Steve, W3AHL wrote:
> I received several inquiries off-list, so I'll summarize some of the answers
> here, instead of replying individually:
>
> No, you can't just multiply the inductance of one foot of wire by ten to
> find the inductance of 10'. Use the calculator.
>
> The inductance of both round and strip conductors increase as they approach
> a ground plane or other conductors. So the free space inductance given by
> the calculator link is lower than it would be in a typical installation.
> It should scale similar to the strip calculator shows (compare a 120" long
> conductor spaced 120" above the ground plane to only 12" above the ground
> plane to get a ballpark idea of how a round conductor would vary).
>
> Based upon actual rough measurements in my workshop, 10' of the following
> conductors spaced about 12" above a concrete floor have the following
> impedance at 10 MHz:
>
> #6 copper wire = 330 ohms
> 0.5 copper pipe = 270 ohms
AWG 6 is .162". Assuming you're talking pipe that's 0.5" in diameter
(not half inch pipe, which is actually .625 inch OD), that's a diameter
ratio of 3:1, for an impedance ratio of 1:1.2, which is pretty small. Is
your impedance including the AC resistance? (at 10MHz, AWG6 wire is
basically a tube, because of skin effect)
> 1.5" copper strip = 105 ohms
The rule of thumb is that for tube, you only get one side, but with
strip, you get both sides, so the 1.5" copper strip is more like two of
those 1/2" pipes (1.5" circumference) in parallel.. so the much lower
impedance is expected.
>
> The impedance scales fairly directly with frequency (so the Z at 1 MHz is
> 10% of the above).
Since Xl = 2*pi*f*L, this is expected
The impedance varies significantly with different types
> of "ground planes".
>
> No, it isn't better to run the ground bus higher above ground just to lower
> its impedance. I don't know why. But you always want your ground bus close
> to ground, short and straight. Every sharp bend significantly increases the
> inductance.
>
>
Steve,
Do you have an equation or other reference for the inductance of a bend?
Last time I went hunting for such a thing, I couldn't find it, and
running some FEM models showed that the inductance of a "less than
quarter turn" bend wasn't all that high. (because the inductance comes
from the interaction of the magnetic field of one part of the wire with
the current flow in another part, and for bends, there just isn't much
interaction). I wasn't interested in deriving an analytical expression.
There are good reasons to not bend lightning conductors or other pulsed
high current wires, because while the inductance isn't high, the
mechanical stresses are.
Jim
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