Tom Rauch wrote:
>
> You imply the tower "charges up", probably because you hear others
> say the same thing.
>
> Voltage drop is a function of resistance and current. E=I*R
don't forget the integral of the electric field along the path.
this is especially important in this case where the electric
field can be very strong under a cloud.
>
> How much current do you suppose flows into the tower?
>
> Let's assume the tower has a resistance of 1 ohm from end to end, and
> 1000 ohms to earth. How much current has to flow in the path to hold
> the tip of the tower at 1000 volts potential more than the earth
> surrounding the tower?
in the strong field under the charged cloud the ground potential
(even without a tower) will rise as the mobile charges in the
ground move in response to the charges at the base of the cloud.
this potential can be quite large over a small area. if you had
two ground rods separated by a couple hundred feet and ran insulated
wire back to a meter you could measure this potential change as
charged clouds approach. (don't attempt this at home, the voltages
can get quite large, especially if there is a lightning stroke near
one of the rods!) because the tower should have a much lower
impedance than the ground it will track the voltage at the ground
connection point, in order to do this current will flow up and down
the tower as the surrounding field changes. these currents can
get large even if the lightning doesn't directly strike the tower
as the charges in the ground and cloud respond to a nearby stroke.
>
> The tower would require just under one ampere of current
> CONTINUOUSLY to have a 1000 volt charge!
not to have 1000 volt CHARGE, but to have a 1000 volt DROP from
top to bottom. in a slowly changing field before a lightning
stroke this wouldn't happen. the voltage from the bottom to the
top of the tower would be virtually constant and equal to the
ground potential.
>
> Is such current possible except during an actual strike?
>
during an actual strike of even a rather modest 10,000 amps
you would get about 10,000 volt drop down that tower. this is
puny compared with the millions of volts that you could measure
from the tower to a distant ground reference (such as is represented
by the power lines into your house). never thought about that?
a common occurance on transmission lines is that a tower
gets struck. as the stroke progresses it raises the voltage on
the tower, but the voltage on the power conductors doesn't change.
at some point the voltage on the tower gets high enough to flash
over the insulators out to the phase conductor. this is commonly
called a 'back flash'. a similar thing happens when lightning
strikes your tower and 'finds' its way out of your shack via the
power lines through your radio.
this gets to the heart of the problem when you talk about
lightning and charged clouds and how they affect towers and
antennas. when you start talking about events on the scale
of charged clouds and lightning you have a basic problem. what
is 'ground'? you can measure large voltages between things
that are supposedly 'ground'. not only because of the resistance
of the dirt, but because of the speed of propagation of the
fields vs the rise times of the waves involved. over small
distances you can limit the differences in the ground potential
by connecting all your grounds together.
i hope this isn't rambling too much, but this is a complex topic
and we are just scratching the surface here.
--
David Robbins K1TTT (ex KY1H)
k1ttt@berkshire.net or robbins@berkshire.net
http://www.berkshire.net/~robbins/k1ttt.html
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