provides one of the optimum sources of illumination.

Diagram of a focal feed parabolic reflector antenna

Cassegrain feed system

The Cassegrain feed system, although requiring a second reflecting surface has the advantage that the overall length of the dish antenna between the two reflectors is shorter than the length between the radiating element and the parabolic reflector. This is because there is a reflection in the focusing of the signal which shortens the physical length. This can be an advantage in some systems.

Diagram of a focal feed parabolic reflector or dish antenna with a Cassegrain feed