Bruce, I am asking why there is apparently such difference between feeding
an ungrounded dipole with coax from an ATU (my shore station) and feeding
an
insulated (hence ungrounded) backstay from an ATU? I work Alaska bareback
in the summertime with that setup and I just can't understand what GTO-15
does that hardline doesn't. If you could explain or reference a document
that specifies the reasoning I would try to correct my misunderstanding.

Thanks,

Jack Painter
Virginia Beach, Va

If I can jump in, the quick answer is that the coax is approximately the
same impedance as the center of your ungrounded dipole, at least at the
frequency for which it is resonant. Thus, from the perspective of the
transmitter and the antenna, the transmission line is "invisible." I'm
exaggerating, of course.

In the case of a backstay used as an antenna, the feedpoint impedance can be
anywhere from a small fraction of an ohm at low frequencies to thousands of
ohms where it approximates a half-wavelength. In those cases, the coax will
most certainly not be invisible and will most likely either burn up or
greatly attenuate your signal (incoming as well as outgoing, actually).

If you tried to end-feed your half-wavelength dipole with coax, you would
see a similar problem because the impedance at the ends is in the thousands
of ohms range.

Hope that helps.

Chuck