If you look at a dish 500 or a dish 1000 you will notice that the LNB's are angled slightly upwards pointing to the focal point of the reflector/dish. So with that said, Does the LNB have to be at that certain height just to have that particular angle? Is this angle a must?
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I'll leave the art of putting more than one lnbf on a dish to others, but I just wanted to comment on the above, ie you say the LNBf is angled upward pointing to the focal point. On a dish with one lnbf, the lnbf won't "point" at the focal point, it will BE at the focal point. I assume that what you mean by pointing to the focal point is pointed toward the center of the dish? (If not, I'm not sure what you're referring to.) But the reason for pointing to the center of the dish is simple, ie just to get the maximum quantity of signal since feedhorns generally get maximum signat from the direction they are pointed. A lot of people are adamant that a prime focus feedhorn should always be pointed at the center of the paraboloid, and some even use laser devices to get this perfect, but as long as the lnbf is at the focal point, the angle of the feed really isn't really that important unless part of the dish is blocked or MISSING (or unless the angle is so great that you're picking up lots of ground noise from around the edges of the dish). I once had the bottom half of my BUD blocked by some big pine trees, and I found that I could improve reception dramatically by tilting my feed up by some 15 or 20 deg, ie aiming at the top half of the dish that was receiving signal above the pine trees. This is very much analogous to the performance of an offset dish. Ie an offset dish is really a prime focus dish where the bottom half of the dish is gone. If you aim the lnbf at the center of the paraboloid, you'd be aiming it off the lower edge of the dish, and you'd be receiving ground noise, and missing considerable signal coming from the dish. That's why the lnbfs are tilted up, just so that they're aiming generally at the dish so that it sees only what's reflected from the dish, and as little as possible of the noise coming from around the dish.
Now, re putting more than one lnbf on the dish, since there is only one focal point of a dish, this is pretty much just a matter of trying to find the best compromise position for the secondary lnbfs. I think it's more of an art than a science. Some people seem to have good results, but I was never much good at it myself.
