Parabolic reflector antenna

- an overview or tutorial about the essentials of the parabolic reflector or dish antenna and its theory and design for high performance applications such as satellite transmission 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 courtesy 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. In this way, the feed system forms the actual radiating section of the antenna, and the reflecting parabolic surface is purely passive.

When looking at parabolic reflector antenna systems there are a number of parameters and terms that are of importance:

• Focus   The focus or focal point of the parabolic reflector is the point at which any incoming signals are concentrated. When radiating from this point the signals will be reflected by the reflecting surface and travel in a parallel beam and to provide the required gain and beamwidth.



• Vertex   This is the innermost point at the centre of the parabolic reflector.



• Focal length   The focal length of a parabolic antenna is the distance from its focus to its vertex.



• Aperture   The aperture of a parabolic reflector is what may be termed its "opening" or the area which it covers. For a circular reflector, this is described by its diameter. It can be likened to the aperture of an optical lens.

Antenna gain

The gain of the parabolic antennas is of paramount importance. Parabolic antennas or parabolic reflector antennas are often used solely for their gain and directivity. Accordingly the gains of the antennas need to be determined as part of the design.

A full overview of the gain equations and calculations of the parabolic reflector antenna can be found via the "Related Articles" link on the left hand side of this page below the main menu.

Parabolic antenna focal length

One important element of a parabolic antenna is its focal length. To ensure that the antenna operates correctly, it is necessary to ensure that the radiating element is placed at the focal point. To determine this it is necessary to know the focal length.

Focal length     f   =

D 2     16 c

Where
    f is the focal length
    D is the diameter of the reflector
    c is the depth of the reflector

In addition to this the f/D ratio is important. 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.

Feed systems

A parabolic antenna is designed around its feed system The design of the feed system is central to the design of the overall parabolic reflector antenna system. There are two basic forms of feed system that can be used for a parabolic reflector antenna:

1. Focal point feed system:   Using a focal point feed system the source of the radiation is placed at the focal point of the parabola and this is used to illuminate the reflector.
   
   



2. 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.
   
   

A full overview of the methods of feeding parabolic reflector antennas can be found via the "Related Articles" link on the left hand side of this page below the main menu.

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 used 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.