OK, I just took a look at the manual for the BN-86. It appears to be
a simple common mode choke. My guess is that it fails because the
common mode impedance is not sufficiently high over the range of
operating frequencies in use. Not unusual.
When I've attempted to stress a choke by putting it at a very high
voltage point and shove a lot of power to it, it will typically fail
by melting the coax. If the heat source (the transmitter power) is
removed soon enough, the melting may not do permanent damage.
I suggest that you build one of the coaxial ferrite chokes listed in
the "cookbook" section of my tutorial. If you run the antenna only
between 14 and 29 MHz, four widely spaced turns through 5 #31 or #43
2.4 inch o.d. toroids would be a good design. That choke will start
running out of gas below 20M. If you run the antenna down to 40M,
you'll want more impedance. Simply add a second choke in series of 5
turns on 5 cores to add some impedance on 40M.
http://audiosystemsgroup.com/RFI-Ham.pdf
What you'll observe when you're done is that the antenna hears a lot
less noise, because it is more effectively decoupled from the
feedline.
73,
Jim K9YC
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