LNB - how far from the dish ?

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Denmark
#1
I have a 1.25 m dish (symmetrical with the LNB held in position by three arms converging from the edge of the dish - i.e. it is not an offset focus dish).

Sadly I havent got a bracket for the LNB, so I making one out of an old bicycle tyre, an empty Heinz baked-beans tin and ½ lb of blue-tack (well not quite, but ...)

Im in doubt about:

- Should the dish focus on the surface of the LNB head or the diode-thingy inside it ?
- If the latter is the case, how far inside is the diode ... is there anyway of knowing ?
- Lastly, but not leastly: what distance should the LNB (diode or otherwise) be from the dish surface, given it is symmetrical and 1.25 cm across ?

Kind regards

Soren - Denmark ... where I hope to get BBC back, now that CH4+5 seem to be going out ...
 
O

Old Fred

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#2
Find the approximate focal point by aiming the dish at the sun and holding a piece of white card in front of it. (This doesn't work too well with a black dish!)

Now fix your LNB so that the entrance to the horn is at the focal point. Make your bracket adjustable because this is only approximate.

The LNB horn should match the f/d ratio of your dish (don't ask - I just read it in a book!) otherwise it might not be collecting signal from the full area of your dish and, if you move it further away, it's no longer at the focal point. Or the opposite can occur where the LNB horn is collecting signal from too wide an area so it collects heat readiation from anything behind the dish. It's "looking around the edge" of the dish, in other words.

Normally a dish is supplied with a matching feedhorn by the manufacturers. Anything else is likely to be inferior.

The "twist" ("skew") position has to be optimised, too, for best "signal quality".

Fred
 
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