On 5/15/2016 12:00 PM, towertalk-request@contesting.com wrote:
From: <john@kk9a.com>
...I'm not sure why more hams don't do what the pros do.
I've been watching the discussion with some interest to see where it
would go, and to see if anyone has come up with a better system than the
3/4" tape over the connectors/butyl seal/2" tape overall system. I've
been a broadcast engineer for over 30 years and have used the 2" wide
Andrew tape and butyl weatherproofing exclusively. It is also important
to tighten with wrenches or pliers to inhibit loosening from vibration
or wind movement. To date I have not had a connection fail due to water
intrusion using this system. If you examine the tens of thousands of
connections on broadcast towers across the world on Heliax and coax
products, this is what broadcast engineers and tower riggers use. Keep
in mind this sealing system has to be in use 24/7 and at thousands of
feet up in the weather and last for years. The very worst I encounter
when dis-assembling a connection protected in this manner is tape
adhesive residue on the connectors, but never moisture or corrosion.
The 2" wide thick tape is an important part of the system; when used as
the top seal it inhibits the butyl from extruding between the wraps of
tape, which can be a problem with 3/4" wide tape. It also allows a more
generous overlap for better adhesion. The high-quality tape Andrews
specs (not always 3M...) will not lose adhesion even after years of UV
and high winds. Of course the last few wraps must be made at low
tension and laid flat without wrinkles, AND making sure the surface of
the tape it is adhering to has no dust, oils or greases to inhibit
adhesion. (see below on using greases)
Packing the connectors with any sort of grease is problematic for a few
reasons: Some greases have a fairly high film strength, and the wiping
action of the mating parts may not fully displace it resulting in a poor
connection, compatibility with dielectrics or seals (BNC/N) is not
guaranteed, and unless the product is guaranteed to be free radical and
moisture free (most are not), you are potentially adding a source of
corrosion. Also, it can bleed, it makes a mess and seems to get on your
hands/gloves no matter what you do so you end up getting it on the
connector and coax. This film can then interfere with the adhesion of
the primary seal you put on the outside of the connectors. An example
of this problem: when installing flanged hardline or Heliax connectors
you must grease the o-rings, but then you must be be meticulous about
cleaning your hands/new gloves before you install any grounding kits
which include taping. Basically, if you do the external sealing
correctly you just plain do not need grease with non-flanged Heliax or
coax connectors.
You must be very careful how you use dual-wall adhesive heatshrink for
use on flexible cables, even the thick marine grade type. It is
important to clean the jacket with a degreaser such as isopropanol
before installing to ensure a good bond. Also, extend the heatshrink
4-6" onto the coax to ensure the zone near the connector remains
relatively rigid even if the bond at the end fails. I have had
factory-supplied FM antenna multi-bay phasing harnesses weatherproofed
with short lengths of adhesive heatshrink tubing fail after being
aggressively flexed by wind in cold weather when the hot-melt glue
became very stiff and brittle. The bond between the glue and coax
jacket then failed and resulted in moisture intrusion. I have since
installed cables with much longer heatshrink and many coax supports to
inhibit any movement. I took a picture of this exact connection after
rework on my website here (note broken phasing network cover due to
falling ice):
https://proaudioeng.com/wp-content/uploads/2016/05/20140822_103713.jpg
For connections with hardware such as lugs under screws and nuts, as
well as sealing exterior cabinet panels we use Scotchseal 800 which in
my experience will give years of life and seals pretty well to metals
and plastics even when exposed to UV and weather.
Remember: bonding occurs at the molecular level which means the
molecules you wish to bond together must be able to interact. Oils,
moisture, and dust can all inhibit tape/adhesive bonding. Cleanliness is
imperative.
Just my $0.02 worth,
Howie - WA4PSC
CE - WXYC-FM
UNC Chapel Hill, NC
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