I will be puting a intenna mast up on the roof, and I plan on
running a wire from the mast to a grounding rod. My house is grounded
via just the water pipe, as far as I can tell, no grounding rod connects
to my electrial service. Do I need to run a wire from the grounding rod
to my water pipe to prevent a grounding loop?

No, Earth ground is earth ground is earth ground,

That's true, _if_ you happen to be living on a solid sheet of copper,
or a superconductor.

The actual DC resistance of "earth ground" across the width of a house
can be quite substantial. This can lead to a significant voltage
potential developing across "ground" under certain circumstances.

For this reason, the U.S. National Electric Code states that each
building may have only a single "grounding system". If you have two
or more ground points (ground rods, grounded water pipes, etc.) the
NEC says that you must "bond" them together with heavy-gauge wire,
precisely to overcome this resistance. 8-gauge is usually required,
and I believe that some municipalities require even heavier wire such
as 6-gauge.

If you don't do this, then under certain circumstances, an electrical
fault can have some very unfortunate effect. Let's say you have a
grounded antenna mast, with its own ground rod, but that you haven't
bonded it to your electrical service-panel ground. Let's also assume
that:

- You've got a DC-grounded antenna (e.g. a copper-pipe J-pole)
attached to the mast

- Your rig is plugged into an outlet, whose ground wire happens to
come loose, and

- Your rig develops an internal hot-to-chassis electrical short.

At this point, you're probably going to be in a world of hurt. The
"hot" current is going to flow into the rig's chassis, through the
coax braid to the antenna, down the mast to the ground rod, and
through the soil to the service panel. The soil resistance is going
to have two very bad effects:

- It can limit the current flow through ground to below the circuit
breaker's trip limit.

- It'll allow the rig chassis, and the antenna and mast, to remain at
a fairly high voltage above neutral.

As a result, you can end up with both a shock hazard, and a risk of
severe heating and fire. 10 amps through your coax could spoil your
whole day.

You only need to ground the antenna to the grounding rod, the water
pipe and antenna ground rod are both on the same circuit. Just think
of the earth ground as a very large negative terminal on a car
battery.

Nope. The two ground points could very well have tens to hundreds of
ohms of soil resistance between them. For purposes of electrical
safety, bond 'em together!

As Bob Pease of National Semiconductor said (in a different context,
also having to do with ground impedance): "You may be able to trust
your friends. If you're very fortunate, you may be able to trust your
government. You cannot trust your ground."

I'd strongly encourage the OP to check the his local electrical
codes to find out what the ground-bonding requirements are in his
area.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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