On 1/10/16 10:17 AM, K6OK via TowerTalk wrote:
Kelly ve4xt wrote:
Here's an interesting question: if the concrete base is an effective
ground connection, do you get more value by placing the rods a rod length
away from the base? <<
Is concrete is an effective ground connection? I have not seen any data
regarding the measured _impedance_ between tower and remote earth via a
reinforced concrete base. I am skeptical of using DC theory or DC
measurements as a proxy for the impedance of a tower footing seen by the
RF-like lightning impulse.
And when you start talking impedance, there's really two things: AC
resistance and Inductive reactance. big conductors/flat ribbons/large
surface area helps the AC resistance, but doesn't change the inductance
much.
It seems codes and practice are in a state of confusion with DC and
impedance. DC resistance traditionally is used to measure the quality of a
ground, yet low impedance is the design goal.
Codes (and formal requirements) want something that is measurable, and
low frequency AC resistance (a few hundred Hz) meets that nicely.
As long as the theory supports the performance at DC and the performance
at 1 MHz being related (and they are), then one can be used as a proxy
for the other.
For example TIA-222 suggests,
in their default grounding system, use of a star ground with wires having a
_surface area_ not less than #00. The surface area language tells me the
code writers were thinking about skin effect and impedance, not DC
resistance.
The code requirements also addresses mechanical strength: that's one of
the reasons for the wire size requirements. A ground in a conduit or
protected can be smaller than a ground that's exposed.
So it's not entirely clear what the "intent" behind the specific code
(or R56, or FAA, or MIL-STD) requirement is. And a lot of that intent
is buried in committee discussions that are not-trivially accessible.
I concur with Grant KZ1W, best known practice is a star ground with rods at
20 feet separation minimum.
73 Jim K6OK
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