On 11/28/2012 10:14 AM, Ashton Lee wrote:
I rebuilt the antenna from new wire, built a two insulator termination at the
end of the horizontal section where the high voltage is,
One thing I observed here several years ago with a dipole with an end
touching tree branches is arcing to the branch, accompanied by scorching
of the wire insulation (white THHN).
Also, a common mode choke whose choking impedance is too low can
overheat if the common mode voltage is high enough. That voltage
depends on the degree of imbalance, which, as Tom observes, is highly
dependent on the antenna system, INCLUDING the feedline and the radial
system (and/or counterpoise). Tom's analysis of Guy's folded
counterpoise design showed it to have significant imbalance, which fried
common mode chokes, but was at least partially corrected by the stray Z
of an isolation transformer.
Overheating in a common mode choke wound on a lossy ferrite core shows
up in the wire itself (the coax shield) and can melt the dielectric,
allowing it to either short, arc, or change spacing. I've done some
experiments purposely intended to observe what happens when the choking
Z is inadequate.
73, Jim K9YC
_______________________________________________
Topband reflector - topband@contesting.com
|