[amsat-bb] Re: AMSAT-BB Digest, Vol 3, Issue 375

[James Whitfield]

Peter Ellis, vk1kep:

I hope that you will get "better" answers from others of the group that have
practical rather than theoretical experiences with large parabolic
reflectors.  My only qualifications on this subject come from experience as
a Geometry teacher and limited amateur astronomy, but for what it is worth,
here are my thoughts.

The simple answer to the necessary "flatness" is better than a quarter
wavelength error.  The general idea is that the radio waves you want to
capture come to the "ideal" reflector and all of them bounce back to a focal
point, and all of the waves reaching that point at the same time, that is
"in phase".  The error of the real surface translates into a phase shift.
If a substantial surface area is a quarter wave behind the ideal and if an
equal area is a quarter wave in front of the ideal, then the waves from the
two areas will be half wave apart, causing full cancelation.  The quarter
wave error is used to indicate that the maximum error difference from a high
point, relative to the ideal, to a low point is no more than a quarter wave.
That way the wave error causes phase shift rather than cancelation.

A greater error probably will provide some "gathering" of the signal, but if
you want to have a parabolic reflector that all of its aperture is
effective, then you need the surface to meet the quarter wave flatness.  As
with optics, the less the error, the better the "image" that can be
resolved.  As a practical matter on radio frequencies, trying to exceed
quarter wave is not worth the effort.

You did not state if the framework was in good enough shape to serve as a
template for the ideal parabola.  If it is not, I can give you a formula,
but I will need more information about the dimensions of the dish.

Best Wishes.

James
n5gui





> Date: Sat, 26 Jul 2008 08:00:26 +1000
> From: Peter Ellis <vk1kep at gmail.com>
> Subject: [amsat-bb]  Dish surface 'flatness'
> To: amsat-bb at amsat.org
> Message-ID: <488A4CFA.6060703 at gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> 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