Dave, all,
That's an excellent bit of work. Very depressing for city dwellers...
Actually, when carried to the extreme, it's good news for city dwellers.
Since the principal losses have to do with vegetation, and that may be
either non-existent or many stories below, antennas stretching between
city buildings can be quite efficient.
I've given the example before of a 350 foot single wire T on 580 kHz
with a 30 foot downlead. It was supported on the ends by two 10-story
buildings, and the feedpoint was on the roof of a 4-5 story building in
the center. There was a LOT of copper cornice work in the area, and it
had been carefully bonded together, and tied to some heavy copper cables
which ran to the basement of the center building, thence to the water
mains. The feedpoint resistance actually dropped over the years as
air-conditioning equipment and piping was added on the roof. The last
measurement of impedance was 2.3 - j240 ohms, as I recall. The antenna
was taken down about 20 years ago.
As Jim points out, the loading coil controls the efficiency in such
cases. If memory serves the tuning setup was about 50% efficient in the
above example. The simplest coupling arrangement is a single coil,
grounded at the bottom, with a coupling tap. It can be analyzed as a
high-pass L network.
Anyway, excellent work, Jim. I hope your results will be recorded on a
web site somewhere.
John Andrews, W1TAG
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