>From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net>
>Sent: May 30, 2008 1:47 AM
>Cc: 'Skip K3CC' <email@example.com>, firstname.lastname@example.org
>Although most damage is normally caused by spikes coming in on either
>the electrical mains or telephone lines, it's not necessary for any
>outside connection when a powerful nearby strike is involved.
>According to the NWS literature a strike a mile away can induce voltages
>as high as a 1000 volts per meter in a piece of wire. As the inverse
>square law still holds, imagine what a strike a 100 or 200 yards out can
ACtually, it's more linear fall off than inverse square. Inverse square
applies when the source is a point (or can be considered as such in the scale
of the analysis.. a 30 foot yagi is a point source to someone 100 miles away).
Lightning is more of a linear conductor.
The other troubling thing about the "volts per meter" statement for induced
voltages is that this isn't how the physics works. Induced voltages are
induced in a loop, and the voltage is proportional to the area of the loop and
the rate of change of the field (more properly, the flux passing through the
loop.. it's orientation sensitive, after all).
The E-field along a stroke is in the tens of kV/meter sort of range, but it
would drop off pretty fast as you moved away.
That means in these cases even relatively short interconnecting
>wires between equipment may be long enough for damaging and sometimes
>lethal voltages to be induced.
The wires have to form a loop. Hence the effectiveness of twisted pair, which
has very small effective loop area.
>Even with every thing floating a spike with a steep rise time can create
>substantial differences in voltage between the ends of relatively short
TowerTalk mailing list