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matching lnb to dish

most lnb's are made for the dish they mount on. it's not just because of the different focus position though. each lnb will have a specific "pickup angle" let's say 80 degrees. if you mount this on the correct dish it will "see" nothing but dish in front of it. if you put an lnb in the same spot with a wider pickup angle, it would be seeing past the dish to the wall or the ground behind it which radiates a lot of background noise. you want the signal to be only coming from the dish which is pointed at cold empty space. ( and the satellite ). if you put on an lnb with a smaller pickup angle it would be seeing only the center portion of the dish and so you would get the signal you might expect from a smaller dish. the old round dishes had a plastic mount for the lnb to slide forward or back and get the distance to the dish perfect. the new elliptical dishes have to keep the lnb in 1 fixed position because you have signals hitting the dish from 2 different angles. if you were to slide the lnb forward or back you might still have 1 lnb catching the signal from the correct angle for the main satellite but not the second satellite. so the 75e lnb has to made for the 75e dish. put a 60e on it and you wind up with a lower ebno due to background noise from behind the dish or a lower signal strength due to the lnb not being able to see the whole dish, or the signal coming in from the 2nd satellite is hitting the side of the feedhorn instead of being directed down it's throat.
 
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