Topband: Peru report....and how to make fireworks with an inverted L

[Jim Brown]

On Mon, 10 Dec 2007 17:06:58 -0500, Jeff Maass wrote:

>Where the previously-insulated reflector wire
>    was making contact with a limb of a bushy
>    tree, the wood was seriously charred.

My wire dipoles are hung up about 120 ft in giant redwoods. For 
nearly a year, one end of the 160 dipole was in contact with 
branches of the tree that supported it. The wire was #12 THHN. It 
worked fine and stayed up. (It was touching the tree because there 
was also a 20/15/10 trap dipole in line with it, and there wasn't 
quite enough room between the two trees to do it "right.") 

Several months ago, I dropped it, found a new home for the 
20/15/10 fan so that the 160 dipole wasn't in the tree, and pulled 
it back up. I had bare copper at the end in contact with a 
thimble, and that 5/16 inch rope of the type sold by DXE and 
others went through the thimble. Somehow, 1.5 kW melted that wire 
and dropped the antenna to the ground. 

I observed that the #12 THHN at the end of the antenna was heavily 
charred, and melted through in many places. But it didn't CAUSE a 
problem -- in fact, it prevented the rope from melting by 
discharging the high voltage at the end of the antenna to the 
tree. When I pulled the antenna out of the tree, the high voltage 
soon (within a week) melted the rope!  

I've since re-rigged all of my antennas with end insulators so 
that no bare wire is in contact with rope. One suspects that 
moisture in the rope supports enough current to cause the heating. 
That might also be a problem at your QTH.   

73,

Jim Brown K9YC