How do I find the focal point of an offset dish?

[izthebye]

I have a glass offset dish about 48" x 52". It's commercial quality with an very solid mount. I don't think I have the correct arms with it. Can anyone direct me to a formula/method for determining the focal point that does not require a Ph.d in math?

Thanks!

[wejones]

Originally Posted by izthebye

I have a glass offset dish about 48" x 52". It's commercial quality with an very solid mount. I don't think I have the correct arms with it. Can anyone direct me to a formula/method for determining the focal point that does not require a Ph.d in math?

Thanks!

There is no good way to find this precisely, other than perhaps using the mirror technique I posted about a year and a half ago. However, there is a way to find a close approximation to the correct focal point.
If you make a few assumptions, and somehow know what the focal length of the dish is, perhaps from the manufacturer's specs, then, I put a calculator at:
http://wejones.ftdata.com/offsetang.htm

This calculator assumes a couple things, but it can tell you if the arm is locating the lnbf at the wrong place.

However, if you don't know the focal length, which I suspect is the situation here, you may have a problem, since the above calculator requires you to know the FL. But if you assume that you have the proper lnbf arm, make the measurements described, and the calculator will give you a distance that the lnbf should be from the top edge of the dish. If this is off significantly, then it would suggest that perhaps you do have the wrong lnbf arm, and if it is pretty close, then you are probably OK.
This is something I just put together in a few minutes, so I'm not positive it works all the time, but it did seem to check out with my dish.
If all else fails, you might wait for a sunny day, and try the mirror method. See:
http://eskerridge.com/bj/FC90-sg2100.html

About 2/3 of the way down the page. If you do this mirror thing, and point the dish at the sun by hand, and move it around watching for where the mirrors reflect the sun, you can get a pretty good idea of where the focal point is. The problem in your case, is that the actual surface of your dish is inside the fiberglass, not at the surface, so the actual focal point will probably be a bit closer than the mirrors predict, but again, it will get you close enough to get a signal, and then tweak the FL a bit while peaking on the signal.

[izthebye]

Originally Posted by wejones

There is no good way to find this precisely, other than perhaps using the mirror technique I posted about a year and a half ago. However, there is a way to find a close approximation to the correct focal point.
If you make a few assumptions, and somehow know what the focal length of the dish is, perhaps from the manufacturer's specs, then, I put a calculator at:
BJ Offset Angle and LNBF Position Calculator

This calculator assumes a couple things, but it can tell you if the arm is locating the lnbf at the wrong place.

However, if you don't know the focal length, which I suspect is the situation here, you may have a problem, since the above calculator requires you to know the FL. But if you assume that you have the proper lnbf arm, make the measurements described, and the calculator will give you a distance that the lnbf should be from the top edge of the dish. If this is off significantly, then it would suggest that perhaps you do have the wrong lnbf arm, and if it is pretty close, then you are probably OK.
This is something I just put together in a few minutes, so I'm not positive it works all the time, but it did seem to check out with my dish.
If all else fails, you might wait for a sunny day, and try the mirror method. See:
FC90-sg2100.html

About 2/3 of the way down the page. If you do this mirror thing, and point the dish at the sun by hand, and move it around watching for where the mirrors reflect the sun, you can get a pretty good idea of where the focal point is. The problem in your case, is that the actual surface of your dish is inside the fiberglass, not at the surface, so the actual focal point will probably be a bit closer than the mirrors predict, but again, it will get you close enough to get a signal, and then tweak the FL a bit while peaking on the signal.

Thank you, Bill!

This should get me off the ground. Unfortunately, there are absolutely no manufacturer's names or markings on the antenna and it did not come with an LNBF. I have seen similar ones at local gas stations (Esso), but they have no names on them either. The only parameter I am certain of is that the mounting pole is level! The dish uses heavy galvanized steel for it's mounting hardware, and I think it will be worth the effort to try and get it to work as I plan to use it for a single sat. I don't think the fiberglass thickness will be an issue. If I can get a signal at all, I can tweak it after.

BTW, what kind of gain do you get from that wooden LNBF?

[wejones]

Originally Posted by izthebye

...

BTW, what kind of gain do you get from that wooden LNBF?

Yeah, that was probably being a bit extreme re precaution. I wasn't sure how much heat the sun's reflection would generate, so I didn't want to risk cooking the real one. I went inside, looking around for something I could mount in the lnbf arm yoke. The wooden thing is actually just something I made which originally had no function. I was just playing around making wooden threads with a gadget I bought, but after making it, I found out that it worked perfectly as a nut-cracker for almonds. But here, I also found that it worked perfectly to screw into the lnbf arm, positioning itself right about where the lnbf would be. So you can always find a use for something with no use, which is why I never throw anything away, which my wife isn't happy about.