Focal length

[MrMylesG]

I saw a couple of diagrams showing the focal length of a Fortec 36" offset dish to be 20.06" and I measured from the front of my c-band lnbf to the dish surface and it was 23 inches. Don't tell me I have to cut off 3 inches of the support arm to get this to work! Argh!

There has got to be another way. Anyone have a suggestion?

MrM

 

[caddata]

An offset dish is really only a small portion of a much larger (virtual) prime focus dish, often times less than 1/4 the size of the virtual dish. Complex math equations would allow you to verify the imaginary point you should be measuring to. Visualize your dish as being a segment of a much larger dish. That's why
the dish is much shallower than a comparable .7 F/D prime focus dish, it's only a portion of the outermost section of the virtual prime focus dish. You could swing an arc equal to the focal length along the polar axis of the dish from the center face of your lnbf and mark it to indicate the center of the virtual dish, but it might be off the surface of the offset dish.

 

[MrMylesG]

Whew!

I guess the best thing to do is to take the TV, receiver outside and hold the lnbf until I get a signal then figure out how best to construct a holder. <Gasp> Not an easy task for me.

Thanks for the response.

 

[Cascade]

MrMylesG, Lowes sell aluminium square tubing which is the exact fit for the Fortec dishes, 3 drill holes, a cut and you'll have a longer focal length.

 

[caddata]

MrMylesG:

Please, please, please don't cut anything. Just draw an x between the mounting bolt head centers on the reflecting face of the dish and use it as your reference point. Use the measurement you get between the center of the lnb face and the mark as your reference dimension. You could even use a wood dowel cut to your reference length. It'll be more than accurate enough eventhough it's not the true focal length.

Harold

 

[vj9999]

On 120cm fortec dish focal length is 720 mm. The only measurment that is exactly 720 mm is the length of the LNB arm.

 

[MrMylesG]

Whew II !!

I have a Fortec 36" dish. Once I get some extra time and after the inlaws leave I should be able to spend some time putting it together. Thanks for the help, guys!

 

[bigOleB]

Anyone have a formula or a test to measure focal length of any given dish?

 

[caddata]

Here's a link that will bog your mind down with calculations:

http://www.vhfcomm.co.uk/otypka.htm

And here's on that uses a knotted string:

http://www.qsl.net/n1bwt/chap5.pdf

It's simple for a prime focus dish. The formula is:

Focal Length = Dish Diameter Squared Divided By 16 Times The Dish Depth

Harold