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
wire.
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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