Shown above on the left is an offset dish with one feed support arm and on the right above an offset dish with three feed support arms.
As a general rule the two types of dishes are simply two ways of doing the same job, however, an offset dish, if well engineered, can have more gain than the prime focus dish. It is never, however, that simple. It is true that petalised offset dishes usually have poor gain but a one-piece well engineered prime focus dish can be better than a poorly engineered offset dish and some very good prime focus dishes can have the same gain as a good offset dish of the same size. As usual you get what you pay for. Cheap prime focus petalised dishes are to be avoided and we don't sell them. Prime focus dishes probably look better than offset dishes as they point upwards. Below we compare the two types of dishes in much more detail. Read on ....
Comparing a prime focus dish with an offset dish.
Which has the most gain? Assuming two dishes have exactly the same area, which is unlikely as dishes of the same quoted diameter are rarely the same (more about that later) but assuming the same area, and (for the sake of this discussion) the same efficiency (same accurate parabolic shape, again unlikely) the offset dish should produce more signal as the arms, feed support, feedhorn and lnb are not in the signal path. However, it is never that simple. A prime focus dish produces a very precise focal point. (Just like focussing the sun with a magnifying glass), the electromagnetic microwaves can be focused to a very exact well defined position into the feedhorn. In addition, any unwanted radiation, (e.g. infra red heat) that approaches the feedhorn from around the edge of the dish are rejected around the edge of the feedhorn by the scalar rings. These rings produce destructive interference and therefore maintain a high signal to noise ratio. The focal point on an offset dish is not as precise and the feedhorn needs to be flaired to collect the radiation. There are no scalar rings and so it is more likely that noise (eg infra red from around the edges of the dish) can enter the feedhorn. It is possible that this effect, (improved signs to noise ratio) can, to a greater or lesser extent make up for the shadows on a prime focus dish due to the feed assembly. Some manufacturers, when they quote dish diameter, quote maximum, others quote average and some quote useable. Does the dish work all the way to the edge or is there a roll off area resulting in a much smaller useable area? Petallised dishes or dishes in several segments tend to have low efficiency as they usually do not form a very good parabola. once assembled they often lack accuracy. The quality and rigidity of the feed support arms can also affect the amount of signal being received Flimsy arms may not hold the feedhorn and lnb in the exact optimum position. It may be possible for the arms to flex to a position that gives better results but then they spring back again. Paying more for a dish usually provides a higher build quality. Petalised dishes are usually cheap and have a very disappointing performance. One piece dishes tend to work really well if manufactured by specialist companies that make quality a priority. Dishes can be made of steel, (steel needs to have a good coating to stop corrosion) aluminium, or fibre glass / plastic. The type of material is not really an issue. What can be a problem is if the material is too thin. If it can be damaged or warped in transit or in high winds. The backplate needs to be very strong. Here steel is ideal and the best covering is "dipped galvanised". All bolts, nuts and u clamps should be stainless steel. Brass is a good material for long threaded elevation adjusters. The gain of a dish should be specified at (at least) 3 frequencies. The feedhorn should be designed for the dish, or have an adjustable f/d (scalar rings on a threaded adjustment) that can be matched to the dish. 40mm universal lnbs may not be a good match. All prime focus dishes should use a prime focus feedhorn with scalar rings. A prime focus or offset matched feedhorn should provide an improvement over an LNBF (An LNBF is an LNB with an intergrated feedhorn, more commonly these days called a 40 mm LNB. Technically only an lnb without an integrated feedhorn e.g. a c120 LNB or a block type should be called an LNB) if the dish size is 1.2m or larger. (Larger dishes have a narrower acceptance angle) Whether prime focus or offset, a matched feedhorn should improve the signal to noise ratio. To summarise, there are good well engineered offset dishes and low cost mass maket offset dishes. There are high quality prime focus dishes and some poor quality low cost petalised prime focus dishes. As usual "you get what you pay for". Which look the best? This is a matter of opinion but the prime focus dish points upwards to the satellite and looks purposeful. When domestic satellite TV started in the UK in 1985 all dishes were prime focus. The offset dish reflects the radiation downwards and so the dish face is more vertical (there is an offset angle). There is a great deal more to this subject than most people realise, hopefully this discussion clarifies some of the differences. Cost, build quality, the right matereials and good design are the key to maximum useful signal (High signal to noise ratio.) quality dishes.