[Elecraft] NOT the feedline

[David Gilbert]

I had an antenna for 160m that had a fairly high SWR at the feedpoint, but
was easy to match to 50 ohms with a series inductor less than 30 feet
further down the line toward the shack.  I put a common mode choke per
K9YC's designs at the antenna feedpoint, though, and even though it was
actually two chokes in series (two large cores with separate windings) that
should easily handle 5 KW under matched conditions, at 1.5 KW it heated up
in less than two seconds to put the QRO Tech HF-2500DX amp into a fault.  I
could have moved the choke to the shack side of the matching inductor, but
I decided it was best to just fix the antenna.  No problems after I did.

Common mode chokes don't need high saturation current to get hot (and
that's rarely the problem anyway) ... high SWR voltages work just fine for
that.  And at high power it can happen in a very brief period of time.

Everyone stay safe,
Dave   AB7E


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