On 4/22/18 3:15 PM, Steve Gorecki VE3RX wrote:
Thickness of the copper strap is not that critical, but the wider it is,
the better. A wide strap but thin, will have less inductance than a thick
strap, but narrow.
wider has Lower AC resistance than narrow, but not lower inductance (at
least not in the order of magnitude sense)
I use a minimum 2" copper strap for outside grounding,
and into the shack. In a small pipe, you can curl the copper strap to fit
the pipe, but never fold it, that adds a lot of inductance to it.
Actually, coiling the strap into a tube won't have a huge effect on the
inductance, neither does folding it.
Think of the strap as a bunch of parallel wires. The parallel wires
each have a magnetic field that interacts with fields of the wires next
to it, and less so with wires that are farther away.
So two wires, close side by side, are very tightly coupled (as in a RF
transformer for instance) and have basically the same inductance as a
single conductor.
What changing the shape has a big effect on is the AC resistance,
because of skin effect. (skin effect has a small effect on inductance)
At lightning frequencies (1 MHz) the skin depth in copper is about 2.6
mils (0.065 mm). The usual rule of thumb is to consider that there's
essentially no current deeper than 5 skin depths (current density is <1%
of that at the surface).
So, from a resistive loss standpoint, at 1 MHz, copper thicker than 30
mils (a layer on both sides that's 5 skin depths thick) is a waste of money.
At 7 MHz, the skin depth is about 1 mil (handy to remember - skin depth
of copper on 40m is 1 mil).
So if you're grounding your vertical - pay attention to the shape of the
conductor.
For lightning protection, worry about mechanical ruggedness, short runs,
and ease of use. There's a reason why stranded AWG 2 wire is so popular
- it's WAY above the fusing threshold for lightning currents. It's also
plenty heavy if something like a power line falls on it. And it's
readily available and not too thick to mechanically handle, but it can
take a lot of abuse - Someone digging with hand tools isn't likely to
miss hitting a #2 buried conductor.
BTW, if you're running wires in a conduit made of metal, all bets are
off with respect to AC resistance and inductance.
Use lightning arresters just outside the entrance, tied to your outside
perimeter ground around the shack. The idea is to stop any lightning before
it enters the shack.
73
Steve VE3RX
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