This article on Ham Radio Station Protection from Polyphaser answers your
question.
http://www.protectiongroup.com/ProtectionTechnologyGroup/media/PTG/WhitePape
rsandTechnicalNotes/1485-013.pdf
It is important to take the cable off at the BOTTOM of the tower. If you
bring it off the tower higher up, the towers inductance will cause a large
transient voltage to develop (dv=Ldi/dt). The cable must be grounded at top
and bottom of tower. A surge device at top to protect inner conductor also.
Then route the cable to the utility entry and ground it again there to the
utility ground rod. A surge device here also, last chance before it goes to
your equipment. Any excess charge at this point will go into utility ground,
raising the potential of house utility ground. Think of your house neutral
wiring riding on top of a wave of this charge. If you also install a whole
house surge protector, then any difference between L1, L2, and Neutral will
be clamped to 180V or so, keeping the potential of any two wires below the
600V insulation limit.
It sounds like your tower is attached to other end of your house. It is best
to keep the tower 30 feet away from any structure, if this is possible, due
to the magnetic field pulse your conducting lightning struck tower creates.
This couples to circuits nearby and may damage them. Think foil wrapper in a
microwave.
In my particular case, I was contemplating a shorter direct run, until I
started reading up on this topic. I ended up routing all the way around my
house and in the final analysis added 60-70 ft from my original plan, but I
sleep better knowing the charge is outside all the way to utility ground
before it has a chance to come inside and the potential of that charge is
minimized by the above grounding technique and the wiring in the house is
also protected. The incremental cost and effort was well worth the peace of
mind.
I did not yet mention the tower should have a ground radial system too for
the earth to absorb the charge. I bring 2gauge tinned copper from each leg
to a ring of 2gauge around the tower foundation, three ground rods on this
ring, and extend 3 radials to 45 ft with ground rods every 16ft, all
cadwelded. The inspector who did the final on my tower inspects cell sites
and said I had done everything rights.
>From a safety standpoint (lighning), is there any reason not to run
>coax
through an attic for the length of a house? My new tower is located at the
opposite end of the house from the shack and I'd prefer to go the more
direct route through the attic. That would save me 25 feet or cable and
would look neater. But if there are issues of safety, they'll take priority.
Tnx/73,
John Harper AE5X
http://www.ae5x.com/blog
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