[TowerTalk] Crank up tower lightning protection

[jimlux]

On 10/11/16 10:43 PM, Grant Saviers wrote:
> To paraphrase, it depends on what the definition of a "good bond" is.
> It seems to me that at the overlapped section ends, two legs are always
> in contact at one level and at least one at the other level.  Wind, tilt
> from gravity, or off center tension from the hoist cables insure the
> tower sections lean against each other. The wear marks I've seen on the
> inside of crank up legs have the galvanizing missing where the "sliders"
> are wearing when the tower is extended.   So my bet is the tower legs
> are the primary ground path, not the cables.  So the path inductance is
> probably about the same as similarly sized fixed towers.
>

The inductance is driven by the length, not the width, of the conductor, 
so I would expect the voltage drop along the tower from a lightning 
impulse to be basically the same for the "through the tower" path vs the 
"through the coax shield" path.

AC Resistance through the tower is another story - the tower is steel, 
not copper, so the bulk resistivity is higher, and because it's steel, 
the skin depth is smaller. There's also the "section to section joint 
resistance" (which I suspect is fairly low). And since we're talking 
about an AC signal here (lightning), the capacitance between the 
overlapped sections is also significant - although I've not even back of 
the enveloped what it might be.

For copper, the impedance of the inductance (1uH/meter - 6 ohms/meter at 
1MHz) dominates over the copper resistance (whether AC or DC - fraction 
of an ohm/meter).  But for steel?

Carbon steel has a resistivity about 10 times that of copper, and if I 
assume a relative permeability of 100, the skin depth is just under a 
0.001" at 1 MHz.  There's probably enough magnetic field during a 
lightning transient to saturate the steel, so maybe a mu of 1 is a 
better approximation - that's 10 mil skin depth, then.
(250 microns) - or maybe we should look at the zinc coating?

I do these kind of things by scaling from something familiar - AWG 10 
copper wire is 0.1" in diameter and has a resistance of 1 ohm/1000 ft.

The steel is 10 times as resistive - AWG 10 wire DC resistance is 10 
ohms/1000 ft, and has a cross section of (.05)^2*pi =0.00785 square 
inches.  A tower has 3 faces that are 12" long, and with a 0.001 skin 
depth, it's got a cross sectional area of 12*3*.001 square inches or 
.048 square inch.  That's about 6 times the cross sectional area of our 
AWG 10 wire.

So, ultimately, the AC resistance of a tower, at 1 MHz, is probably 
around 1.6 milliohm/ft or 5 mOhm/meter.

The inductance still way dominates at 6 ohms/meter.