> 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.
I've been using both DC-4 and DC-5 for 25 to 30 years (I retired from the
company that makes it)
>
> 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.
I don't use anything under the flooded heat shrink.
>
> 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.
>
I only use the silicone dielectric greases through VHF. I've had them break
down at UHF. Also if an arc does develop they break down into SiO2 (Silicone
Dioxide better known as sand, or quartz)
> ========
>
> 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).
I stick it on directly. Never had a problem so far.
> -- 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.
Three M has been making flooded heat shrink
http://www.rogerhalstead.com/ham_files/connectors.htm for at least the last
ten years.
>
> 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!
It works quite nicely when applied directly. Yes it does take a bit of work
to remove, but most of the glue peels off with the tube. I need to include
some photos in the above page to show how it comes off.
> 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 wouldn't use anything other than flooded after this many years.
OTOH I always figure connectors are a "one time use" and then discard them.
I now use crimp on connectors for most instalations along with the proper
tools.
> 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
And I do my best to make sure there is no grease on the cable or connector.
<:-))
I want the tube to be sealed/glued securely to the coax jacket.
However, if you don't want the glue to stick to the connector, a thin film
of that dielectric grease should keep it from sticking.
> 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
I purchase the flooded 0800/022 in 4' lengths for about $10. You can do
quite a few connectors with 4' of the stuff.
On boxes such as antenna matching networks where the connector is a chassis
connector the user has to be careful that the connector inside is sealed to
keep water from going through the connector into the cable. This is where I
seal the chasis connector with liquid electrical tape inside the box and
flood the inside of the connector (PL-259) with DC-4 or 5.
> 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
I have and they were with the exception of one where water had migrated
through a chasis connector and down some 9913. It was still shiny on the
outside though. Old, silver plated connectors still shine after many
years. That is saying a lot as any exposed connector on top of the big tower
will be thouroughly etched with in a year or so due to the corona discharge
up there. The system http://www.rogerhalstead.com/ham_files/Tower29.htm
gets hit regularly with an average of 3 verified hits per year. How may it
actually gets I don't know.
http://www.rogerhalstead.com/ham_files/Tower30.htm
> 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.
I've never been able to use one sucessfully if there was a noticable breeze.
I just haul up a long extension cord on a rope and use the electric heat gun
which also has one of those curved atachments.
73
Roger (K8RI)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|