DTV dish focal point.

[ShortCircuit]

I gotta project coming up and was wondering where the focal point on a DTV dish is? I know its on the lnb, but is in on the surface of the round plastic cap or is it underneath the plastic cap? Is the cap just a watherproof thing?

Ken

 

[RadioRon]

That's a good question. Ideally, the focal point would be inside the feed, a small distance from the plastic cap. Trouble is, it is really hard to say exactly where because this depends on the feed design. No question that it is not at the plastic though.

If you want to find the focal point of the dish for some experiments you could do so using optics. One way is to cover the dish surface with reflecting foil (carefully formed to sit right on the surface of the dish) then aim it at the sun. Next take a small piece of something black, like paper or whatever and move it around where you think the focal point is and observe the focused spot of light get larger and smaller. Where smallest, that's your focal point. I guess you have to be fussy to make the foil smooth.

 

[marcbarker]

cover the dish surface with reflecting foil (carefully formed to sit right on the surface of the dish) then aim it at the sun. <..> observe the focused spot of light get larger and smaller..

I was just going to suggest that exact same thing, then when I scrolled down I read it!

A tip for when setting up a dish for something, if you're looking for a signal, it's easier to find if the LNA is defocused a little so as to broaden the beam a little. One the station has been found it can be refocused.

 

[BrownOut]

take a small piece of something black, like paper or whatever and move it around where you think the focal point is

Make sure that "piece of something" is non-combustable!