Parabolic reflector antenna

- an overview or tutorial about the parabolic reflector or dish antenna, and RF antenna design that is widely used for high gain applications such as satellite transmissions and reception as well as microwave links.

The parabolic reflector or dish antenna has been used far more widely in recent years with advent of satellite television (TV). However the dish antenna finds uses in many radio and wireless applications at frequencies usually above about 1GHz where very high levels of RF antenna gain are required along with narrow beamwidths. In many professional applications these parabolic reflectors or dish antennas are used for satellite as well as for radio astronomy and it is used in many microwave links, often being seen on radio relay towers and mobile phone antenna masts. In all these applications very high levels of gain are required to receive the incoming signals that are often at a very low level. For transmitting this type of RF antenna design is able to concentrate the available radiated power into a narrow beamwidth, ensuring all the available power is radiated in the required direction.

The Goldstone parabolic reflector antenna
Image coutesy NASA

Parabolic reflector basics

The RF antenna consists of a radiating system that is used to illuminate a reflector that is curved in the form of a paraboloid. This shape enables a very accurate beam to be obtained. The antenna exists in two basic forms. These are termed the focal feed reflector where source of radiation is placed at the focal point of the parabola and this is used to illuminate the reflector.

An alternative form of feeding the RF antenna design is known as a Cassegrain reflector system. Here the radiation is fed through the centre of the reflector towards a hyperboloidal reflector which reflects the radiation back onto the paraboloidal reflector. In this way it is possible to control the radiation more accurately.

Diagram of a focal feed parabolic reflector antenna

The gain is a function of the diameter of the reflecting surface, the surface accuracy, and the quality of the illumination from the radiator. Despite these factors it is possible to estimate the gain of the antenna which can be deduced from the following formula:

G = 10 log10 k (pi D)^2 / lambda^2

where
G is the gain over an isotropic source
k is the efficiency factor which is generally about 50%
D is the diameter of the parabolic reflector in metres
lambda is the wavelength of the signal in metres

From this it can be seen that very large gains can be achieved if sufficiently large reflectors are used. However when the antenna has a very large gain, the beamwidth is also very small and the antenna requires very careful control over its position. In professional systems electrical servo systems are used to provide very precise positioning.

To provide the optimum illumination of the reflecting surface, the level of illumination should be greater in the centre than at the sides. It can be shown that the optimum situation occurs when the centre is around 10 to 11 dB greater than the illumination at the edge. Lower levels of edge illumination result in lower levels of side lobes.

The reflecting surface antenna forms a major part of the whole system. In many respects it is not as critical as may be thought at first. Often a wire mesh may be used. Provided that the pitch of the mesh is small compared to a wavelength it will be seen as a continuous surface by the radio signals. If a mesh is used then the wind resistance will be reduced, and this provides significant advantages.

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.

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

Summary

For most domestic systems a small reflector combined with a focal point feed are used, providing the simplest and most economical form of construction. This is the form that is most widely sued for satellite television applications. These antennas may not always look exactly like the traditional full dish antenna. For mechanical and production reasons the feed is often offset from the centre and a portion of the paraboloid used, again offset from the centre. This provides mechanical advantage. Nevertheless the principles are exactly the same.