At 11:49 AM 3/31/2003 -0800, Michael Hatzakis, Jr wrote:
>No, as part of the complex negotiations to get approval from the very local
>authorities (wife) I decided to move the tower to a less exposed part of
>the yard (and you thought zoning boards were tough...) Which means there is
>at least 60 feet from tower base to shack. Not ideal. So, I was planning
>on multiple grounds rods at the tower base and a few just outside the
>baseboard of the new room. One of the issues this raises is one of ground
>loops, tower base vs shack? Are there issues of having two ground paths on
>the same circuit when ther is non-negligable impedance between them?
You need to separate "grounding for lightning protection", "RF grounding",
and "safety ground" as concepts. If you're driving the tower as a
radiator, then the ground at the tower is the "RF ground". The ground at
the shack would presumably be for "safety", and one would assume that you
have sufficient chokes/baluns/filtering/decoupling to make sure that the
outside of the cable doesn't provide a significant RF path. The RF on the
inside of the coax is part of the circuit, just like any other part of the
feedline system. The safety ground is what makes sure that when you are
standing there in bare feet on the damp concrete floor, you don't get a
shock from touching the rig.
For what it's worth, multiple ground rods within one ground rod length of
each other don't markedly improve the ground resistance. The notional
resistance for a single 10 foot, 5/8" diam ground rod is roughly rho/335
ohms where rho is the ground resistance in ohm-cm. A ground rod resistance
of 30-80 ohms is a representative range for common soil, etc.
What closely spaced multiple ground rods DO buy you is a)redundancy, in
case of a failed rod or rod connection and b)reduced current density at the
rod, in case of a high power fault (in the power distribution business,
they use multiple rods to reduce the chance of boiling the water out of the
ground when a fault occurs). If you're sending less than 50 Amps or so
into the rod, you probably don't need multiple rods, from a current density
standpoint.
If you're worried about high fault currents and step potentials (which the
commercial folks do worry about), then perimeter grounding rings and so
forth, are important.
IEEE Std 142-1991 has a whole lot on grounding in it. That's the industrial
grounding spec. There's also the "emerald book", another IEEE spec (which
number I can't recall, and I don't have the book in front of me) which
covers grounding for "sensistive equipment."
Note also that the kinds of grounding reflected in the NEC is somewhat
different than what the IEEE standards call for, particularly with respect
to multiple ground points. I suspect that any reasonably well documented
and explained approach will stand muster with the local regulatory authorities.
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