John Kemker wrote:
> Aluminum has 60% of the conductivity of copper.
For DC resistance.. for RF, it's actually better (skin depth is thicker
because the conductivity is lower)..
That's not "almost the
> same." In order to get the equivalent conductivity out of aluminum, you
> have to make it thicker, which isn't always the best choice, based upon
> the skin effect of lightning conduction.
Not sure about that.. AND, the resistance of the plate is insignificant
in this application for a couple reasons:
1) it's a short distance, so the resistance is small, in an absolute
sense. A foot of AWG10 copper wire has a resistance of 1 milliohm.
Either aluminum or copper sheet will be less. If you're carrying a full
20kA lightning strike through the plate the voltage drop across the
plate will be in the few volts range. A MUCH bigger problem is that the
whole plate has to be connected to something to dissipate the lightning
energy, so it will rise quite high. (but that's what good common
reference plane voltage is all about)
2) The domininant thing you worry about in transient suppression is
inductance, and the inductance of copper and aluminum will be
essentially the same.
For good mechanical
> properties, screw your copper to MDF and utilize that material's
> mechanical properties. That's what PolyPhaser does. That's what is
> recommended by I.C.E. as well. Engineers who design lightning
> protection systems for large installations generally recommend copper
> ground plates, not aluminum, for that very reason.
"Engineers who design lightning protection systems" have lots of other
requirements and different budgets than hams. Just because something is
done in typical commercial installations doesn't mean it's appropriate
or advisable for hams. Different application, different requirements.
You need to know WHY those Engineers are doing what they do. For all
you know, they have a company policy or regulation they follow, and it
was developed back in 1910, when copper was a lot cheaper than aluminum.
Example: I'm sure all hams use only Hard Drawn Copper wire or copper
clad steel for their wire antennas, and in the sizes required by the NEC.
> Of course, not every engineer designing lightning protection systems is
> going to specify copper for ground plates. Also, there's lots of people
> out there using aluminum who have not had problems with lightning. This
> is just my opinion, based upon the information available to me. Make
> your own decision.
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