[amsat-bb] Re: Dish surface 'flatness'
THOMAS F DAVIS
tfdavis at snet.net
Fri Jul 25 20:12:15 PDT 2008
I assume that the dish is a paraboloidal reflector. Can you determine the focal length and the f/ ratio? What was the original size of the mesh holes?
I checked with Skolnick's Radar Handbook, and there was no information with regards to your question, just equations on how to create the paraboloidal form. From my experience with optical paraboloids I can state that it is widely expected that such mirrors have a deviation over their diameter of no more than 1/4 of a wave (hmm, where have we seen that number in antenna design before?). Due to the Rayleigh limit, little gain is added if the accuracy is increased beyond 1/8 of a wave.
The ARRL Antenna Handbook 19th edition states on page 19-19 regarding "Parabolic Gain Versus Error": "Each time all the errors are halved, the frequency of maximum gain is doubled".
Hope this helps.
Peter Ellis <vk1kep at gmail.com> wrote: I am now the proud owner of a very 'second-hand' 3m mesh-covered
The mesh covering is metaphorical, rather than actual. I watched the
dish on its back while as a group of small children rolled the dish back
and forth and trampled holes in the mesh. That's the reason I was able
to ask for it. "Please take it away," was the manager's reply to my query.
Now, I need to re-mesh the dish, but I need to decide what 'flatness'
('true-ness') the surface should be. There has to be a formula, that
encompasses things like: frequency of operation, curvature or focus
I've done some looking online, but can't find it readily.
Suggestions, and discussion, please?
Please note: If you're going to also make product, brand name, or
supplier suggestions, then be generic as suppliers and products in VK
(Australia) will differ from where most list members will be sited.
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