[TowerTalk] Ground resistance

[Gary Schafer]

> 
> >I could put a cooper plate panel to the right side of the shack
> 
> Every piece of wire connecting things together is an inductor that
> stands in the way of effective lightning discharge. The longer the
> wire, the greater the inductance. When lightning hits, is is quite
> likely to arc from one point to another than to follow those long
> wires.
> 
> The most dirty words in lightning protection are LONG and
> INDUCTANCE. More copper plates solve nothing except acting as a
> place to bond all the antenna ground connections together, and
> those connections are only effective if they are very short.

The purpose of the copper plate is to give a low impedance connection
between all lines connected to it. (short low inductance)

> 
> Another point. When you tie the coaxes to a common ground bus or
> to your RAT switch, the shields are now grounded together. The
> Polyphaser or equivalent simply adds protection for whatever is
> connected to the center conductor (that is, it shorts the center
> to the shield with a lightning strike).
> 
> And remember that the nature of coax is to act as a common mode
> choke at RF, so when you short the shield to ground, the center
> conductor is pretty close to ground too. It isn't a PERFECT common
> mode choke, so perhaps 1% of the lightning strike is present
> between the center conductor and ground. So if the lightning is 50
> kV, the center conductor might be 500 v above the shield. Enough
> to blow something connected, but probably not to arc across the
> switch.

Unfortunately the center conductor is not at or near shield potential
because of propagation qualities of the coax. Lightning energy will
propagate on the coax like any other signal.

Also even if the center conductor is shorted to the shield at the antenna
with a grounded type antenna feed the lightning energy will still find its
way to the center conductor from the energy that is carried by the outer
shield. This happens because of "transfer impedance". At low frequencies, of
which includes a large part of lightning, the skin effect that normally
keeps energy on the outside of a cable from penetrating the shield falls
apart and allows current on the inside of the shield. 
The energy on the center conductor can be equal to that on the shield. But
because of the propagation delay in the coax the energy on the center
conductor arrives later than the energy on the outside of the shield. That
gives a potential difference between the center conductor and shield at the
shack end of the line.
This is why it is as important to protect the center conductor as it is the
shield.

73
Gary  K4FMX