Many free to air satellite TV enthusiasts can easily tell the difference between an Prime focus and and Offset dish, they can even correctly predict which satellite a certain dish is facing just by looking at it, even from a moving vehicle.
Well if you do not know the difference between a Prime focus and an Offset dish then continue reading. Take a look at the image just below the heading, it shows two satellite dishes that you will know which one is which very soon after you know the difference between Ku Band and C band frequencies.
Prime focus Dish
A Prime focus dish is a reflector whereby the waves received are concentrated on a central point in front of the dish. The prime focus is usually in the same plane as the most central point on the reflector itself It can also be viewed as a concave reflector like the image below tries to explain.
A prime focus dish is used to receive C band signals and is usually big in size. The smallest size C band dish in Kenya is 1.8 meters wide and usually goes for around Ksh6500 (around 65 US Dollars). These type of reflectors are usually big and heavy and require a considerable amount of space to install them. Here is a hint if you replace the C Band LNB with a special type Ku band made for prime focus dishes then you can receive Ku band signals. Because of their big size, prime focus dishes are usually affected by wind, that is why some dishes like the one in the image above are made of perforated metal or mesh. But you cannot receive Ku band waves with such a (mesh type) dish, this is because the Ku band waves with their higher frequencies and shorter wavelengths will just penetrate the big holes and not be reflected at all.
An offset dish just like the name suggests reflects the signals to an offset position usually around 20 degrees offset. The reflective area can be viewed as a parabola. The back of the LNB on an offset dish usually points at a false location (not the satellite aimed by the dish). The LNB and LNB arm of an offset dish does not in any way block the waves the dish is receiving unlike the LNB and LNB arms of a Prime focus dish.
Offset dishes are usually small in size and are used to receive Ku band waves. Most pay TV providers usually use Ku band frequencies because it usually makes the equipment at the consumers end cheaper. A small size 65cm offset dish usually goes for around Ksh1200 (about13 US Dollars) in Nairobi. There is nothing like a perforated offset dish like in the case of Prime focus reflectors. Offset dishes have an advantage that they are cheaper, lighter and can be installed almost anywhere provided there is a clear view of the sky.
I believe by now you know that the smaller dish in the image is an Offset dish while the larger dish is a mesh type prime focus dish.