Topband: Potting compound for beverage transformers.

[R. Kline]

Many years ago, I potted a toroidal balun used on a 10/15/20 meter quad
using epoxy. The result was a significantly larger VSWR.  The potting
compound had completely changed the characteristics of the balun.
Afterwards, I  realized that with its increased dielectric constant,  the
epoxy had increased the capacitance between all parts of the balun.  Since
that time, I never use potting on such things. Instead, I've depended on
small plastic boxes for weather-proofing (being sure to leave a few 2mm
holes or their equivalent at the bottom for moisture drainage).  Of course,
the effect of increasing capacitance should be much less pronounced at 1.8
MHz, still it is something to consider.  Having overlooked an insulation
problem, I found it  necessary to weatherproof the feed connection point of
my homebrew 20M beam.  I decided that I must try to encapsulate the
problematical point, and I chose to do it with polyurethane foam.  The
reasons that I chose this material were (1) the resulting foam had "closed
cells", i.e., each little bubble was isolated from all of the rest, so there
could be no transmission of water through the material. (2) with a lot of
air, the resulting foam should have a much lower dielectric constant than
solid materials, and therefore the increase in stray capacity should be
minimal (3) polyurethane itself had reasonable rf loss characteristics - not
the best, but acceptable (4) the material was readily available at hardware
stores in a convenient can - just press the top and the foam comes out of
its little plastic tube. The unused portion in the can may be used at a
later date.  Having used this material for thermal insulation, I was well
aware of one bad quality.  When exposed to sunlight it disintegrates.  In
order to overcome this limitation, I painted several coats of polyurethane
paint  onto the external surface.  I chose polyurethane paint because it is
one of the best paints for exposure to ultraviolet - perhaps, second only to
expoxy paints. I should mention that it is one of the paints recommended by
the UK Civil Aviation Administration for external use on aircraft.  I used
black paint because, all else being equal,  it is the best color for
stopping ultraviolet penetration.  I should mention that (at least in 4X)
polyurethane paint is commonly available in paint stores and is only
slightly more expensive than other common paints.  The result of sealing the
20M beam's feedpoint was satisfactory, even though the effect on tuning was
significant, and the driven element tuning had to be readjusted.  The bottom
line is:  It is necessary to consider both RF and environmental (weathering)
characteristics when choosing materials for RF.
73,
Riki, 4X4NJ

----- Original Message -----
From: "George & Marijke Guerin" <gmguerin at voyager.net>
To: <topband at contesting.com>
Sent: Wednesday, November 05, 2003 7:37 PM
Subject: Topband: Potting compound for beverage transformers.


> Topbanders:
>
> Does anyone have a good source of small quantities of potting compound
> suitable for Beverage and Pennant transformers, etc.? Maybe 4 ounces at a
> time (100+/- grams)
>
> Thanks in advance,
>
> George    K8GG
>
>
>
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