Three major types of antenna construction are commonly encountered for small aperture earth station applications in Australia. These are: prime focus, offset fed and dual reflector.
Figure 2: Prime Focus antenna [Ref 2].
Figure 3: Offset fed parabolic antenna [Ref 2].
Figure 4: Dual reflector antenna [Ref 2].
For antenna sizes up to about 1 metre offset feed offers higher efficiency. Above this it is difficult to make a low cost, stable construction and prime focus dishes tend to dominate. Dual reflector dishes are little used for small aperture TVROs (one of the main reasons is that the subreflector obscures too much of the dish's effective aperture).
Another antenna type often mentioned in US satellite text books is the spherical antenna. The spherical description refers to the side on cross sectional profile which is spherical (the shape of the dish seen from the front does not necessarily have to be circular it may be rectangular). The antenna shape is designed to accommodate several LNCs. This type of antenna may be able to simultaneously receive signals from several satellites at different points on the geostationary arc. However the need to accommodate multiple LNCs requires the antenna to have a long focal length. In some designs the feed(s) are mounted separately from the dish. This may be impractical if the dish has to be located in confined areas.
Figure 5: Comparison of parabolic and spherical dishes. Parabolic dish focuses signal energy of single satellite onto feed. Gentler curvature of spherical permits reception from several satellites [Ref 3].
The spherical antenna is not generally used in Australia. It has poor efficiency compared with parabolic or offset designs because the dish shape does not focus all of the incident signals into the feed. In addition there is dish blockage due to extra LNCs and mounting paraphernalia also reduces efficiency.
More exotic antennas such as microwave Fresnel lenses and phase array antennas have been demonstrated but have not yet been widely used for satellite television reception.
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