Re: NEC-LIST: short towers near tall trees

Many years ago I was experimenting with a two element 40 meter phased
array. It was composed of two quarter wavelength vertical radiators with
moderately good ground systems, spaced a half wavelength apart, in my
back yard. The radiators and ground systems were, as close as I could
make them, identical. A friend made some measurements of field strength
as I excited each element in turn. The unused element was open circuited
at the base, and a directional wattmeter was used to make sure the
applied power was constant. I was startled to discover a 10 dB
difference between the two! And this was a capably made measurement, not
some estimation from an S-meter. Standing at each antenna and looking
toward the friend's house, the only apparent difference was that a stand
of a few Douglas fir trees of approximately a quarter wavelength height
were in the path from the weaker antenna. They were in my yard, roughly
a quarter wavelength from the weak antenna.

Receive signal measurements showed at least 6 dB difference between the
two antennas, only in the direction through the trees, providing added
evidence that the trees were responsible for the attenuation.

Not long afterward, I moved the weak element and its ground system a
quarter wavelength toward the strong one and repeated the measurements
with the friend. The path from the weaker element went through the edge
of the stand of trees, but no longer through the center. The measured
difference between the elements dropped to 4 dB.

The only way I can think of to absolutely prove that trees can have this
profound an effect would be to set up a test giving similar results,
then cutting down the trees and remeasuring, but that's a test I've
never had the opportunity to do. In the meantime, I'm convinced that I
have observed 10 dB of attenuation of a vertically polarized HF signal
caused by absorption by trees, although I can't point to exactly what
criteria must be met to effect this level of attenuation.

Roy Lewallen

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