David:
I had been unable to find coax shield grounding kits for RG-213, the
material I used in all my tower runs. (Most of the kits I found were for
hardline-type shields of larger diameter than -213.) So I homebrewed my
own.
I found some heavy gauge copper flashing at the local scrap yard. You
might be able to find some new stuff at building supply stores - it's used
quite frequently as roofing flashing for upscale houses - not mine BTW. For
each connection I cut a piece of flashing 3 inches by 2 inches. I folded
the flashing around a 9/32 inch drill bit so that the final dimensions were
about 2 inches by 1-1/2 inches. The 9/32 inch drill bit was just the right
size to slip snuggly through a piece of jacket from my -213 cable without
splitting it.
I squeezed the flashing tightly around the drill bit with a handtool
used to put creases in sheet metal. I forget the tool's name but it's a
Wiss HS-1, available at Home Despot. It looks like a spring-loaded pliers
with jaws shaped like a hammerhead shark's head. I was able to get the
copper flashing to fit tightly around the full circumference of the drill
bit, with a flag of copper sticking off the side.
From Harger Lightning Protection I purchased a dozen lengths of #2
tinned solid copper wire (same as the rest of my ground system) with tinned
two-hole lugs pressed on one end of each wire. This special order cost $5
for each assembly, but I didn't have to purchase the $600 crimping tool! I
drilled two 3/8 inch holes in the flag part of the flashing, close to the
coax, to accommodate the two stainless bolts in each lug. The long lug and
its two large-diameter bolts/nuts/washers squeeze the flashing tightly
against the coax shield.
On each piece of coax where I wanted to apply a ground connection, I
removed a 2-1/2 inch piece of insulation (don't nick the braid.) I spread
open my homebrew clamp and folded it around the braid, applied the ground
wire and hardware, and tightened the nuts. To protect this connection from
the weather, I first took a piece of Scotch 2200 mastic pad. Each pad is
about 3 inches by 5 inches and is really sticky black material. I folded
this pad around the copper connection, making sure it contacted the coax
jacket above and below the flashing. I squeezed the edges of the pad and
around the bolts to make sure the connection was sealed. Then I finished by
covering the entire wad of material with several layers of Scotch 88 tape
and some rubberized tape.
I connected the end of the #2 wire, sticking out of the wad of tape, to
my tower with Harger ground clamps. These clamps aren't made for tubular
tower members, but for angle steel, like my Trylon tower.
Also, my antenna runs are 135 to 150 feet long. I chose RG-213 for
everything, even the V/UHF vertical at the mast top, because the losses were
less than 1 dB for the HF antennas, as recommended in W3LPL's web pages.
For the vertical, I figured that I could tolerate the higher line loss
because this antenna is used strictly for local repeater work, definitely
not weak-signal UHF stuff.
For your radials, you might consider wrapping the leg(s) with a small
piece of stainless steel flashing, then holding the copper wires in place
against it (on the outside, not against the tower leg) with a stainless hose
clamp. Use anti-oxidizing compound under the flashing and the hose clamp.
(I just clamp the radials to the #2 ground wires that come off the bottom of
my tower; I use small hose clamps for this.)
If you need further clarification of any of this, drop me a note.
GL es 73 de
Gene Smar AD3F
-----Original Message-----
From: David <k4zzr@bellsouth.net>
To: 'Tower Talk (mail list)' <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Thursday, July 25, 2002 1:23 AM
Subject: [Towertalk] Coax ground at tower
>Hello... My transmitting equipment will be between 1.5-50Mhz fed with
50ohm
>LMR-400 coax cable using UHF-PL259 connectors. According to Polyphaser, I
>need to know exactly which product do I use at the top of the tower and the
>bottom of the tower to provide the ground to my coax shield? I've seen
>websites showing the use of "cable grounding kits" but not sure where and
>what kind I should get? I've seen what appears to be the coax attached to
a
>UHF bulkhead splice connector with a clamp on it that's attached to a
ground
>wire connected to the tower leg. I guess I need some specific product
>numbers if any of you have done this successfully. This will be on a US
>Towers HDX-555 crankup galvanized tower. Incidentally, I'm already using
>ICE lightning arrestors on a copper SPG bulkhead.
>
>Another question....the total run from shack to antennas will be about
>115'.....should I use LMR-400 up to the top of the tower and then use
>LMR-400UF (ultraflex) for the remainder to the antennas so there's less
>twisting stress to the coax around the rotator? Or should I just stick
>with RG-213 for the whole length?
>
>And finally, which product do I use to attach my copper ground radials to
>the galvanized tower legs?
>
>Thanks for any advice,
>David, K4ZZR
>
>
>_______________________________________________
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