Hi --
I use high-dielectric silicone for three tasks around the station:
a) a lubricant for O-rings on watertight connectors and gasketed boxes.
A film spread on with a finger helps make a clean seal.
b) a flooding compound on connector faces, including coax connectors. A
healthy squirt on the connector face and a film on the connector threads
before mating helps remove air/moisture from the connector during
mating, and keeps them out.
c) a flooding compound under UV-resistant heat shrink over connectors.
I have started experimenting with heat shrink tubing to protect coax and
other connections out in the field. More on this topic below.
Silchem (www.silchem.com) sells high-dielectric silicone in 8-oz
squeeze tubes for $12 + shipping on their website. Anyone can buy
tubes, cartons of tubes or even pails of the stuff off their website.
The electrical specs of Silchem SM7704 are:
DIELECTRIC STRENGTH (50 MIL) 710 V/MIL
DIELECTRIC CONSTANT (1 kHz) 2.9
DISSIPATION FACTOR (1 kHz) 0.0025
VOLUME RESISTIVITY (OHM-CM) 7.5 x 10^13
I haven't done extensive testing on this stuff yet, but so far it
seems sufficiently promising that perhaps others will be interested to
test it at their sites.
========
Now about heat shrink:
I have long been waterproofing coax and other connectors outdoors
with the three layer method:
-- inner layer of Scotch 88 tape or, for very lumpy connectors,
Teflon tape. The purpose of this layer is to keep the next layer from
sticking to the connector body (which would result in a mess when it
comes time to open up the connection).
-- middle layer of vinyl mastic tape, extending past the edges of the
inner layer, providing the waterproof seal.
-- outer layer of Scotch 88 tape, extending past the middle layer,
for UV protection. (Vinyl mastic, self-vulcanizing tapes, etc all
become brittle after a period of UV exposure.)
This procedure works well, but:
-- it is time-consuming to apply.
-- it is difficult to apply to connections on a box; e.g., a coax
connector attached to an outside switch box. Frequently these boxes
have multiple connectors, leaving little space to maneuver the tapes...
and it is nearly impossible to seal a chassis-mount SO-239.
Recently I came across a UV-resistant heat shrink tube with goop
inside. When shrinking the tube from the middle out towards the two
ends, the goop squeezes air/moisture out and provides a watertight
seal. What was new (to me) about this particular heat shrink was the
UV-resistance; past versions were made for direct burial of the cable
joint in the ground, where UV is not a problem.
Unfortunately, the goop is a heat-sensitive glue, which rules out
using it directly on a coax connector that one would wish to open up in
the future! While one could apply an inner layer of Scotch 88 to keep
the glue off the connector, one is back to a time-consuming multi-step
application process involving wrapping tape (sometimes in awkward places).
Other "dry" (no glue or other stuff) UV-resistant heat shrinks exist,
including versions pre-shrunk over a spiral plastic tape that can be
pulled out to cause the tube to shrink down onto the connector (clever -
avoids using a heat source on the tower).
But so far I haven't found a combination of (a) UV-resistant heat
shrink with (b) a non-glue flooding compound.
I've started testing the following two-step solution:
-- smooth out a lump of high-dielectric silicone around the connector
body and a bit of the adjacent cable (or SO-239 threads), forming a 1-2
mm thick layer. Accuracy is not important here. A little practice is
sufficient to judge how much is "enough".
-- carefully move the heat shrink tube over the connection, and apply
heat from the center of the connector body out towards the end of the
tube. The tube shrinks down over the high-dielectric silicone, filling
any voids and forcing the excess out the ends of the tube when
finished. Wipe off any excess with a paper towel or rag.
Finished!
This seems quicker and neater than the three-layer tape method, and
seems to be a better seal for coax connections to SO-239s on boxes. I
will admit a length of UV-resistant heat shrink costs a little bit more
than 2 meters of electrical tape and a meter of vinyl mastic tape, so
not everyone will want to trade time for cash. But even folks who are
pinching pennies may want to try this on SO-239 and other chassis-mount
connections outdoors, where the 3-layer tape method is more difficult to
apply successfully.
Caveat: I haven't done this long enough to say "connectors which
were sealed for 10 years were opened up and found to be in perfect
condition". But this approach seems promising enough that perhaps
others might want to consider trying it.
73,
-- Eric K3NA
p.s.: Oh, about heat sources on the tower. "Hot air tips" and "shrink
attachments" are available for many hand-held propane or butane
torches. The shrink attachment provides a curved shield a few inches
opposite of the flame end of the torch. The torch on a low setting
shrinks well, and the curved shield causes heat to reach all sides of
the heat-shrink tube evenly (without having the move the torch to
different sides), while protecting any adjacent cables or materials.
Get a self-igniting torch -- much easier to start in a breeze.
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