[TowerTalk] Ground resistance
Roger D Johnson
n1rj at adelphia.net
Thu Jun 8 14:06:27 EDT 2006
Jim Brown wrote:
>> Electricians and power companies are mainly interested
>> in a measurement at 60 Hz.
> WRONG! Earth grounding for power systems is solely for protection
> against lightning and other potentially destructive on the power
> line. Earth ground should NEVER carry current (except under a
> lightning strike or power spike condition), and those are going to
> be RF currents, not 60 Hz.
I think you are reading too much into my statements, Jim. I'm trying
to keep it relatively simple for Gary until he comes up to speed. It's
my understanding that most ground measurement devices use a low
frequency (100 to 300 Hz). The result measured is not necessarily
indicative of performance at RF frequencies.
>> Hams with vertical antennas are interested
>> in the resistance at the frequency of operation.
> Wrong again. Vertical antennas do not work against an earth
> ground, they work against a conductive plane that may or may not
> be earth. The earth itself is rarely sufficiently conductive to do
> this in an efficient manner, which is why we install ground
> radials. A ground rod (or a collection of ground rods) is quite
> unlikely to improve the RF performance of a vertical antenna.
> Those ground rods ARE important for lightning protection.
Again, you are making it more complicated than I intended. Since Gary
is interested in lightning protection, I didn't feel it was necessary
to elaborate on vertical antenna ground planes. I just wanted to convey
the idea that the RF resistance, at the frequency of operation, is
usually the most important factor in a vertical antenna installation.
>> lightning has a large RF component
> Correct. IEEE data shows the energy spectrum of lightning to have
> a VERY broad peak around 1 MHz, with lots of energy a decade or
> two above and below that frequency. Thus the INDUCTANCE (and thus
> the length) of the connection to earth is often of FAR greater
> importance than the resistance.
> Jim Brown K9YC
Agreed. I also want to stress the importance of proper bonding in
any lightning protection scheme.
Remember the USS Liberty (AGTR-5)
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