I'm looking at making a parabolic reflector for a wifi link.

I'm trying to work out the distance required between the radiating
element and the reflector.
Should it be an odd number of quarter wavelengths, or an even number?

(An odd number of quarter wavelengths would be required if there is a
180 degree phase change on reflection, and
and even number would b required if there is no phase change).

So... is there a 180 degree phase change at the reflector?

TIA