On Sun, 28 Oct 2007 09:48:18 +0000, a wrote:
a wrote:
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?

Thanks for the replies.
I agree that the radiating element must be placed at the parabola focus
to give minimal beamwidth, and that this condition is met when the
radiating element is placed at the focus (which is given by D^2/16d).

The point remains that I can still choose the parabola parameters to set
the focal length to whatever is desired.
Should I choose them so that the focal length is an odd or even number
of quarter wavelengths?

You are getting the use of a _parasitic_ reflector mixed up with what
amounts to a _mirror_ reflector in your case.

It matters _not_ what the distance is from the driven element at the
focus to the surface of the parabolic reflector -- in wavelengths,
inches, centimeters, or furlongs.

What you could do is mount a _parasitic_ reflector outboard of the the
driven element -- and _this_ would be positioned approx 1/4 WL from the
driven element. Tho', how much gain it might add is questionable...

73
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
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