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Re: helix location vs focus

Hi Cliff,

The dish I'm using is a pretty standard BBQ-style grill,
covered with window screening.  I think these things have
an F/D of about 0.3 - 0.4 or so.  The focal length is 10",
with a top-to-bottom length of 32", so that matches.  The
feed is central, not offset.

So, it sounds like I may have an odd-ball canned helix.  I
re-measured the position of the feed, and 10" gets you from
the dish to the lip of the 2" deep 4" diameter can.  But, I
didn't re-experiment with the feed position when I removed
the support stick.  Perhaps I should...



>From: cbuttsch@slonet.org
>Reply-To: cbuttsch@slonet.org
>To: nx7u@arrl.net
>CC: "Greg D." <ko6th_greg@hotmail.com>, amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] helix location vs focus
>Date: Mon, 10 Jun 2002 16:52:19 +0000
>I've been experimenting with the focus location as well.  The length
>of the can does make a difference and I have yet to determine just how
>long the edges of the can should be.  When a helix is used, two inches
>seems about right.  When a patch is used, then an inch and a half seems
>better as the patch is so much closer to the ground plane.  The helix
>phase center does seem to be one turn in from the end closest to the
>reflector, while the patch appears to be at the surface of the patch.
>      Scott and Greg, what F/D are you using and are your dishes center
>fed or offset fed?  The goal of this is to obtain the minimum amount of
>terrestrial interference which in my view is the limitation of mode S.
>Cliff K7RR

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