K4SAV wrote:
> Having the cables exit the tower above ground is a really bad idea.
> Voltage drop down the tower is very high during a strike. Anything
> exiting the tower above ground will see a very large voltage impressed
> on the cable at the point it exits the tower.
But how high? And what needs to withstand that potential?
The voltage would depend on whether resistance or inductance dominates.
A 30kA strike on a 1 ohm tower would only be 30kV over the whole length
of the tower.. call it on the order of 5kV over a 10 foot length out of
a 60 foot tower.
A 30kA strike with a rise time of 2 microseconds on a 20 microhenry
tower (1 uH/meter) would result in a voltage across 10 feet of about
3E6*30E3/2E6 or 45kV.. so for the direct strike scenario, the
inductance dominates.
That translates into very
> high currents on the cable. Those currents go toward the shack instead
> of into the ground at the tower base.
If the cable has a low enough impedance relative to the path through the
tower.
Considering the example above, where the di/dt makes the voltage 45kV.
If the cable path has some extra inductance (say by coiling the coax a
few times), you could make the path along the coax very much higher
impedance than through the tower, so the majority of the current might
flow through the tower. Using 30kA as your stroke current, say you
wanted to limit the current through the jacket to 1kA (peak).. you'd
need the coax path to have 30 times the inductance of 10 feet of tower.
12 turns, one inch apart on a 12" diameter form would be about 28 uH.
12 turns, 1/2inch apart on a 8" form is 23 uH. The former is about 10
times the inductance of the tower to ground from 10 feet up (I'd guess
the tower is <3uH) (I used Wheeler's approximation for inductance)
I chose 1kA as a round number, and on the assumption that the fusing
current of the jacket is quite a bit greater than the AWG13 center
conductor of a RG213 style cable. 16 AWG wire has a fusing current of
about 100150 A steady state. AWG13 is twice the area and fusing current
runs roughly as the area (for small changes in area). This is also a
short pulse at 1kA (lasting, say, 50 microseconds)
So, at least the coax won't melt, and if the voltage is 45kV across
those 12 turns, it's about 3kV/turn. If the turns are spaced a bit
apart, it won't arc over.
>
> Other effects:
> 1. The cable at the point of exit, looses its mutual inductance with the
> tower which increases the currents on the cable.
But, one can fairly easily increase the inductance of the cable, which
would reduce the current. So I think that effect is a wash.
>
> 3. The cable is now more suceptible to direct strikes.
>
Yes, but it's probably well below the top of the tower, so the tower
provides some protection (whether you use the rolling ball or cone of
protection idea, either way, a cable coming off at 10ft on a 60 ft tower
would have to go fairly far to be a "good" lightning target)
> 4. The cable is also more susceptible to induced currents due to the
> large loop area.
That is, I think, the biggest problem.
>
> 5. There are no good points.
Not having to dig a trench, not hitting your head on the cable when you
walk under it, etc.
There are places where mechanical constraints force doing things like
this. On commercial installations, they'll have a cable tray from tower
to building, and presumably, that tray carries a lot of the lightning
current.
Several folks (N6RK does this, I think) have array setups where the
feedline is open wire line carried across a field at some feet above the
field (so that animals can graze underneath)
VOA's big curtain array in Delano,CA is fed by open wire line supported
well off the ground. It might be interesting to look at what sorts of
lightning protection strategies they use. (Although.. they have a big
enough budget to take a "if it doesn't hurt, why not do it" approach)
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