I'm at work, so I can go a bit further with this thread.
Generally, corrosion is a problem when the atomic potential
(nobility) exceeds +/- 0.3 Volts. The lower this value, the
longer corrosion takes to occur, or it can become a non-factor.
According to my chart (which is on page 18 of our "Grounds"
book), the difference between aluminum and zinc (galvanizing) is
(-)0.90 Volts. Personally, I don't think it's enough of a
difference to worry about and anything you might do may make
matters worse. As an example, offsetting and/or jacketing may
increase the likelihood of a side flash due to the potential
difference between the cable and the tower leg. You would
probably have to offset 6-9 inches to minimize the likelihood of
this side flash. The best thing to do is to bond the hardline to
the tower at the top and bottom. If the tower is greater than
150', bond at the 150' point and every 75-100' above that point.
(Well, some ham towers are >150'.) At the risk of being flamed
for commercializing, PolyPhaser makes appropriate products for
this purpose. Contact me at 800.325.7170 for more information
(however, give the receptionist a company name rather than just
saying you're an individual or a ham; without getting into the
subject too deeply, all hams are being summarily sent to AES.)
The catalog's also on-line at <www.polyphaser.com>.
Hope this helps!
Bob Wanderer AA0CY
Senior Applications Engineer
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com