[TowerTalk] from towers to shack

Mon Dec 9 01:49:02 EST 2013

On 12/9/2013 12:08 AM, 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.
>

Many of us ground the shield at both the top and bottom of the tower

>> 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.
The length of the line has capacitance and inductance. This will Round 
off the leading edge of a pulse, and attenuate the pulse depending on 
its duration. It will also reduce the amplitude by radiation

Buried coax has capacitance to ground as well as between the center 
conductor and shield.  I believe the polyphaser page devotes some time 
to this and treats the tower as a separate object if it's more than 150 
or 200 feet from the house. So the transients are smaller at the house 
due to grpunding at the tower and buried coax.

As to how to treat the suppression at the shack entrance, that is still 
the same, it just doesn't have to work so hard.
I ground the shield at the top and bottom of the tower.  It goes through 
a grounded bulkhead and polyphasers at the shack entrance.
With all the direct strikes my 100' 45G and antennas have taken, I've 
only lost one polyphaser.

There were 17 visually verified strikes the first 6 years the tower was 
up, yet none since then.
Whether the neighbors no longer pay attention, or there really are fewer 
strikes, I don't know.
Rarely was the station disconnected.  Now I have patch panels, at least 
in the shop. http://www.rogerhalstead.com/ham_files/boat12.htm
It was too difficult to get at the cables in the den. By the time I 
heard the thunder and could get to the connections, it was no longer 
safe to work with them.

 From the antennas to the grounding bulkhead is 200 feet. by then the 
voltage between the center conductor and shield has been reduced quite a 
bit.
Antennas to the rigs in the den is 228 feet.

>
> 73,
>
>    ... Joe, W4TV
>
>
> On 12/8/2013 11:59 PM, Jim Lux wrote:
>> On 12/8/13 8:46 PM, Joe Subich, W4TV wrote:
>>>
>>>> I think as a practical matter, whatever transient suppression you do
>>>> at the entrance to the shack is going to be the same whether the
>>>> cables are laying on the ground, suspended 10 feet off the ground,
>>>> or buried a couple feet deep.

The same methods are used, but the voltage should be substantially less 
at the shack entrance than at the tower.
>>>
>>> Not if the cables are coming off the tower 10 - 15' high unless one
>>> has a grounded metallic "cable bridge" above the cables and connects
>>> the cable shields to the bridge/tower where they leave the tower.
>>>
>>> With cables exiting the tower some distance above ground, the lightning
>>> protection at the shack entrance will be much more critical.
>>>
>>>
>>
>>
>> But what would you do differently, transient protection wise?

You can calculate what the capacitance and inductance of the coax do to 
the pulse.
The capacitance from the shield to ground through the jacket is large, 
but I've not measured it.

>>
>> 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.
As I listed 17 direct strikes to the tower and antennas with the rigs 
still hooked up.  I lost one polyphaser and no rig damage. Prior to the 
grounding system installation, I lost one 2 meter rig and one computer, 
but that was in just around a year's time frame.
According to my understanding of the polyphaser page, the "power" is 
greatly attenuated by the time it gets to the shack on a 200 ft run.
>>
>> A strike right to the cables? Is that particularly likely, given that
>> there's a tall tower and a not as tall house next to it?

As the shield is usually connected to the antennas and tower, a strike 
to anything up there is a strike to the cables, but the tower is well 
grounded (to a substantial grounding network in my case. Over 600 feet 
of bare #2 CadWelded to 32 or 33 8' ground rods with several yards of 
concrete serving as a UFER ground. This should shunt most of the strike 
to ground, although there should still be quite a bit left.

73

Roger (K8RI)

>>
>> Conventional transient suppression techniques work for phone and power
>> lines, which run above ground for miles.
>>
>>
>>
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