"a" wrote in message
...
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?

Here it is, exactly:

f = D-squared divided by 16-times-d,

where f is the focal length of the dish (your "distance between the
radiating element and the reflector"),
D is the dish diameter, and
d is the depth of the dish

Example; If your five foot dish is a foot deep, D-squared is 25 and
16-times-d is 16; Divide 25 by 16 and mount the radiator (or "feed" in dish
parlance) 1.56 feet away, about 1' 6 3/4". The example is a fairly deep
dish; the shallower the dish, the longer the focal length.

See also http://www.satsig.net/focal-length-parabolic-dish.htm but note he
uses letter "c" for the depth.