26 Jul
2008
26 Jul
'08
9:59 a.m.
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,