DaveC wrote:
On Sun, 5 Jun 2005 04:23:34 -0700, Russ Renaud wrote
(in article ):
No, I'm talking about replacing the feed with my own feed. "Adjustable" means
that depending on how much of the dish I want to illuminate, I can change the
length of the feed's waveguide to extend beyond the feed point as described
he

http://tinyurl.com/ar9es


What kind of horn are you planning to use for 2.4 GHz? DirecTV dishes
are usually fairly small - the more efficient feedhorns are fairly
large, at least for 2.4 GHz if you have a .7 f/D DirecTV dish.


I haven't decided, yet. Options are cantenna-type feed, or patch panel-type.


The simple answer is to place the phase center of your 2.4 GHz feed at
the focal point of your dish. What is the phase center of your 2.4 GHz
feed? Maybe this is the question you want to ask. If so, the answer is
probably here.


Well, this I think I know. I'm just trying to determine what the feed point
is for the *existing* feed. That gives me a point on the hardware to attempt
to duplicate with the new feed. This is different and easier than aiming for
a point in space (the dish's feed point).


http://www.qsl.net/n1bwt/contents.htm


Yes, a great resource. I discovered this last month. While it is my "bible"
for designing this project, it doesn't contain the answer to this question.

OK. I think I understand. Assuming you are using the DirecTv dish, and
you want to replace the DTV horn with your own horn or dish feed, the
'phase center' of the DTV horn is about .25 lambda from the mouth of the
horn.

So, you place the phase center of your replacement feed at the same
point as the DTV horn's phase center.

Again, assuming you are using the DirecTv dish, neither the cantenna
nor a patch antenna will provide ideal illumination for a DTV offset
dish, which has an f/D of around .7

If you are OK with linear polarization (is the antenna for WiFi use?),
I suspect a biquad would provide better illumination than a cantenna
or a patch. Not that the latter two wouldn't work, it's just they
aren't optimum for that f/D

Russ