[TowerTalk] from towers to shack

[Jim Lux]

On 12/8/13 9:08 PM, Joe Subich, W4TV wrote:
>
>> Consider a direct strike to the tower? The cables being above or
>> below ground don't materially change the voltage waveform, at least
>> as selection of the suppression devices are concerned: it's kilovolts
>> you need to deal with.
>
> Cables "tapped" to the tower are coming off a voltage gradient from
> several hundred KV at the top of the tower to zero at ground (if the
> tower is properly grounded).  At 10 feet on a 100 foot tower, one had
> several 10s of KV .. not easily dealt with using simple devices.

But is it that high?

The inductance of the tower is about 1 microhenry/meter, as is the 
inductance of the cable.

So say you tap off 3 meters above the ground, and you have a 3 meter 
horizontal run, and then, the entrance point to the shack is 1 meter 
above the ground..

So we have 3 uH from the tower tap point to ground, then 6 uH on the 
horizontal and vertical components of the cables, but the input to the 
shack is only 1 uH from ground, so it's like a 1:6 voltage divider.


And, then, if you have a breakdown device at the entrance point, it 
breaks down at say 1 kV, and there's significant voltage drop from the 
tower tap point to the entrance point.

It's kind of like standing next to Van de Graaff generator charged to 
500kV.  If I take a spark from it, it's not like my body sees the full 
500kV.




>
>> Conventional transient suppression techniques work for phone and
>> power lines, which run above ground for miles.
>
> Phone and power lines are not connected to lightning rods several
> times the height of the lines.


Yes, but they ARE the highest thing above the ground for a long distance.