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Re: [RTTY] Topband: Foam

To: "Brad Rehm" <bradrehm@gmail.com>, "Rick D" <n6pe@pacbell.net>
Subject: Re: [RTTY] Topband: Foam
From: "brian" <brianb@brianboschma.com>
Reply-to: brianb@brianboschma.com
Date: Tue, 30 Oct 2007 15:46:01 -0500
List-post: <mailto:rtty@contesting.com>
Hi Rick,

A non conductive foam will alter the dielectric constant of the region in
which it is applied. Try searching "dielectric lens or antenna", probably find
an interesting set of data on the characteristics. Likely someone like GE
sells a foam that is rated for dielectric, but the density of the foam is
going to play a role. So, the more air in the expansion the closer the
dielectric will be to air. 

Here is an experiment I have done that yields a measurable result:  Take a two
meter vertical, measure the point of resonance by finding the lowest SWR on a
SWR meter. Now wrap the antenna with a foam, I used a fairly dense foam used
for construction of marine foils (rudders/keels/fins). Re take the
measurement. In my case I saw about a 2 mhz lowering of the resonant point,
indicating the antenna was surrounded by something with a dielectric constant
larger than air.

If you place a foam in the coil of a resonant circuit you would likely also
see the frequency of resonance pull downward.

Of course depending on the foam your mileage will vary and you may also see a
lowering of Q of sensitive circuitry/antenna's/cavities.


Brian n6iz.

"Brad Rehm" <bradrehm@gmail.com> wrote:

> > Anyone know the electrical/RF characteristics of the various expanding
> > foam products?
> > Rick Darwicki  -= N6PE =-
> Rick,
> We have several measurement products in our EMC lab that were filled
> with foam by the manufacturers to keep out moisture and mechanically
> stabilize them.  This has made servicing them almost impossible,
> though.
> I wouldn't use the foam because of this.  I built a preamp many years
> ago for mounting on a tower.  I weatherproofed the die-cast box as
> well as I could and then put a big bag of silica gel in with the
> electronics.  The silica-gel came in the packing materials for a
> computer.  I baked it for a few hours at 150-200°F to remove moisture.
>  After 7 or 8 years, the preamp was still shiney when we took it down.
> Brad, KV5V
> _______________________________________________
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> Topband@contesting.com
> http://lists.contesting.com/mailman/listinfo/topband

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