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

Nigel Gunn G8IFF/W8IFF nigel at ngunn.net
Sat Jul 26 00:59:10 PDT 2008

Surely a signal that reflects off a half wavelength dent will be one wavelength late, half a wavelength getting to the dish and half a 
wavelength coming back. So a dent (or bump) 1/4 wavelength will cause a 180 deg shift. I would suggest that your dish needs to be better than 
(my rule of thumb) 1/10 wavelenth accuracy.

> 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, 

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