----- Original Message -----
From: Tom Rauch <w8ji@contesting.com>
To: <W7TS@ibm.net>; <towertalk@contesting.com>; <K7LXC@aol.com>
Sent: Saturday, June 12, 1999 6:30 AM
Subject: Re: [TowerTalk] Force12 C-3 and Omni-VI+ Static Build-up
>
> > Why not do the right thing by putting a lightning arrestor like a
> > Polyphaser at the point where the coax enters the shack connected to a
> > couple of ground rods? It's not a lightning protection *system* but it
> > should protect your radio frontend from these static buildups.
> >
> > This is probably the easiest option as well since it's at ground
> > level
> > and you don't have to fool with the antenna.
>
> Polyphaser and other lightning arrestors do almost zero for
> protecting HF receivers. The problem is they have a breakdown
> voltage of about a kilovolt, and the receiver has a failure voltage of a
> few hundred volts or less.
The grounding relays certainly do a lot, at the point where the remote
switchbox goes. Good first line of defense -- probably enough for a lot of
induced pulses. They will take care of the static from the antenna. Close
lightning, however, is a different can of worms.
If one takes a direct hit on the tower, or very close, and the ground
saturates at the remote switchbox point, then the coax shield, and the
center conductor by induction, will arrive at the shack with a voltage to
settle. If the shield is grounded at the house entrance point, that will
help drain the shield pulse. However, unless the center conductor is also
grounded at this point, it will carry a differential on to the rig.
If a *DC-blocking* arrestor is used at the house entrance, in addition to
the ground of the shield, then the rest can be dealt with by a coax switch
at the rig that grounds the incoming and leaves the HF rig disconnected.
This, in a thimble summary, is what Polyphaser recommends. It is also a very
good reason NOT to use a voltage-on-center-conductor type switch .... the DC
blocking arrestor can't be used.
The switch operating lines to the remote location will also have to receive
arrestor treatment at the entrance point. These arrestors fire at much lower
voltage, since you are protecting a 24v circuit, or less, and are a greater
protection.
*HOWEVER*, even if grounded at the house entrance, a rig-killing
differential can STILL be present, if the coax house entrance ground is not
at or directly connected to the power service entrance ground. As the
lightning pulse spreads out, (we are talking about a period measured in
microseconds) the ground closest to the strike is at a raised potential to
the ground farther away. THE GROUND ITSELF will use whatever pathways it
finds convenient in it's microseconds race to spread out and dissipate.
One unfortunate path of ground potential equalization, since the stuff under
the house is drier and often laced with less conductive construction debris,
is this... Read and shudder...
The path of ground equalization comes across the coax shield to the ground
at the house entrance. Blocked from direct propagation forward by basement
or less conductive material under the crawlspace, it goes up the shield of
the coax to the rig(s) and everything tied together by coax of any kind, to
the green wire connection in the electrical system, and on over to the power
service entrance ground. The ground equalization DOES go around the house,
but it takes LONGER to do so. In the time between the arrival of the outer
equalization around the house and back up the green wire from the power
ground, lies opportunity for all kinds of odd impedance and distance related
destructive wierdness. The path through the house is high impedance relative
to a robust ground bonding, and allows a way for the pulse to build up to
destructive potentials headed across.
So, for this very short period of time, the *chassis* of your rig can be at
a high potential above the inner circuitry, and a lot of other things in the
house, as well, including above both neutral and high leads in the power
wiring. The rig then gets killed with a *backwards* zap, a very short-lived,
but very chip-and-transistor-killing-capable backwards zap.
In this circumstance, the common ground has become the enemy, because there
was not a direct path from the coax entrance ground to the power service
entrance ground. Depending on exactly how the power wiring physically runs
inside the house, this could fry TV's, etc, even if the HF rig itself was
physically disconnected. All there needs to be is a path from the incoming
shield to something connected to a green wire in the power wiring.
You may say, what about the station ground? Frequently the station ground
and the coax entrance ground are one and the same, aren't they? In this case
you don't even need the shield of the coax to create the differential. This
is NOT to say that you shouldn't have a station ground, just that it needs a
good, robust bonding to the power service entrance ground, one that is the
most attractive route to a ground equalization pulse.
The best condition, of course, is that the power entrance, telephone
entrance, coax entrance, and station ground are all one and the same.
There has been at least one recent TowerTalk post of a rig-frying episode I
would attribute to a ground equalization differential.
Get the Polyphaser book, understand everything they explain, and do the
work. Using a grounding switch like the RCS-8V at the remote switch point is
a good first step, but don't stop there.
73, Guy.
>
> The solution I use is relays, that automatically disconnect the
> center conductor. The RCS-8V does this, and the relay shorting
> bar goes to ground giving excellent protection.
>
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