On 4/18/11 8:00 PM, Gary Schafer wrote:
> Hi Mat,
> Copper strap is better than wire as it has lower inductance and lower
True when you're talking about the same cross sectional area of
copper.. Inductance doesn't change all that much, but resistance
changes a lot.
2/0 is about 0.36 inches in diameter (0.10 square inch, roughly) to
get the same copper area in Georgia Copper's "standard" thickness
(0.022"), you'd need a 5" wide piece.
If you start looking at skin depth, though.. at 1 MHz (a nice round
number for lightning discharges).. the skin depth in copper is about
That 0.36 inch diameter is about 1" in circumference, so the "effective
cross section" for 1 MHz is about 0.0026 square inches. A 1/2" wide
piece of copper strap would have the same effective cross section.
Copper stuff is basically priced by the pound, so for the same AC
resistance, the strap is 1/10th the price of a round wire. And if you
go up in frequency, the trade is even more attractive: you could use an
even thinner strap, as long as the width of the strap is more than half
the circumference. So, for a HF antenna ground, or a conductor carrying
high RF currents...
However, that 1/2"x0.022" strap is mechanically a lot less rugged than a
3/8" diameter copper wire, and when it comes to handling high peak
currents with high di/dt, you want mechanically rugged.
> Do not run any wires or strap out of the concrete base below grade.
> Someone mentioned that they ran wires thru the concrete base and out the
> side below grade to conveniently connect the radials to the tower.
Indeed, that would be a bad plan.. below grade penetration = microscopic
cracks for water to infiltrate in
> Radials should be bonded directly to each tower leg with a longer radius
> from tower to earth over the concrete base. Don't make a neat sharp bend
> over the edge of the concrete. No "ground rings" around the tower.
Again, this is more for mechanical reasons (with electromagnetic forces
from a lightning pulse) than in an attempt to reduce inductance. The
inductance of a half or quarter turn of wire is not much different than
the inductance of the same length of wire in a straight line. However,
a sharp bend has larger magnetic forces on the wires (the forces try to
make the radius of a bend larger).
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