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Re: [RFI] RFI Digest, Vol 58, Issue 27

To: tomcox@iquest.net, n6pe@pacbell.net
Subject: Re: [RFI] RFI Digest, Vol 58, Issue 27
From: dgsvetan@rockwellcollins.com
Date: Tue, 30 Oct 2007 14:39:57 -0500
List-post: <mailto:rfi@contesting.com>
I have my doubts that you will find published electrical data on the 
expanding foam products.  I agree with Tom's suggestions (below) about 
doing no more than wasting one set of traps, but there might be an even 
less expensive approach:  if I understand your trap construction 
correctly, the foam will be in contact with the wire and the ceramic caps 
that are to be enclosed within each trap structure.  Therefore, the most 
important electrical characteristics would seem to be dielectric constant 
and loss tangent.  (Yes, corrosion that can result when using products 
containing acetic acid is a problem, but it is not, strictly speaking, an 
electrical problem.) 

If you have a way to read capacitance, or can borrow a capacitance meter, 
take one of the ceramic caps that will be used in the traps and measure 
its actual value with the meter.  Now, encapsulate that ceramic cap with 
the foam you propose to use and let it set up and dry.  Clean off the cap 
leads so that you can make contact with them and measure the capacitance 
value again.  By how much did it increase?  (I'll assume that the effect 
of the foam dielectric constant, whatever it is, will be to increase the 
effective value of the ceramic capacitors because the foam replaces the 
air that would normally exist between the cap's leads if it were not 
encapsulated.)  You'll need to consider that new value when designing the 
coils for the trap.

Next, apply some of the proposed foam to a microwave oven safe plate or 
container.  You don't need a lot - a glob of about an inch or so in 
diameter should be enough.  After it sets up and dries, stick the foamed 
plate or container into a microwave and "shoot the juice" to it.  Keep a 
very watchful eye, especially if you see smoke or color change within 
seconds of applying RF to the sample.  The idea here is to get an idea of 
the loss factor (tangent) of the particular foam in which you are 
interested.  If it gets real hot, you don't want to use it.  (That's one 
reason that PVC is not used for the dielectric in coax cables.  It's loss 
factor is higher than for polyethylene.) 

Assuming that you now have found at least one foam product that doesn't 
have too much RF loss and it did not shift capacitance value by more than 
5% or so, you are ready to make your traps.  One more caveat, however:  if 
the foam is going to fill the spaces between coil turns, that emans you 
will be placing a dielectric material between the coils, effectively 
increasing the capacitance between turns.  This can also result in 
detuning after you have totally assembled the traps.  Given that thought, 
it may be better to just leave the trap capsules unfilled (air only) with 
a moisture drain hole on the bottom and some sort of weather tight seal at 
the top end. 

Good luck with the project.  Please let us know what you decide and how it 
all works out.  Thanks.

- Dale


Tom Cox <tomcox@iquest.net> 
Sent by: rfi-bounces@contesting.com
10/30/2007 12:22 PM


Re: [RFI] RFI Digest, Vol 58, Issue 27

Don't know about RF characteristics, but some of those products have a 
strong acetic acid smell as they cure. They might be corrosive to 
copper. You can buy silicone caulk that is (supposedly) low in acetic 
acid. I don't know if foam is available similarly. You might try a test 
batch in a PVC capsule with some copper wire, and see how it looks after 
a week or so. If it eats the wire, RF characteristics are moot.

As to RF characteristics -- If you find foam that doesn't corrode the 
copper, and you can afford a realistic test, build a trap in PVC, run it 
on an analyzer (MFJ259B or similar), and run it again after filling  it 
with foam and allowing it to cure. Wasting one set of trap components is 
better than wasting a bunch. If you share your results with the list, 
many will probably appreciate it. Good luck.

Tom, KT9OM

rfi-request@contesting.com wrote:
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> Today's Topics:
>    1. Foam (Rick D)
> ----------------------------------------------------------------------
> Message: 1
> Date: Mon, 29 Oct 2007 18:21:28 -0700
> From: Rick D <n6pe@pacbell.net>
> Subject: [RFI] Foam
> To: Topband <topband@contesting.com>, SCCC <sccc@contesting.com>,  RTTY
>                <rtty@contesting.com>, RFI <RFI@CONTESTING.COM>
> Message-ID: <47268718.5020401@pacbell.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> Anyone know the electrical/RF characteristics of the various expanding 
> foam products?
> I am building some traps for my 160/80/40 L and was considering filling 
> them with foam. Traps are made from old RO water filters with #12 wire 
> and HV ceramic caps mounted inside.
> Thanks,

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