The change in an overheating core has a swing to it and you can watch the
SWR change, in a continuous fashion, while you are transmitting, while arcs
exhibit sudden change.
What you call "imbalance" was on purpose, but true. The more the radiating
length of an antenna is shortened, the more severe the voltage is at the
end(s). Enough so that a motorcycle at a W3LPL open house, with a KW in a
motorcycle trailer, even the rounded end of the 8 foot whip on SSB peaks
would crackle and sizzle with corona, visible in broad daylight. Quite the
show.
Even with full size devices, toward the ends of dipoles, vees, etc, the RF
voltages are well in excess of insulation on the "THHN" insulation on wires
from home improvement stores.
But I'll vote with Tom's likely assessment, arcs in connectors, etc. You
may have some surprisingly high voltages in the UNselected antenna plus
feedline.
73, Guy.
On Wed, Nov 28, 2012 at 1:56 PM, Jim Brown <jim@audiosystemsgroup.com>wrote:
> 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.
>
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