Peter, A dent in dish parabola is just one factor. You also need to consider 1. Overall parabola shape. 2. Feed choice for best illumination. 3. Feed Alignment. In a well designed dish antenna the efficiency rarely is greater than 50% (-3 dB from ideal) Spending a little time learning how a dish works and all the considerations will give you a far better antenna.
Art, KC6UQH
-----Original Message----- From: amsat-bb-bounces@AMSAT.Org [mailto:amsat-bb-bounces@AMSAT.Org] On Behalf Of James Whitfield Sent: Saturday, July 26, 2008 3:05 PM To: amsat-bb@AMSAT.Org Subject: [amsat-bb] Re: AMSAT-BB Digest, Vol 3, Issue 375
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@gmail.com Subject: [amsat-bb] Dish surface 'flatness' To: amsat-bb@amsat.org Message-ID: 488A4CFA.6060703@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
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