October 28th 07, 01:01 AM
posted to rec.radio.amateur.antenna
|
|
external usenet poster
|
|
First recorded activity by RadioBanter: Dec 2006
Posts: 1,169
|
|
antenna reflector
"Sal M. Onella" wrote in
:
"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.
Sal, more more complicated for offset feed... Owen
|