On 12/29/11 5:11 PM, Larry Banks wrote:
> I have been reading all of these posts and feel perhaps I can add to the
> conversation.
>
<snip of stuff>
Interesting comments from Joel and Dean, although neither actually
provides any quantitative assessment (or, really, much of an assessement
at all).. Joel repeats what is really more of a speculation from Floyd
Koontz:
"Even though the vertical array wires were 20 feet or more from the
trees, the trees seemed to be absorbing the signal. IN northern
locations, the trees drop their sap to the roots in the winter (DX
season on the low HF bands), and many stations have good luck with
verticals hung from trees. Here in Florida the sap stays up all year
round and the trees are quite conductive (and longer than lambda/4 at 75
meters). Vertical polarization simply doesn't seem to work in my yard"
Floyd was comparing signals against a horizontal reference dipole at 80
ft, and the 10dB difference he observed between H and V pol could be due
to tree absorption or something else. In any case, it wasn't a well
controlled experiment, and the connection between "difference in
performance" and "tree absorption" is pretty tenuous. I suspect that
the difference he observed is not a "antenna efficiency" effect, but,
rather, a "propagation through forest" effect. And for the latter, the
water content of the trees probably isn't as big a deal as the "grid of
dielectric posts".. so there would be little or no seasonal variation.
(something that could be tested in a modeling code, or analytically)
Gene Zimmerman made a comment: "Everyone appears to have an opinion on
this subject but definitive scientific works are more difficult to find
- at least on the Internet."
Indeed, this is the case, even broadening beyond the internet. I think
it's safe to say that it's basically something that has not received
very much rigorous study.
And the basic reason is, I think, that a quick analysis, like that which
has been done over the past couple days, shows that there is a very
small effect from a tree on the efficiency of a vertical radiator.
-
>
> If I remember, the URL for the paper may no longer be a good one,
> unfortunately. This link is the same paper:
> http://www.its.bldrdoc.gov/pub/ot/ot-78-144/ot-78-144.pdf
>
>
this is the very useful report I cited yesterday (or was it the day
before).. It's really a summary of the literature as of the late 70s,
and for the HF aspect, he's citing the data in Tamir's papers. Those
papers (citation and abstract below) are quite complete, but deal more
with propagation through a forest, rather than near field effects (i.e.
what happens if the antenna is hanging from a tree branch next to the
trunk). Furthermore, there's not much attention to the sky wave
propagation effects: the papers focus on relatively short distance
propagation (few tens of km). The papers are worth reading,
nonetheless, since they're what *everyone* since cites, particularly the
first one, where he introduces the concept of the dielectric slab model.
Dr. Tamir's website: http://www.poly.edu/user/ttamir
Tamir, Theodor, "On Radio-Wave Propagation in Forest Environments", IEEE
trans on Ant and Prop, v.AP-15, #6, Nov 1967
Abstract-Propagation of electromagnetic waves in forest environments at
medium and high (1-100 MHz) frequencies is examined for the case where
both the transmitting and receiving points are situated within the
vegetation. A dissipative slab in the presence of a reflecting
ionosphere is employed to describe the forest configuration. If the
effect of the ground-forest interface is disregarded, the radiated field
of an arbitrarily oriented, small dipole is found to consist primarily
of two separate waves: a lateral wave which skims along the tree tops,
and a sky wave which is produced by a single-hop reflection at the
ionospheric layer. These two field constituents are compared and their
domains of preponderance are calculated for a large range of the
pertinent parameters; it is then found that the lateral wave plays the
major role since the sky wave is restricted to a narrow frequency band
and its amplitude is appreciable only at large distances. The
lateral-wave field is examined in detail and is shown to yield a simple
physical picture for the propagation mechanism in forests. Its features
are found to be qualitatively consistent with the field behavior
reported in the literature and the quantitative aspects agree well with
the available experimental data. The observed variation of the field
with distance, the height-gain effect, the vegetation factor, the basic
path loss, and depolarization effects are separately examined and are
all shown to express merely one or another of the intrinsic properties
of a lateral wave. The ground-proximity effect produced by the presence
of a planar-conducting ground is also estimated and shown to be of minor
importance in most cases.
and
Tamir, Theodor, "Radio wave propagation along mixed paths in forest
environments", IEEE Trans on Ant and Prop, v25,#4, Jul 1977
The propagation of radio waves is examined for communication paths that
may be partly within a forest and partly in regions outside the
vegetation. Analytic results are found for simple canonic geometries in
which the fields can be described in terms of ray-trajectories. By
viewing a realistic forest environment as a combination of such canonic
cases, it is possible to evaluate radio losses in complex situations by
using a ray-tracking approach. The pertinent fields can then be
expressed in terms of relatively simple analytical expressions, which
hold well for frequencies between 2 and 200 MHz.
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