David,
The intent of saying "net Z" was to include the length of the grounding
lead(s), plus remembering that any significant bends will appear as a turn
on an inductor. With regard to physical object size relative to
wavelength, that is why I mentioned a frequency range of about 100 kHz to
10 MHz. Net Z will vary considerably because of the spectral power
distribution within a strike, making estimates very difficult (or at least
complex).
For towers and other grounded objects, the most critical Z will be that of
the grounding system closest to the object itself. In most tower
installations, that should be the ring ground installed around the tower
base, with at least 1 ground rod per tower leg, and a tie line going to the
main station perimiter ground (if available). Physics says that most of
the lightning strike current will flow into/out of that tower ground
because it is closest to the struck object and has least length; hence,
lowest Z. Of course, coupling to other parts of the system, such as the
coax and control line cables, will occur and part of the strike current
will flow along the shields of those cables, terminating at the shack
entrance panel (and that is WHY hams need to properly terminate their line
protectors into or onto some form of entrance panel that is well-grounded).
Fast risetimes are a good reason to minimize any bends or kinks in the
grounding wires.
Good point about the voltage differential across a grounding wire (or
system). Hey, it's real wire and has real resistance. The developed
voltages are directly proportional to the length of that wire and its Net Z
(actual Z + DC resistance). In a real strike situation, if "just" 10,000
amps flow in a coupled circuit, that's 10 kV per 1 ohm of Z in the path.
Nice "tingle".
73,
Dale Svetanoff, WA9ENA
Sr EMC Engineer
E-N-A Systems, LLC
Specializing in shielding applications, system grounding, and lightning
protection
> [Original Message]
> From: David Robbins <k1ttt@verizon.net>
> To: <rfi@contesting.com>
> Date: 9/18/2012 9:56:21
> Subject: Re: [RFI] ISOBAR
>
> the one thing that many people forget is that it is not just the
impedance, its the distance. The size of a typical shack, house, and
grounding system, is NOT small with respect to the wavelengths in play with
significant energy, so you can't use a simplified impedance to model the
protection. You must take into account the rise time vs distance between
connections and understand that just because you have a ground wire
connecting two things they can be at significantly different voltages
during a lightning stroke.
>
>
> Sep 18, 2012 10:47:14 AM, svetanoff@earthlink.net wrote:
> >Since all of the station equipment is tied (bonded) to
> >the one ground system (or should be), the lower the net Z to ground means
> >the less voltage developed differentially between grounded connections
and
> >power feeds, antenna inputs, and so forth. ness of that approach.
>
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