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Re: helix location vs focus



Hi Scott,

Interesting...  My can is 2" deep, with a 3 turn helix inside.
I wonder if the can moves the hot spot about the same amount,
regardless of how deep the can is?  Note that my "mouth of the
can" is not exact; I could be off by 0.5" either way.

Greg  KO6TH


>From: Scott Townley <nx7u@arrl.net>
>Reply-To: nx7u@arrl.net
>To: "Greg D." <ko6th_greg@hotmail.com>
>CC: amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] helix location vs focus
>Date: Sun, 09 Jun 2002 23:27:46 -0700
>
>My experience/modelling indicates that just a bit beyond the "mouth" of
>the "can" is about right; for my 1-1/2" high can and 2-1/2 turn helix
>the focus seems to be about 1.9" down the helix from the ground plane.
>The depth of the can (if used) does make a difference; my experimental
>set indicates about 0.4" past the can.
>YMMV :-)
>I have been hoping to publish all my modeling results etc. but lack o'
>time.
>
>"Greg D." wrote:
> >
> > >   I read somewhere (can't remember) that you should place the
> > >   bottom (i.e. feedpoint) of the helix at the focus of the dish.
> >
> > I seem to recall someone on this BB experimenting, and finding
> > that the hot spot was at the end of the first turn for a simple
> > helix.  My own experiments seem to put the hot spot at the mouth
> > of the can, for "canned helix" antennas.
> >
> > The best thing to do is to create (or locate) a weak signal
> > source, and adjust it for best signal.  Then, let the rest of
> > us know where it is!
> >
> > Greg  KO6TH
> >
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