Need Help understanding length of Feed Horn Mount Legs

[travisnj]

I have a 10' Mesh, the arms were uneven and measured bend to bend on 1/2" EMT conduit of 41 3/8" length... Now I have read and found my F/D... My intersection point of the strings to the back of the satellite (reflector) is 25 inches this would be d. Therefore F= (120*120)/(16*25) which equals 36 inches.

So, how to determine the proper length? Do I measure 36 from the back of the reflector to the tips of the scaler ring? to the back of the scaler ring?

I know this question may be bizzare but I really have tried very hard to find and solve my issues and so far have only been a "fizzle" of failure...

ANY assistance is GREATLY appreciated!!!!!

 

[sergei]

Are these the original arms that came with the dish? If they are then I'd say there most likely the correct length and by moving the LNB/LNBF in or out of the scalar should get you the correct F/D position. Also remember, even if the tripod legs have the same length, that does not mean the feed is centered, so you might have to "bend" the feed back into center of dish, or adjusting the tripod legs to get the feed centered in the dish.

 

[travisnj]

Legs

The legs for this dish were made from EMT pipe, this dish was made by a small company that came and went, they used houston tracker polar mount, bent their own frame for the mount, I am positive that the arms were not "engineered" to fit, they were bent onsite and installed... I just don't know how to go about making sure of the length.... I would like clarification of whether or not you should measure 36" from the "tips" of the scaler to the back of the reflector? or do you measure from the back of the scaler to the back of the reflector? So from the "tip" or the "flat" part of the scaler to the reflector would be one question I have..

Any comments are appreciated~!

 

[FaT Air]

Focal length should be measured from approximately 1/4 inch inside the throat of the feed. Depending on the F'D setting the scalar could be 2 inches behind the sweet spot of the focal length. Because of all the inaccuracy in measurements and construction, fine tune the feed focal length (tweak) as necessary. Once you find the "Sweet spot" calculate F/D, and adjust/modify assembly to place the scalar on the feed at the correct position while keeping the feed throat at the "sweet spot".

 

[sergei]

The legs for this dish were made from EMT pipe, this dish was made by a small company that came and went, they used houston tracker polar mount, bent their own frame for the mount, I am positive that the arms were not "engineered" to fit, they were bent onsite and installed... I just don't know how to go about making sure of the length.... I would like clarification of whether or not you should measure 36" from the "tips" of the scaler to the back of the reflector? or do you measure from the back of the scaler to the back of the reflector? So from the "tip" or the "flat" part of the scaler to the reflector would be one question I have..

Any comments are appreciated~!

I'm curious as to what your dish looks like, so could you post some pictures of what you have.

 

[Anole]

I've always found this an inexact science.
I guess C-band isn't quite so critical, so people get away with a lot of slop.

This Chaparral web page discusses focal length and F/D.
If you scroll down about half way, there's a table of how far the feed should stick out of the scalar.
Perhaps you can apply that info to your very different hardware.

Theoretically, the best dishes I've seen to allow for easy optimization, are:
- some offset dishes ...
- some button hook buds ...
...where the LNB mount holds the feedhorn directly, and the scalar is only mounted to the feed.
That way, you independently adjust the feed for peak signal withOUT the scalar....
... then install and tune the scalar for best signal quality.

Most dishes mount the scalar at a fixed location, and you move the feed in 'n out.
That's pretty stupid, because as you tune the focal distance, you're simultaneously tuning the F/D ! - :rant:
That's how I spell, interaction.
Talk about a nightmare.

edit:
Well, come to think of it, there's a dish that is pretty reasonable.
I've looked at the lnb mount on the Paraclipse. (thanks for all the pix, Stogie)
You set the scalar and feed for proper F/D (as in the Chaparral info above), then...
The adjustment the dish gives let you move the whole assembly toward or away from the dish on long screws.
If you can trust your F/D setup, this approach gets the job done properly! - :up