sat. dish "sweet spot" ?

[sevengoals]

OK my education continues.

is there such a thing as a sweet spot on the dish. (like with a tennis racket)
should i plan to have the "important" LNBs in the middle of the dish or is it not that important where the LNBs are positioned on the sat-dish as long as every thing is correctly aligned. (dish, lnbs.etc)

cheers and thanks in advance for any advice.



i'm so excited.
If all goes well i may be able to at least see the Adelaide Test Highlights this weekend.
what joy, what bliss!

[Jimdefruit]

The lnb's should be on a slightly curved bracket so that each points at the centre of the dish.

Have a look at this post http://www.satellitehelp.co.uk/forum...pic,665.0.html

[sevengoals]

^^^thanks.
so the set up is called "multi-block".

ok looking through the posted thread i get the feeling that it's not so important because of curve of the arm has them more or less pointing towards the centre.

[BGonaSTICK]

It's actually very important.

Just taking a single LNB setup - the dish will be aligned exactly on a single satellite.

If the dish were a perfect parabolic curve, the signal would be focused on a pinhead at the focal point of the dish. This is where you put the feedhorn of the LNB. Job done.

Move the LNB away from that point, and you start having problems. More than a few millimeters, and you'll start noticing a real-world difference. A drop in signal quality, a loss of signal etc.

Now, lets imagine an exaggerated scenario to help visualise this. One dish, two single LNBs.

Two satellites, twenty degrees apart in the sky, one at 10 degrees East and one at 10 degrees West. We'll assume they are of equal power with identical footprints etc. for the sake of this discussion.

To receive both of these satellites, you would point your dish due South, i.e zero degrees.

Now because our mid point is zero degrees, the multi-LNB arm will in fact be horizontal, but this is an unusual situation. Normally it will be tilted slightly to accommodate the slope of the satellite arc.

Now, because the two satellites are coming at the dish from an un-natural angle (i.e. from either side) each creates its own focal point on the other side of the dish. The western satellite will have a focal point slightly on the eastern side of the dish, and the easterly satellite on the west.

That's how multi-LNB setups work. The signal from the various satellites is refocused in a series of hotspots across the end of the LNB arm.

So yes, positioning is critical!

[sevengoals]

^ wow,now i get it.
It's a series of " hotspots" across the face of the dish depending on were the Sats are.
Now i also see the importance of the size of the dish relative to the number of LNBs

cheers!

[BGonaSTICK]

Yes, that's right. Hotspots focused closer to the centre of the LNB arm are more efficiently focused too, and the further away from that ideal the next hotspot is, the less efficiently it'll be focused.

For that reason, in many multi-LNB setups, you try and arrange to have the dish pointing at the weaker satellites so that they are more easily received, whereas more powerful satellites like Astra1, Astra2, Hotbird etc. can be focused further out and still provide sufficient signal.

It's a case of trying to average everything out, so that all satellites are receivable. No point having some crashing in and some right on the point of pixelating.