[NLRS] loop yagi driven element corrosion

[Chris Elmquist]

I've mentioned this to a couple guys and am kind of perplexed about this
issue...

I have two DEMI / Directive Systems loop yagis on my tower at about the
35' point.  One is 33 elements on 902 and the other 45 on 1296.

While doing some work on other antennas on the tower, I noticed that
the driven elements on both of these yagis are extremely discolored
(ie, black).  All the other elements on both yagis are nice and shiny
aluminum color.

The little rigid coax that comes down to meet the N feedpoint connector
from the driven element is also black on both yagis.

The yagis a fed with a length of Superflex that goes down the boom to
the mast, down the mast a few feet forming a rotor loop, and then
meet a run of 7/8" Heliax (air line under positive pressure).

These antennas seem to still work as well as when they were put up but
I'm concerned about the discoloration since it suggests some kind of
dissimilar metal action or some other kind of active attack against the
metals--  but I'm not sure where or how that would be happening.

And no, I'm not running 1 KW+ on these bands :-)  30W on 902 and 60W
on 1296.

Tower is aluminum and all these feedlines are copper of course but
all jackets are still intact and no copper is meeting the aluminum.
Everything's taped and sealed really well and so I'm having a hard time
explaining what process would cause the entire element to turn black
with no similar situation occuring on the other elements, boom or other
non-loop yagis on the tower.

Any ideas?

Chris
-- 
Chris Elmquist N0JCF