Yep, I've been informed that what I've done isn't such a Good Thing and I've
performed a naive (not to code) fix.
Among the problems I now face is distance: one antenna (a vertical dipole) is
about 300 ft from the shack, and the tower is about 100 ft away. The tower is
grounded, but I've not bonded that ground to any other grounds. The vertical
dipole does not use a ground reference, though I could bond the 3 ft support it
sits on to ground with a ground rod. Likewise, I have rx loops set up in K9AY
fashion that are grounded at a single point. Everything in the shack is
grounded to its own ground rod, which is now tied in only through the AC mains
ground (does not meet code). To meet code, I need to bond every ground rod
together with Big Wire of some kind, which means about 500 ft of bare copper
wire will be required. It looks like about 8 AWG solid wire was used for the
mains ground, though I'll have to measure it to be sure. Should I ever do this
again I'll build the antennas first, before any additional construction occurs,
so that the ground system is defined. Of course, that
means that I'd better build it like I want it to be permanently, because
changes and additions will be very expensive.
An aside: as a research meteorologist, I see how radars are grounded and
protected from lightning in field experiements : they use huge Litz-like wire,
and sharp spikes at intervals on every structure. The actual ground net is
trenched and then the wires are welded together with thermite (use of thermite
requires an explosives permit). They also used many, many ground rods at
intervals along the grounding net, and all conductors were bonded with welds.
That was the ground system for the old MIT/LL FL-2 Doppler radar and it alone
ran a bit over $100k in 1986. This worked relatively well, since the radar took
several direct hits without damage. A particularly large discharge did get
through, however, and destroyed various hardware including the data acquisition
DSP system that worked at a low IF. That was a very expensive lightning
strike...
Obviously, I have additional work to do...
Kim N5OP
----- Original Message ----
From: Jim Brown <jim@audiosystemsgroup.com>
To: amps <amps@contesting.com>
Sent: Monday, March 5, 2007 12:01:45 PM
Subject: Re: [Amps] Old Power Cord
On Sun, 04 Mar 2007 13:50:34 -0600, Kim Elmore wrote:
>I've gone to some pains to isolate my station RF ground from the
>mains ground and it isn't always straightforward...
This is a VERY BAD thing to do. ALL grounds within a building, MUST,
by LAW be BONDED together by a low impedance path (that means BEEFY,
DEPENDABLE, and SHORT). This is a requirement because it is the
SAFEST thing you can do, both for lightning safety and safety for
personnel. If this doesn't work for you, you need to fix your
station, especially your antenna system. And if you've read
somewhere that you should make that separation, burn that book!
73,
Jim Brown K9YC
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