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Re: S Band Helix



Using a form for a helix has two effects. It shifts the resonant frequency
down and it increases losses. The shift can be compensated for by decreasing
the helix diameter. The loss increases with frequency but some materials are
better than others. I've used fiberglass pipe as a form at 2.4 GHz and it
had lower losses than PVC and greater mechanical strength. For a short
helix, PTFE (Teflon) tube might work and would have very low loss. Losses
can also be reduced by reducing the area of the material inside the helix by
using thin wall pipe.

Another approach is to use 3 small rods with a non-conductive support at the
open end. This can reduce frequency shift and losses and still support the
helix.

73,

John
KD6OZH

----- Original Message -----
From: "Robert V Johnson" <w7lrd@juno.com>
To: <amsat-bb@amsat.org>
Sent: Thursday, June 16, 2005 02:06 UTC
Subject: [amsat-bb] S Band Helix


> Hello
> I'm doing some "stuff" with 2.4 ghz helix'.  I have seen some designs
> that use a plastic type pipe  (pvc) for a form.  I bought a 1.2 ghz
> helix on ebay that is a commercial unit and is enclosed in a large pvc
> type pipe.  Do these forms have a significant impact on the performance
> of these antennas?  I have read conflicting information about this.  I
> want to establish a general consensus of what is "right".
>
> 73 Bob W7LRD
> Seattle
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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