Date: Fri, 18 Jun 2010 16:38:04 -0600
From: EZ Rhino <ezrhino@fastmovers.biz>
Subject: Re: [TowerTalk] SPG or whatever it is called now, to coax
entrance ground question
Since you're starting from scratch, and since you apparently want to do this
right the first time, what about doing a complete ground system like we read in
the Polyphaser papers (and elsewhere, including ARRL articles) in which you
surround the entire house with copper, with grounds rods at the corners and
interspersed between? And how about using copper strap (lower inductance)
rather than cable? Just a thought. Sure is nice to do this before the
landscaping and concrete go in.
Chris
KF7P
## at one of the telco's I worked at back in the 80's... it was done very
similar to that. 18' rods were driven in....every 18' around the perimeter
of the building [aprx 40' x 60']
Everything was cadwelded. To grnd the short microwave tower on the flat roof
[8' tower..close to the edge of the roof, 10' diam Andrew dish] ... they used
2-3" wide copper strap.. abt 1/8" thick... which
ran down the outside of the building.. and bonded to the perimeter buss system.
Then all the ashpalt + concrete poured..after the fact.
Telco's are different of course. They are after < 2 ohm grnd in most cases.
At one site.. by a lake... they dug 2 x 450' deep holes [6" diam each]
and ran a copper tube
dead center, down the middle of each one. The space, surrounding the copper
tube was then packed with granular carbon. Forget the distance between the
2 x 450' holes.
At one mountain top microwave site [ helicopter access only] the mountain top
was solid granite rock. All the concrete had to be delivered via helicopter [
+ diesel fuel for the 3 x gen sets, that ran 24/7/365]
The 'grnd' consisted of 200 x 10 ga solid wires.. like radials... going out
150-200' in every compass direction. One day, the site took several
lightning hit's... and the place burnt to the grnd !
You could visibly see the darkened, burnt, etched black marks.. where the 200
x radials..used to be .
Depending on where you are located... and which time of the year you normally
get lightning.... you have to be careful with frost penetration. [same applies
with free standing concrete tower bases]
In most cases, 8' long, copper clad steel grnd rods will suffice. If 2 ga
bare, stranded Cu wire is used, it too should be down into the grnd a foot or
two... which is not always practical. I use the
2 ga bare cu..to bond the 3 x grnd rods at base of tower.... to the SPG just
outside the basement.. so they are at the same potential. In this case, bare
wire in the dirt is used vs insulated wire.
In some places like northern + interior of BC... frost penetration goes down a
long way... like several feet. At -17 to -40 deg C.... the top 3-4' of dirt
will feel like concrete. It takes a long time
to thaw out too.
later.......... Jim VE7RF
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