[amsat-bb] Re: Dish surface 'flatness'
antonio at qualcomm.com
Fri Jul 25 21:35:16 PDT 2008
At 03:00 PM 7/25/2008, Peter Ellis wrote:
>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
Focal length, f/d, etc are irrelevant. The thing that is relevant is
wavelength of your signal. You want the variations in the dish's
surface to be small relative to a wavelength. 1/4 wavelength is
ok. 1/8th wavelength is better. Better than that matters not much.
Its not to hard to get an intuitive feel for what is going
on. Suppose you have a dish that is a perfect paraboloid, except for
one little dent that's a little farther from the focus. Suppose the
dent is 1/2 wavelength farther from the focus than the rest of the
dish. The signal that reflects off of that dent will be 1/2
wavelength late, in other words 180 degrees out of phase with the
rest of the signal, and will therefore SUBTRACT from the rest of the
signal. That isn't good. That's a clue that 1/2 wavelength is too
much variation. You can go thru the same sort of thinking with a 1/4
wavelength dent and see that the effect is much smaller, etc.
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