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Re: N3IYR AO-40 antenna



Being one who criticized your dish before, let me re-iterate that
you have a very good product-- especially from a mechanical standpoint.  My 
comments were that a helical feed on a 3' dish isn't
necessairly the "best" combination for AO-40.  It is a good compromise, the 
only problem being that helix feeds tend to have greater sidelobes than horn 
or patch feeds do, especially with no "cup" or choke ring at the feed.

Having said that, the amount of gain required to properly illuminate the 
approximate 0.7 f/D ratio on a offset feed makes a horn or patch feed 
impractical physically.  Many have stated that patch feeds work well on 
offset fed-dishes despite the obvious over-illumination at that f/D ratio, 
however.

The reason why, I believe, that either type of feed works well on an offset 
fed dish is that because of the 22.5 degree offset, the feed sidelobes in 
the helix case and/or the over-illumination in the patch feed case is, under 
most circumstances, pointing at the cold sky.   Therefore G/T is improved in 
most cases with an offset fed dish in satellite (not pointed at the horizon) 
use.

In fact, I have read that the 18" dish that the DSS systems use has the same 
G/T ratio that a 24" prime focus would have.  Part of the reduction in size 
of the DBS type dishes have is due to the fact that the sidelobes of the 
antenna are more favorably places.  This advantage vanishes when the dish is 
pointed at or nearly at the horizon.

So I still claim that your antenna is a compromise, but a very appropriate 
one.  A prime-focus dish with a G3RUH patch of the same size should 
outperform your helix fed setup, however, based on G/T calculations and 
theory.  But, since 39" TVRO Ku-band offset-fed dishes are so cheap due to 
MPEG-2 DVB use in most of the world (with North America being sort-of an 
exception) it makes very good finacial sense to go with the design you have. 
  And often those Ku dishes are of better quality than the prime-focus 
reflectors due to the high consumer demand.

I only took issue with your website before because you were claiming "the 
best" in many cases.  This is dangerous when it comes to engineering as "the 
best" often depends on many factors or trade-offs.  You may argue that you 
have the best price-vs-G/T combo out there and have a valid argument.

Remember in engineering:  "Good Enough is often much better than perfect".

And I'd say your antennas are probably better than "Good Enough"...

Fred W0FMS


>From: "Glenn Rollins" <grollins@attbi.com>
>To: <amsat-bb@AMSAT.Org>
>Subject: [amsat-bb] N3IYR AO-40 antenna
>Date: Tue, 28 May 2002 10:08:17 -0600
>
>Hello gang,
>I have received many inquiries about the antenna I have developed for AO-40
>and I thank all of you for your interest. It has been very time consuming
>and hard work, but the payoff is the satisfaction knowing my efforts have
>made AO-40 more enjoyable. I have resisted posting anything to this BBS in
>fear that someone would be upset with me using it as a forum for personal
>gain. The truth is that the work and cost involved in making these antennas
>available far exceeds the monetary gain I may accumulate. I developed this
>antenna system because of my frustration with my "grill" dish that is now
>land fill. I attempted to make my web site both informative and useful. I 
>am
>however having a hard time getting the word out about this antenna since I
>want to keep the price down for everyone by avoiding advertising. Sales are
>slow and I sure need the room in the tool shed. HI HI. I'm not sure as to
>how long I can continue to offer this antenna because of the above 
>mentioned
>reasons. If you have any suggestions on how I can let more people know 
>about
>this antenna, please let me know. It does work as good and better than I
>mention on my site. I also have added new detailed graphs of it's known
>performance and a nearly 3D X-ray of the feed. This is not just a dish with
>a feed. Everything including the shape and size of the dish is designed
>specifically for S-Band work. Earlier, I read some comments about my site
>and antenna by some critical people on this BBS. Time and proof have pushed
>those aside and my concerns have disappeared. I can go on and on and baffle
>you with numbers and technical gibberish like SOME sites selling THEIR 
>dish,
>but the bottom line is who crosses the finish line first, and I don't smell
>any exhaust.
>I did contact AMSAT headquarters and I was told there would be not harm in
>mentioning my dish here, so there you go.
>My very best wishes to all. Hope to meet you on the "bird".
>
>http://www.n3iyr.com
>
>Glenn Rollins  N3IYR
>
>----
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org




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