A copper shield can protect from "electrostatic fields", IE a Faraday
shield, but not magnetic fields. Grounding, and in particular the single
point ground can bleed off voltages be they from magnetic or
electrostatic fields.
A nearby lightning strike can develop tremendous magnetic fields that
will induce substantial voltages and currents in *any* conductor whether
it's inside a shield (coax) or not. Conversely I believe the shield
should serve as a shorted turn in the one plane.
Lightning does not need to follow the phone, or power lines into a
house. It will if that's the easiest way in, but the lightning can
induce voltages into every piece of wire in a house and particularly
wires that are not connected to anything such as old AC wiring that is
no longer used. Old network cables, extra RG6 runs in the walls for
future expansion, old twin lead that used to go to an outside antenna;
these are all great antennas for picking up substantial voltages. A
lighting strike a mile away can induce as much as 1000 volts per meter
of wire. So just disconnecting the cables from outside sources is no
guarantee against lightning damage.
Here my antennas are all at DC ground. It's why I use baluns in addition
to common mode chokes on wire antennas. All coax shields are grounded to
the top and bottom of the tower. The feeds to the slopers are grounded
to the tower where they reach the tower. They are grounded again where
they enter the junction box that contains the 6-pack and again at the
SPG bulkhead where they enter the house and that's where the arrestors
are located. Most of my coax runs to the house are close to 228' over
all. To the shop they are roughly150'.
There are a number of reasons for locating the arrestors at the
bulkhead with one being code. Another is any pulse on or in the coax
will be diminished in strength the farther it goes due to both
inductance in the run and capacitance to the grounded shield. I figure
that putting the arrestors at the house entrance will give them the
weakest pulse to work with and thus make them more effective along with
standing a better chance of survival.
This has worked for me. Whether it's the best way? I don't know, but it
also meets code.
73
Roger (K8RI)
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