'Prime Focus Antennas
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The basic design principle of the parabolic curve can be incorporated into antenna designs in a variety of ways. Dishes with a focal point directly at the front and center of the reflector are called prime focus antennas.
Prime focus antennas are easy to construct and point toward the desired satellite. There are two main design disadvantages, however: the feedhorn and feed support structure block part of the reflector surface and the feedhorn must look back at the dish at such an angle that it can also intercept noise from the "hot" earth located directly behind the reflector. The feedhorn's illumination of the antenna must be attenuated or tapered to minimize noise contribution from the perimeter of the dish. This design necessity acts to reduce the antenna's efficiency.
Prime focus antennas use two different types of feedhorn support bracket. A three or four-legged support provides a rigid support structure for the feedhorn and LNB over the center of the dish and at the distance specified by the manufacturer. The main disadvantage of this structural approach is that it may be difficult to make minor variations in the focal length, that is the distance from reflector center to the lip of the feed opening. The buttonhook structural design uses a single support member to position the LNB and feedhorn. This tubular leg can usually be slid in and out of a clamp or bracket at the center of the dish, allowing the installer to fine-tune the focal length. However, the buttonhook support may not always position the feed at the precise center of the dish, especially when the feedhorn is weighted down by multiple LNBs.
Motorized dishes may experience feedhorn movement when the antenna is moved from one satellite to the next; heavy winds can also temporarily move the feedhorn away from the antenna's focus. Guy wire kits are available which the installer can use to provide additional structural rigidity to the buttonhook support if required for a given installation.
Offset-fed Antennas
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The dish design of choice for most digital DTH systems is called an offset-fed antenna. Here the manufacturer uses a smaller subsection of the same parabolic curve used to produce prime focus antennas, but with a major axis in the north/south direction, and a smaller minor axis in the east/west direction.
With the offset-fed design, the feedhorn is no longer positioned at the front and center of the reflector but rather offset to the bottom of the dish. However, the feed would be centrally located if we extended the parabolic curve of the offset fed dish to the full length of a prime focus parabola.
The offset fed antenna design offers several distinct advantages over its prime focus counterparts. There is no feedhorn blockage, an important consideration when the antenna aperture is less than one meter in diameter. Moreover, the offset angle at which the feedhorn tilts up toward the reflector is such that if the feed looks over the antenna's rim it will see the cold sky rather than the hot earth. Due to these advantages, the offset-fed antenna can achieve higher efficiency levels than prime focus antennas can generally attain.
The low inclination angles required by offset antennas also may be beneficial in certain climate zones. In tropical or semi-tropical environments, rain will not collect inside the reflector. In cold weather climates, snow will slide off of the antenna surface rather than accumulating inside the reflector.'
