Rick,
The general rule is to minimize the distance from your station ground to
the electrical service ground, to minimize rise of potential difference
in surges or strikes. But, in general, you don't want the surge/ strike
to prefer to go from the electrical panel ground back to your equipment
ground. A way to disconnect your antennas, control cables and power at
the rig is a good way to minimize any chance of spreading the current
paths in a fault to electrical ground. Many radio stations use a
central grounding panel where power, telcom and RF all come into the
building from outside, together, to group these cables to minimize
potential differences between them in case of a fault. For lightning
effects, remember that you are dealing with thousands of amps for a very
brief time, and even very low resistance conductors have a big voltage
drop along a length due to Ohms Law and the extreme amount of current.
For a mast, put a ground rod at the mast base to provide an easy path to
ground, of lower impedance then your feedlines and controls. Even put
multiple rods out close to the antenna mast or tower, to spread any
possible hit to earth as quick as possible. If the mast is some
distance from the house, don't run a large conductor between them to
bond, the coax shelds would be a lower impedance conductor than wire, if
you bond them to an earth system just outside the building. This is the
reason many radio stations use a copper or aluminum panel to pass thru
feedlines. That panel gives you a single point ground to attach to
earth. While having a VHF beam may not be adversely affected by all
metal guy wires; usually for HF, you break up guys with egg insulators,
or use synthetic guys to avoid stray resonances.
A review of the white papers on the Polyphaser site should guide you to
good solutions.
-Stuart Rohre
K5KVH
|