In article Gj4qc.43044$pJ1.24343@lakeread02,
Jack Painter wrote:
Remember that the military does two things. One is NVIS which
has the antenna mounted about 1/4 wavelength above ground. This
gives a good ground effect from 2 MHz to 12 MHz out to 300 miles.
The other is ALE, automatic link, which switches frequencies
automatically to insure the best path. An antenna with tuner
would just be too slow.
David, even for purposes of this discussion, that is a bit oversimplified.
Perhaps that type of broadband temporary antenna installation is acceptable
for field units, but have never seen one of them in a permanent
installation.
My wife and I recently spent several weeks touring around Peru (just
got back last week).
I was intrigued to observe a _lot_ of modest-size HF wire antennas
scattered around... and these all appeared to be of this same basic
"terminated, folded dipole" variety. I saw a couple at the Cuzco
airport, one at a village on the Amazon, one or two in Aguas Calientes
(the village right below Machu Picchu), etc. They were usually
mounted in an inverted-V configuration, and appeared to be permanent
installations rather than short-term field-use installations.
The applications were probably NVIS, with working ranges out to a few
hundred miles.
I can't say for certain that they _were_ T2FDs, as I had no way to go
up on the towers and confirm that the device at the center of the
upper part of the dipole was a terminating resistor. I've never seen
a folded dipole with an open-circuiting insulator at the center of the
upper span, though.
My guess is that these antennas were chosen because of their "one size
fits all" nature - they can be installed and used without trimming,
don't require SWR-matching at the transmitter, and their relatively
low efficiency may not be an issue in these applications.
--
Dave Platt AE6EO
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