On Thu, 19 Apr 2007 02:36:26 +0000 (UTC),
dold@84.usenet.us.com wrote:
>Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> wrote:
>> Sigh. Sorry, I goofed. I was looking at a variety of DBS dish
>> reflectors on other sites and got your construction mixed with another
>> one that did NOT use a can type feed.
>The tin can was cut to 3-1/2 inches, which seems too short to me.
>with a 4" diameter can, I think the radiator should be at 1.7", but I would
>like to see the can at 5.5" as a standalone can.
3.5 inches might be a good length. With dish feeds, the idea is to
design the feed illumination angle so that it exactly covers the dish
area. No more, no less. In this case, there's no interest in
maximizing the gain because the dish does most of that, not the feed.
However, if he were trying to squeeze every last dB of gain out of the
antenna, feed gain would be important.
See:
<http://www.w1ghz.org/antbook/contents.htm>
<http://www.w1ghz.org/antbook/chap6-3.pdf>
See 6.3.1 above for details on the spillover (overspray) problem
and the introduction at:
<http://www.w1ghz.org/antbook/chap6-0.pdf>
for general clues.
New stuff:
<http://www.w1ghz.org/antbook/conf/high_efficiency_prime_feeds.pdf>
>Should the radiator be at the focal point, or the mouth of the can
>(properly sized for its own radiator)?
Just inside the mouth of the can is at the focal point. The exact
location is the "phase center" of the feed. See fig 6.1-1 in:
<http://www.w1ghz.org/antbook/chap6-1a.pdf>
I gave up trying to calculate the exact location and just move the
feed in and out until the gain is at maximum.
>And where might we assume that the radiator is on that adapter? It looks
>like it might be in the middle of the bore, which is good, but that's an
>awfully large hole in the side of the can, which I thought Jeff didn't like
>in building a wire cantenna.
I have a DWL-120 (somewhere) which is what the author used. It looks
like a loop antenna, but it's not. It's just a 1/4 wave (3.13cm)
piece of wire inside the plastic loop. The problem is that I don't
remember which side of the loop had the antenna. If I get ambitious,
I'll tear it apart and see. Meanwhile, you're correct. The antenna
is *NOT* in the center of the loop and therefore in the canter of the
can. That will cause a lopsided illumination pattern.
>Bob Alston's coffee can
>http://members.cox.net/tulsaalstons/Computer.htm
>Clarence Dold's coffee can
>http://www.rahul.net/dold/clarence/u...42-800x600.jpg
My (paper) coffee can:
<http://802.11junk.com/jeffl/antennas/coffee2400/>
I could probably scale the model to match the tin can in the DBS dish
construction, and see how it looks. (yet another project).
My hacked version of Ivor Hewitt's Excel spreadsheet for generating an
NEC2 "card" suitable for feeding to 4NEC2 antenna modeling software:
<http://802.11junk.com/jeffl/rf-calc/Coffee-can05.xls>
Unfortunately, it has some problems that I've been too lazy (or too
inept) to fix. Caveat Emptor and let me know if you find any (more)
bugs.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
jeffl@comix.santa-cruz.ca.us
#
http://802.11junk.com jeffl@cruzio.com
#
http://www.LearnByDestroying.com AE6KS
Build WiFi Range Extender from 18" Parabolic Dish, construction pictures + instructions 
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