[amsat-bb] Re: Dish surface 'flatness'

[THOMAS F DAVIS]

Hi Peter:

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.
Tom W1TFD
Peter Ellis <vk1kep at gmail.com> wrote: I am now the proud owner of a very 'second-hand' 3m mesh-covered 
skeleton dish.

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 
distance, etc.

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.

With thanks,
Peter
VK1KEP
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