Are all dishes suitable for multiple LNB's

[dohouch]

Have to date rescued from scrap sky system( mesh dish and receiver), 105cmX97cm solid metal dish no boom, and now my gaze has landed on long disused larger dish measured it as being 130cm vertical diameter, can't measure horizontal as dish is wall mounted at 6ft of the ground.

1, So my question is are there dishes which are unsuitable for multi LNB's?

2. How do I distinguish between suitable and unsuitable?

Also this dish seems in good condition and probably will cost me, what would be fair? I reckon Euros 60 max(£40)

rgds
doh

[BGonaSTICK]

No, certainly not all dishes are suitable for multi-LNB, but most are to a greater or lesser extent.

There's a lot of science involved in the precise construction of dishes and illumination patterns/shapes of LNBs which is frankly over my head and doesn't interest me, but in general, a 'standard' taller-than-it's-wide offest dish will do a reasonable job.

Prime focus dishes are not the best choice for multi-LNB in the traditional sense, although some configurations do marry C-band and Ku-band LNBs in a professional assembly. Sky-type dishes which are wider than they are tall are also not good for use with an off-the-shelf 40mm neck-clamp universal LNB. Try saying that when you've had a few beers!

Other than that, dishes that are of the 'offset' construction type (i.e. LNB boom arm at the bottom, and about 5%-7% taller than they are wide) should work fine if you can buy or make a multi-LNB arm for it.

As for price, the value of a dish is not really related to it's size. Some huge dishes are still pretty cheap, whilst some smaller dishes very expensive. The money goes on the quality of the dish face. The more mathematically perfect the surface, the more signal and less 'noise' at the focal point of the dish.

Multi-LNB is all about compromise and averages. My basic approach to it is that as long as you go for fairly standard components, just make sure the dish is plenty big enough. As long as you are nowhere near the threshold of reception acquisition, you have enough margin to experiment.

Always go for a bigger dish than you would normally use based on your location and you can normally get away with murder.

As I've said, I'm not the world's foremost expert on the subject, but that's my fairly novice take on it.

STICK

[Newbie1001]

Newbie here again.

Why is it the taller than wide shape of dish is better for a multi LNB installation?

If LNB's are mounted near horizontally, logic would suggest a wider than tall dish shape would reflect signals into left-most and right-most LNB's better?

[Jimdefruit]

Hi Newbie,
If you were near the equator then that would be true. But as you probably live a bit north or south, the satellites occur on an arc that appears high due south and lower near the horizon to the east and west.

A motorised dish moves in an arc from horizontal when pointing East, roughly 19 degrees ish at due South and horizontal when pointing West.

[BGonaSTICK]

If you imagine a perfectly round dish, with the LNB positioned in the center, it would be pointed directly towards the satellite, and the dish face would be at exactly 90 degrees to the radiated signal. Don't forget that the incoming signal is actually more like a cylinder than a thin beam as far as your dish is concerned. This layout is known as a Prime Focus dish. It's simple, and it works.

It has one main disadvantage though. The LNB is actually blocking some of the signal, so that there is a dark spot of 'signal shadow' in the middle of the dish face behind the LNB.

Now imagine an offset dish, where the LNB is much lower on a support arm. It's out of the way of the incoming beam now, so no loss. The dish face is, however, no longer square-on to the incoming signal 'cylinder'. The dish actually appears to be pointing much lower in the sky than it is. The beam comes in from higher up, and bounces lower and into the LNB at the focal point.

Now, if you were to place your eye at the point where the LNB is (in theory! Don't do this unless you want retina damage!) and looked back up that imaginary line, your eye would see a perfect circle!

Have a look here:

http://www.satellitehelp.co.uk/forum....msg520#msg520

If you were to design a dish specifically for multiple LNBs, what would it look like?

Well, scientists say it would look nothing like you might expect. In fact (of course) there is such a beast, and it produces a neat 'line' of focus so that you can arrange a dozen or more LNBs along it with amazing results. It actually has a secondary reflector, and is known as a toroidal dish.

There is an example here:

http://www.satellitehelp.co.uk/forum...p?topic=1159.0

Hope this helps.

STICK