[TowerTalk] grounding (again)
jimlux at earthlink.net
Sun Apr 22 18:29:49 EDT 2018
On 4/22/18 3:01 PM, Jeff DePolo wrote:
>> What I am planning to do is to add a 0.25" aluminum plate on
>> the inside (between the studs) in the shack. The plate will
>> be about 12 inches wide (the KF7P plate is ~ 12 inches wide)
>> I want to tie the two plates together with some copper strap.
>> Any thoughts on the thickness of the strap?
> The thickness of the strap has very little effect on inductance.
>> My thought is to use 1 inch wide strap, and use as many
>> straps as I can, across the tops of both plates. Doing that,
>> I'll have to make "flag" folds (triangle) on several of the
>> straps to get them to line up with the 2 inch PVC pipe, but I
>> don't think that will be an issue.
> You didn't say how long the run length was between the exterior aluminum
> plate and the interior plate so it's hard to gauge what effect using strap
> versus wire would be. But generally speaking, you have to go to a fairly
> wide strap (like, 3" or more) to make a big difference in inductance as
> compared to something like #2 wire. I did the math on this a long time ago,
> but as I recall, 4" strap has about half the inductance as #2 wire for a 12"
> length. 1" strap doesn't have appreciably less inductance than #2 round
4" strap will have almost the same inductance than AWG 2 wire.
If you take a bar with constant cross section (1 sq cm), and you change
the width and thickness (so the DC resistance stays the same), the
indcuctance changes from 1.4 uH/meter at "square" to just about
1uH/meter at 20 cm wide and 0.05cm thick.
What will be better with strap is the AC resistance (skin effect) - if
your conductor is in the ground circuit of a 1/4 wavelength vertical,
strap is worthwhile, from a loss standpoint.
But not from a "voltage reduction for lightning" standpoint - the
voltage is almost entirely determined by the inductance, which is
weakly, at best, related to the shape of the conductor.
Take a AWG10 wire (0.1" in diameter, 0.001 ohms/foot) That foot of wire
is 0.3 microhenry (at 1 uH/m) - at 1 MHz, the impedance is
0.33E-6*1e6*2*pi or about 1.8 ohms - which is enormous compared to the
0.001 ohm DC resistance.
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