The way most people do it is to put a stainless steel shim between the
zinc tower and the copper wire and clamp it. Polyphaser makes clamps
that work like that.
That said, doing it this way doesn't make any sense to me. You have at
least three goals in avoiding galvanic corrosion.
1) Maximize the contact area of the least noble metal exposed to the
electrolyte (water),
2) Minimize the potential difference between the metals in contact, and
3) Keep the water out.
1) The least noble metal in this case is the zinc tower, but the contact
area is the same for both metals because water may collect within the
junction but the whole tower is not going to emersed in a liquid. So
item 1 (in the case of what is in contact with the tower) is a moot point.
2) The least nobel metal will corrode. If you look at the small list
below, you will see this is the zinc (your tower- not good). In order to
avoid this you should put something as closely matched to the tower as
possible, and let whatever comes next do the corroding (you can always
replace that, if need be).
In the galvanic series, in flowing sea water, in order from least noble
to more noble:
zinc is around -.98 to -1.03
aluminum alloys are -.76 to -1.0
copper is -.3 to -.37
tin is -.3 to -.33 and
stainless steel varies a lot depending upon the alloy and may be -.57 to
0.0.
Looking at this table, I can't figure out why people use stainless steel
next the their towers. I would think an aluminum alloy would be a better
choice. After going through this exercise, I decided to use one of the
standard AC entrance panel aluminum alloy ground lugs (Home depot,
Lowes, or any electrical house). I think some of these may be tin
plated, others are not. Don't use the tin plated ones. I used an
oversized clamp because I liked the robust clamping screw it had. I bent
the end of my #4 ground wire into a tight J loop, which I placed under
the screw to gain more surface contact area. When you tighten the
clamping screw, the copper wire actually squashes into the aluminum a
little. These clamps work well under a tower bolt after you file off
the little ridge on the bottom of the clamp, and you will probably have
to drill the mounting hole larger.
I coated the aluminum alloy to copper junction with an anti-oxidant and
sealed it against water. I used duct seal to encapsulate the whole thing.
Jerry, K4SAV
Barrie Smith wrote:
>I have my 70' crank-up tower down for rotator and antenna repairs at the
>moment.
>
>I've never bothered to ground the tower in the past, but I hope to have it
>functioning as a shunt-fed vertical when it goes back up.
>
>The base is steel. The tower is galvanized steel. I'd like to use two, or
>more, of the nine bolts that secure the tower to the base as attachment points
>for my ground and radial system.
>
>I've been told that only certain metals are suitable for such an attachment.
>I think stainless steel was one. Are there others?
>
>Also, how does one get from the stainless attachment material to the (mostly)
>copper radials?
>
>I have a length of one inch copper tubing that I'm planing on making into a
>loop around the tower and then soldering radials onto.
>
>Help appreciated!
>
>73, Barrie, W7ALW
>
>
>
>
_______________________________________________
See: http://www.mscomputer.com 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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