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Re: Re: S band helix



Hello everyone,
There is one thing that strikes me in Ed's mail. That
is the diameter of 1 inch.
All the helix antennas for 2,4 GHz I have made have a diameter
of approximately 43 mm - that is much more. They are
center supported all of them. No PVC pipe :-)
73 OZ1MY
Ib
----- Original Message ----- 
From: "Howard Long" <howard@howardlong.com>
To: "Edward R. Cole" <al7eb@acsalaska.net>; <G0MRF@aol.com>;
<amsat-bb@amsat.org>; <w7lrd@juno.com>
Cc: <k5oe@aol.com>
Sent: Thursday, June 16, 2005 2:53 PM
Subject: RE: [amsat-bb] Re: S band helix


> Hi Ed
>
> > I would question the effects if mounted on a piece of tubing
(symetrical,
> > non-conducting plastic).  Helix antennas are inherantly wide-bandwidth
> > antennas with 15-30% BW.  As such, dielectric shift in resonance should
> > have minimal effect on resonant frequency (and gain).  I have a 5-1/4
turn
> > helix feed for my dish wound around a 1-inch diameter PVC pipe with a
> > 3/8-inch bolt running inside as support.  I use a short piece of brass
> > flashing on the first 1/4-turn for matching.  It worked quite well on
> AO-40.
>
> Although I admit David's a neighbour of mine, I independently came to
> exactly the same conclusions as him. True, a helix is inherently
broadband,
> but using a piece of PVC tubing as a permanent support and former has a
very
> significant effect, and these effects must be taken into account when
> designing and constructing the helix.
>
> ISTR that I had another similar conversation with Jerry K5OE who
> coincidentally is now an even closer neighbour of mine as M/K5OE. From
> recollection I believe he took the PVC's delictric effect into account
when
> designing and making his helices.
>
> There are some very useful hints in Ed Krome's Mode S - The Book from
> AMSAT-NA.
>
> Also I have some practical hints of my own (with others' help) on helices
at
> http://www.g6lvb.com/60cm.htm and
http://www.g6lvb.com/quadruple_helix.htm.
>
> 73, Howard GW6LVB/P
> ----
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