Date: Fri, 18 Jun 2010 15:22:38 -0700
From: jimlux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] SPG or whatever it is called now, to coax
entrance ground question
>
It doesn't really matter.. The goal is to make sure that the two points
are at "reasonably" the same potential. If it were me, I'd snug it right
up against the foundation: 1)less likely to stick a shovel through it
when gardening or driving something into the soil and
2) concrete is usually higher conductivity than soil.. might as well take
advantage of it.
### say what? I thought concrete was a a good insulator?? One of the tower
manufacturer's
in Canada [L+R] told me yrs ago, sometimes they would tie the grnd
rods/cadwelds into the
rebar for the base of a tower... prior to concerete pour. Then I heard a year
later.. where a tower took
a hit... and pieces of concerete were flying every where ! They had jack
hammered the grnd rods in..
at the bottom of the tower hole/base... and cadwelded em, via buss bar.. to the
bottom ends of the
welded re-bar cage. The theory was....the grnd rods would be super deep.. and
below the frost line. In order
for the above scheme to work.. the path is down the tower legs.. then through
the concrete.. then into the rebar..
then from rebar into the grnd rods. That was a large distance between rebar
cage.. and tower legs.
## I tried the same stunt yrs later... but with a twist. The 1st section of
tower went right to the bottom of the hole.
3 x rods jack hammered in at bottom of hole.... and cad welded to all 3 x tower
legs. Then concrete was poured.
Rebar was NOT tied into the grnd rods or tower legs. If the tower gets hit
with lightning... the path is then straight
down the 3 x tower legs... and right to the bottom of the hole [5' deep]... and
continues on.. via 3 x grnd rods.. and goes
down another 8'. The tops of my grnd rods were 1" away from each tower
leg.
## there was a fellow who posted here several yrs back... who had > $1000.00
worth
of poly phaser equipment. He took a lightning hit... and destroyed a kilobuck
worth of
polyphaser gear.
## I wouldn't trust concrete to conduct anything.....esp high strength 30-40
mpa
concrete [4350-5800 psi] Higher strength concrete is way less porous to water
ingress as is.
later.......... Jim VE7RF
>
> At the coax entrance I'm going to place two weatherproof boxes with the
> Polyphasers in them from the two towers. How far away or close to(or on the
> house?) should I place these boxes?
How are you getting the coax into the house? through the back of the
box? down into the ground and then back up into the house? If the
polyphaser melts down, you want the burning crud to fall somewhere other
than on your siding. Beyond that, think about resistance to mechanical
damage: bumps, lawnmower/wheelbarrow/ladder strikes. They should be
some distance from any wiring inside the wall (phone, network, cabletv,
power), so that if there *is* a flashover from your antenna system, it's
not flashing to something that will carry the damage elsewhere. 6"? a
foot or two? There's some requirement in the code, but I don't have it
handy to lookup, and besides, most ham installations aren't "to code"
anyway (e.g. those Polyphasers aren't "listed antenna discharge units"
and I'll bet you're not running your coax in metallic raceways/conduit
the entire distance from the antenna, either)
Think practicality.. accessible for service, not where it will get
banged into or forgotten, not where it will propagate damage.
>
>>From this point I will also tie into the two towers with a couple of more
> runs of the 4 gauge copper. One will be about 32 feet the other 75 feet.
Sounds reasonable...
>
> Any and all advice, or suggestions please as I plan on digging tomorrow
> morning.
Make sure you lay extra runs of conduit, bigger than you think you'll
need. In some sense, the extra hours you have to work at your job to
pay for the conduit are probably less than the hours you'd spend digging
the trench a second time. (although, it *is* easier digging it back
up.. usually fewer rocks to remove, if nothing else)
>
>
> 73 Jeff kb2m
>
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