"Owen Duffy" wrote in message
...
"Sal M. Onella" wrote in
:


"a" wrote in message
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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.

Sal, more more complicated for offset feed... Owen

Yeah, I thought of that and I have no ready backup for offset. I have
played with both kinds and I'm happy that my two offset feed dishes each
came with a feed-mounting arm already in place. ;-)

My two offset feed dishes have the feed 22.5 degrees offset from the
structural centerline of the dish; the angle of peak signal is therefore
22.5 degees off in the other direction. (Avoiding UP, DOWN, LEFT & RIGHT.)
I think that's a standard.

Given that an offset dish is, I think, more properly described as a
parabolic section, rather than a parabola, I would use the edge of the dish
nearer to the feed for the "f" measurement. (This is intuition with no
backup. I'm naked here, so don't look.)

Speaking of dishes, I have two, a 7-foot and a 10-foot, to give away. San
Diego, California area. So far, no takers as I cast the net ever wider.