On Thu, 12 May 2005 22:28:26 -0700, DaveC wrote:

DirectTV 20" dish, offset fed, f/d~0.7, to be used for 2.4 GHz wlan link.

Offset feed dishes have different f/D ratios depending on
polarization. I just did a quicky measurement of an RCA (Thomson)
dish which yields 0.67. Close enough.
http://www.qsl.net/n1bwt/app-5a.pdf

As I understand the issues, the best feed is one that -- for a given focal
length -- will "light" the dish with as spherical a wave as possible, that
has a broad enough dispersion angle to fully "paint" the dish without picking
up off-the-edge noise.

Exactly. See:
http://www.qsl.net/n1bwt/contents.htm
for how it works. Be sure to read chaper 6 on feeds.

I've looked at all types of feeds, from cantennas to bi-quads to patch to
dual-dipole. All have their pros and cons.

Biquad should work.
http://www.trevormarshall.com/biquad.htm
http://martybugs.net/wireless/biquad/
-3dB beamwidth of the biquad is about 60 degrees. Using a protractor
to eyeball my pizza dish, it wants a 70 degree angle. Close enough.

I'd like to hear from anyone who has done this before about what feed they
chose and why.

See:
http://www.weijand.nl/wifi/
Have fun aiming the dish.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558