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[TowerTalk] Copper strap v. Al question

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Subject: [TowerTalk] Copper strap v. Al question
From: (Jim Lux)
Date: Thu Jun 19 19:46:09 2003
Depending on why that strap is there, the DC resistance of the material is 
probably less important than the inductance. The inductance is determined 
by the shape (low, because it is a large flat ribbon, here), not the 
material. If you're looking at 4 parallel pieces of 4" wide strap that is, 
say, .030 inches thick, and 12" long, the DC resistance for copper is only 
about 0.023 ohms, and for aluminum, 0.036 ohms.. neither one is 
particularly high.

Skin depth goes as the square root of conductivity.. At 1 MHz, in copper it 
is .066 mm (about 0.0026 inch), and in aluminum, it's 0.0043 inches. For a 
large flat sheet (almost an idealized situation...) the AC resistance will 
go as the square root of conductivity (skin depth is sqrt(resitivity) 
deeper, so net result is resistivity/sqrt(resistivity))  This makes wide 
flat sheets of aluminum less bad than they would be at DC.  (By the way, 
that fairly thin skin depth means that using 1/4" plate isn't worth it)

As you say, though, electrically bonding aluminum reliably in a "homegrown" 
fashion is a bit tough. Commercially, you can weld it, for 
instance.  Copper could be soldered, a manufacturing technology with which 
most hams are familar and reasonably proficient. TIG welding aluminum is a 
bit more exotic.

At 10:51 PM 6/19/2003 +0000, Rob Atkinson, K5UJ wrote:
>I have a question re the copper grounding strap:  When visiting a small 
>local 250 w. AM broadcast tx site last year, I noticed the base of the 
>tower insulator on the ground side was strapped to the radial ring with 4 
>sections of what appeared to be aluminum flashing.  The insulator sat on a 
>square concrete base and the straps, one on each side, came down off the 
>insulator base where the spark gap was to the buried radials.  Each Al 
>strip was about 4" wide.  I was surprised to see a metal other than copper 
>being used.  (For no reason that makes any sense, I expected to see the 
>whole concrete base covered with copper--my ham tendency to want to 
>over-engineer everything.)
>I may have been mistaken but I don't know what else it could have been--if 
>it looks like aluminum, walks and quacks like it, it's aluminum.  Is Al 
>flashing an acceptable substitute for copper?  It's easier to find and 
>cheaper.  I imagine it presents bonding oxidation problems however.
>Rob Atkinson
>MSN 8 with e-mail virus protection service: 2 months FREE*
>See:  for "Self Supporting Towers", "Wireless 
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
>any questions and ask for Sherman, W2FLA.
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