Greetings to you Larry,

Any length of wire is a 'valid' antenna. i.e., it is sure to work.

If it's a short wire it will have sharply resonant frequencies and
will not be broadband.

If it's long enough, and its normal attenuation is high enough, it
behaves as a transmission line and its input impedance will eventualy
converge on the line Zo. Depending on height above ground, on wire
diameter, and on the general environment, Zo will be roughly resistive
between 500 and 600 ohms.

You then have your wideband antenna. A 9-to-1 matching transformer
reduces the impedance to about 60 ohms which is very nicely between 50
and 75 ohms. So you can take your pick of what impedance receiver you
use.

If the antenna is not very long in terms of wavelengths, or even if it
is, it can be terminated, at the remote end, to ground with a 560-ohm
resistor and then the antenna will have an input impedance of about
560 ohms all the way from DC up to many, many MHz. Which is wideband
enough to keep everybody happy.

Unfortunately the antenna's radiation pattern is many-lobed in the
direction of the wire. When terminated it is highly uni-directional
which is unlikely to be of much use to most people. It will nearly
always be in the wrong direction. Which is why it is not very popular
except for static, specialised, point-to-point LF communications.

It's really quite simple. Try not to be distracted by
over-complicating experts on reflected and non-reflected power and
standing waves.

Actually there is a name for such a wideband antenna. But I can't
think of it at the moment. It may be the early symptoms of Alzeimers.
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Reg, G4FGQ