Parabolic Reflector Antenna: Dish Antenna
- the parabolic reflector antenna or dish antenna is widely used in many areas where high gain and narrow beamwidth are required.
The parabolic reflector antenna or dish antenna has been used far more widely in recent years with advent of satellite television.
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.
Although more difficult to manufacture than some simpler antennas, its performance means that the parabolic reflector antenna or dish antenna is widely used in a number of applications.
The Goldstone parabolic reflector antenna
Image courtesy NASA
Parabolic reflector antenna advantages
As with any form of antenna, the parabolic reflector antenna has a number of advantages and disadvantages. These need to be considered against those of other antenna types before selecting he one that is optimum for the job.
Some of the major advantages of the parabolic reflector antenna include the following:
- High gain: Parabolic reflector antennas are able to provide very high levels of gain. The larger the 'dish' in terms of wavelengths, the higher the gain.
- High directivity: As with the gain, so too the parabolic reflector or dish antenna is able to provide high levels of directivity. The higher the gain, the narrower the beamwidth. This can be a significant advantage in applications where the power is only required to be directed over a small area. This can prevent it, for example causing interference to other users, and this is important when communicating with satellites because it enables satellites using the same frequency bands to be separated by distance or more particularly by angle at the antenna.
Like all forms of antenna, the parabolic reflector has its ,limitations and drawbacks:
- Requires reflector and drive element: the parabolic reflector itself is only part of the antenna. It requires a feed system to be placed at the focus of the parabolic reflector.
- Cost : The antenna needs to be manufactured with care. A paraboloid is needed to reflect the radio signals which must be made carefully. In addition to this a feed system is also required. This can add cost to the system
- Size: The antenna is not as small as some types of antenna, although many used for satellite television reception are quite compact.
Parabolic reflector antenna applications
There are many areas in which the parabolic / dish antenna may be used. Its performance enables it to be used almost exclusively in some areas.
- Direct broadcast television: Direct broadcast or satellite television has become a major form of distribution for television material. The wide and controllable coverage areas available combined with the much larger bandwidths for more channels available mean that satellite television is very attractive.
However as signal levels are low, directive antennas must be used to provide sufficient gain while being able to receive signals from only one satellite in the visible sky. The parabolic reflector antenna is able to meet these requirements and has the added advantage that it would not be as long as a Yagi or equivalent gain and directivity.
- Microwave links: Terrestrial microwave links are used for many applications. Often they are used for terrestrial telecommunications infrastructure links. One of the major areas where they are used these days is to provide the backhaul for mobile phone / cellular backhaul.
- Satellite communications: Many satellite uplinks, or those for communication satellites require high levels of gain to ensure the optimum signal conditions and that transmitted power from the ground does not affect other satellites in close angular proximity. Again the ideal antenna for most applications is the parabolic reflector antenna.
- Radio astronomy: Radio astronomy is an area where very high levels of gain and directivity are required. Accordingly the parabolic reflector antenna is an ideal choice.
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.
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.
A parabolic shape has the property that paths taken from the feed point at the focus to the reflector and then outwards are in parallel, but more importantly the paths taken are all the same length and therefore the outgoing waveform will form a plane wave and the energy taken by all paths will all be in phase.
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. Read more about the focal length
- 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.
- Gain: The gain of the parabolic reflector is one of the key parameters and it depends on a number of factors including the diameter of the dish, wavelength and other factors. Read more about the parabolic reflector antenna gain.
- Feed systems: The parabolic reflector or dish antenna can be fed in a variety of ways. Axial or front feed, off axis, Cassegrain, and Gregorian are the four main methods.Read more about Parabolic reflector feed types.
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.
By Ian Poole
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