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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