> There's an interesting article at:
> http://www.gordon.army.mil/AC/Fall02/BCTSOP2.htm
>
> which discusses various HF comm modes, with a fair amount
of detail on
> NVIS. In particular, go about 2/3 of the way down to
where they start
> talking about horizontal dipole antennas. There's a chart
showing
> performance (straight up) for halfwave dipoles over
"average" and "perfect"
> ground at various heights and frequencies (all the way
down to 0.1 ft). and
> a discussion of preferred heights (30 ft is recommended)
> http://www.gordon.army.mil/AC/Fall02/dffig15.jpg
So they show the same thing we said here.
As the antenna is lowered, efficiency takes a big hit and
signal levels at all angles decrease. I didn't see anything
about adding a ground screen below the antenna, which would
improve signals substantially with an antenna placed over
poor soils.
It would be nice if people doing emergency communications
work could be educated. They need to learn the efficiency
penalty of using an 18 inch high antenna! I hope people
contact QST.
73 Tom
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