As Tom, Rudy, and Richard have noted, this stuff is potentially
important, but awfully difficult to quantify for a variety of reasons.
One is that what's happening in the trees is not the only thing that is
happening when the seasons change. Out here along the Pacific coast, we
have a rainy season when the soil is increasingly well saturated
(thankfully, for 160M operation, from Nov to April) and a dry season
when it is not. So when I see seasonal variations with vertical antennas
in my very dense redwood forest, the obvious question is, how much of
what I'm seeing is soil, and how much of it is sap? :) Almost none of
my trees are deciduous (shedding leaves in the fall), so I don't see
those changes.
I loved Tom's story of hoping that he would have a measurement
opportunity and circumstances made it impossible.
I have three verticals for topband. One is an 86 ft tall Tee, more or
less in the center of a large clearing (~ 90 ft radius) and a lot of
radials on the ground. The other two are sloping wires fed from a base
about 40 ft from a 120 ft tower that supports the top of the wires,
which are insulated from the tower. One wire slopes east, the other to
the west, both have four quarter wave radials elevated about 20 ft, and
there are ten quarter-wave radials on the grounded tower. The tower
supports the small 3-3l SteppIR, and the clearing is barely large enough
to turn it (roughly 24 ft radius) -- that is, it's in the middle of very
dense, very tall (150 ft) redwoods.
NEC predicts gain in the range of 4-5 dBi for these two slopers, but I
am not yet convinced I am seeing that.
More anecdotal stuff -- the 160M Tee is the only vertical of several
I've tried for 160, 80, and 40 that I find even slightly useful. OTOH,
my horizontal antennas for 80M-6M, nearly all of them at 110-120 ft,
play very well.
What I take from all of this is an hypothesis (and that's all it is)
that trees like redwoods and tall pines have a much greater effect on
vertical antennas when they are in the near field, and that up to about
6M, the radiation from horizontal antennas doesn't seem to be strongly
attenuated. I do OK on 6M Es, MS, and tropo, considering the fact that
the ridge above me to the north, east, and SE gives me a horizon of
about 7-8 degrees. But attenuation increases with frequency, so 2M and
up is almost a lost cause, and it doesn't matter much whether the
polarization is horizontal or vertical.
One experiment I'm thinking of, based partly on the comments I've seen
here, is to string a horizontal rope strung between two trees with
pulleys at each end, a pulley that can support an end-fed vertical
dipole using a method that Rudy inspired -- a ferrite coax choke as the
lower end insulator a quarter wave from the end of the coax, with the
center conductor connected to a quarter wave wire. For the test, you
simply move the vertical dipole so that it is very close to one of the
trees, then vary the spacing to the trees.
This sounds fairly easy and simple, but I've got a lot of large and
small towers and antennas with more or less vertical feedlines that can
interact with the antenna being tested, tree climbers cost something
like $800/day, and it could take more than a day to rig it, so it's a
non-trivial exercise, but I believe it's worth doing. Based on the
heights at which I could rig things, I would probably have to do this on
40M, although I MIGHT be able to do it on 80M with non-resonant somewhat
shortened dipoles.
As I'm certain any well trained engineer who has attempted it (or even
thought seriously about it) will tell you, MEANINGFUL antenna testing is
a non-trivial exercise.
Thoughts?
73, Jim K9YC
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Topband Reflector
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