[NLRS] loop yagi driven element corrosion

[Dr. Gerald N. Johnson]

If not copper the driven element is sure to ba a copper alloy since 
soldering to aluminum is difficult. The rigid coax is sure to be copper 
or tin or silver plated copper. What the feed and driven element needs 
is a cleaning to bare metal and a coat of clear Krylon or Rustoleum to 
protect them from corrosion. There is a chance that the black is some 
sort of protective coating. I wouldn't prefer a black coating because 
most black pigments are based on carbon that is a poor conductor. I 
prefer clear to be an insdulator.

73, Jerry, K0CQ

On 9/14/2016 8:56 PM, Chris Elmquist wrote:
>
>
> 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