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RE: 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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