[TowerTalk] bonding to Rohn tower

[jimlux]

John Kemker wrote:
> Polyphaser has stainless steel hose clamps that have a section that fits 
> underneath the ground strap/wire/braid and protects from bimetallic 
> corrosion. (not the exact term for it, I know, but I can't recall the 
> exact term off the top of my head)
> 
> Georgia Copper (www.georgiacopper.com) sells them and also sells wide 
> copper strap to connect them with and Polyphaser strap-to-ground rod 
> clamps.  Only problem I've found is coming up with any sort of cadweld 
> or other brand of mold to attach strap to ground rods with. 


Then why not use what most people use for lightning grounds: stranded or 
solid copper wire/cable.  There's already plenty of inductance in the 
whole tower, so adding a fraction of a microhenry by going from strap to 
wire for few foot run isn't going to make a significant difference.  If 
you're worried, use a bigger cable.

The cable is more rugged, to boot.

I'm not sure I see the value in using a copper strap for a tower ground. 
Yes, you'll see copper bar stock used as a tie point or as a ground 
reference at the back of a bench, or on the wall behind a row of racks, 
but that's mostly because it's easy to drill and tap a hole to connect a 
wire to it where needed, rather than some sort of wire/wire tap or 
splicing. And that's something like 1"x1/8" bar stock, typically.


Yes, folks will be happy to sell you thin copper ribbon. After all, if 
YOU were in the copper business, you'd encourage folks to buy it. 
Copper ribbon in sizes like 1"x0.02" is used in applications like 
transformer windings for switching power supplies.

And sure enough, it might be lower RF impedance at 1 MHz than a 4/0 
cable (maybe, maybe not.. 4/0 is almost half an inch in diameter, so the 
circumference is 1.5". Skin depth at 1 MHz in copper is about 0.002", so 
the AC resistance of the 4/0 and a 1" wide copper strap is about the 
same.  Inductance is going to be pretty close, too.

Copper strap will probably be cheaper (there's less copper than in the 
same length of 4/0 or 2/0), but if you're going through gyrations trying 
to find a connector to bolt it all together, maybe you're solving the 
wrong problems?

Or, you could use a thicker piece of copper, and just drill a hole and 
bolt it to the tower (with appropriate hardware, and suitable washers.. 
probably stainless steel would be a good idea). Somehow, a hoseclamp 
style clamp doesn't strike me as a good long term solution.  Hoseclamps 
are designed to go on something resilient (hose?) that can provide a 
constant spring force.  The clamp itself isn't elastic or springy, so 
over time, it WILL loosen from temperature cycling. If you look at the 
compression clamps used for ground rods, you'll see they're a very 
different design.  A couple bolts squeeze the rod between the jaws of a 
clamp.  And even those need to be inspected and retorqued every year.



By the way, you don't need 4/0 or 2/0 to carry the lightning current. 
AWG 6 or 8 is probably big enough.  The current is high, but doesn't 
last very long.  Look at the grounding conductor on a wooden power or 
telephone pole. They're not running 2" wide copper strap or big 1/2" 
copper cables..  It's a solid AWG 10 or AWG8 (guestimating by eye from 
the last time I saw one a few weeks ago). The reason for a physically 
large conductor is for mechanical strength.