There are measurement techniques to see how a ground behaves under lightning
type impulses, but they aren't exactly portable.
If you get a ground that is acceptable by the NEC for AC safety that is good
enough. You aren't going to dissipate a lightning strike in any ground you
might build anyway... the real protection against lightning damage is making
sure that all conductors are at the same voltage which is the job of the
single point ground and arresters.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Roger
D Johnson
Sent: Sunday, January 10, 2016 16:57
To: Towertalk Reflector
Subject: Re: [TowerTalk] [Bulk] Re: [Bulk] Re: copper or galvanized ground
rods in red SC clay
On 1/9/2016 9:38 PM, Grant Saviers wrote:
> Roger,
>
> Do you propose that it is not necessary to achieve the recommended 5
> to 10 ohms of resistance in a tower ground system?
Recommended by who and for what purpose?
> What is your measurement technique to determine if the actual
> construction is likely to be effective?
There is no measurement technique that I know of. Just use good RF design.
>
> Grant KZ1W
There are factors in the design of a lightning ground system that are not
relevant in a power line safety ground. generally speaking, a safety ground
is not usually a good RF ground but a good RF ground is usually a good
safety ground.
Another factor not usually considered by hams is that conduction through the
ground is NOT the same as conduction through a copper wire! A copper wire
has a copious supply of free electrons and will happily conduct increasing
amounts of current until it melts. In soil, the conduction is by ions from
metallic salts in the earth. The supply of ions is finite and when all are
being used to conduct electric charges, the resistance rises sharply. The
soil has gone into "saturation". A lightning ground system has to be
designed to dump the current from a strike into a large "volume" of earth
capable of conducting the strike current.
We've all seen the illustration in ARRL Handbooks that show the doping of
ground rods with rock salt or Epsom Salts. This is to increase the number of
ions available for conduction.
Different soils have different
amounts of minerals, each case is individual. Water content also affects
conductivity as does temperature.
To sum it up...there is no "one size fits all" solution.
73, Roger
>
>
>
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