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Re: [TowerTalk] Connectors and grounding on tower

To: <rcblumen@centurylink.net>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] Connectors and grounding on tower
From: "Jeff DePolo" <jd0@broadsci.com>
Date: Thu, 9 Nov 2017 13:53:03 -0500
List-post: <mailto:towertalk@contesting.com>
> I read in the ARRL book that I should wrap butyl rubber tape over 
> connectors with Scotch 88 tape. Some people say to put 88 on 
> first with 
> sticky side up followed by butyl tape. Others reverse that.

"Tape and taffy" has been the standard way of weatherproofing connectors in
the commercial world for the better part of 50 years.  Applying a later of
Scotch 88 first is known as a "courtesy wrap" - it makes getting the
connection open again in the future much easier as compared to applying
butyl tape directly to the connectors.  When properly courtesy-wrapped, a
single axial slit with a utility knife should allow the entire tape+taffy
protection to peel off easily.  

If you use good tape (such as Scotch 33+ or Scotch 88), there's no reason to
apply the tape inside-out as it will still come off cleanly decades later,
unlike cheaper tapes that become brittle or have adhesive which stays stuck
to the connector while the tape peels off.  And of course, wrapping the tape
on the connectors the normal way makes for a better weatherproofing seal as
compared to wrapping tape inside-out.

The correct application is the courtesy wrap followed by butyl (and only
enough to provide a seal, not a big "blob" which will ultimately flow from
solar heating), followed by three wraps of 88 in "up, down, up"
shingle-style layers, with the tape pulled taught enough while wrapping to
prevent any wrinkles or gaps, but not so tight that it will cause the butyl
to flow out from under it when it gets hot.  The butyl should be applied as
a fairly thin wrap not a big "blob"; stretch it a bit as you're applying it,
overlapping the wraps slightly.  Then "massage" it to get out any pockets of
air that may have been trapped, and form it into a nice smooth
tapered-cylinder kind of shape, devoid of any bumps or valleys, so that the
outer layers of tape can be applied smoothly with no wrinkles or voids.

> Finally, I found that I had 5 rolls of THIS:
> 
> https://multimedia.3m.com/mws/media/37648O/scotch-linerless-ru
bber-splicing-tape-130c.pdf

I've been using rubber splicing tape like Scotch 130C, Scotch 23, and
similar products from Plymouth (both with a liner and linerless), for 20+
years.  I was turned onto it by a friend who was an engineer at a power
plant.  They used it for direct-bury underground splicing.  It is very good
when applied properly.  It should be applied in layers, and there should be
a good UV-proof tape as the outer layer (again, three layers of Scotch 88).
Rubber splicing tape, especially linerless like 130C, definitely has a shelf
life and will stick to itself and become impossible to get off the roll
after a few years.

> Also, many times I've read to install and ground coax 
> connectors at BOTH 
> the top AND bottom of the tower. 

You don't ground *connectors*, you ground cables.  The shield interface on a
UHF or type N connector is not a reliable path for conducting lightning
current.  Ground kits designed to work with your specific brand/type of
cable should be used to ground/bond the shield of the cable.  They should be
installed at the top of the vertical run, bottom of the vertical run (before
the horizontal bend), and again at the bus bar or grounding window at the
point of entry into the building.  If the vertical run is more than 200',
additional ground kits should be used at intervals no greater than 200' to
minimize the potential between the coax and the tower itself to prevent
arc-over along the length of the cable.

> So, once my cables leave the 
> polyphasers in the antenna box 3 feet from the tower base 

Polyphasers don't belong out by the tower unless the tower is in very close
proximity to the building.  The longer the distance is between the protector
and the equipment being protected, the less effective it is.  You want the
arrestors and all of the equipment to be *at the same potential* during a
lightning event; any L or R between where the arrestors are located and the
equipment is counter-productive.  

Putting arrestors at the base of a tower seems to be something that started
in the ham world in fairly recent history, without any good engineering
basis best I can tell.  Same thing for using connectors as a means for
bonding.

                                --- Jeff WN3A


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