[TowerTalk] antennas in the forest
Jim Lux
jimlux at earthlink.net
Tue Sep 21 13:57:48 EDT 2004
Reading on, especially the papers by Tamir (1967 in particular) and
Cavalcante..
If your antenna is buried in the forest (i.e. it's below the tops of the
trees) then the ground parameters of interest are those of the forest, and
the actual earth parameters are less important. epsilon is typically 1.05
- 1.15, so there is a refractive effect in the propagation. Sigma
(conductivity) is typically 0.05-0.15 mSiemens/meter, so it's not
particularly lossy, compared to earth. You can compute the "refractive
index" of the forest for ray/optical kinds of computations. Tamir gives the
following:
abs(n^2-1) = abs( epsilon - 1 -j*60*sigma*lambda) = approx 60*sigma*lambda
Given a sigma of 0.1E-3 and 20m, I get abs(n^2-1) = .12, or n = 1.06.
(Using Snell's law, sin(critical angle) = 1/n, tells you when total
reflection will occur. For that n=1.06, it means that if the angle 70
degrees from vertical (i.e. 20 degrees elevation or closer to horizon),
then you'll meet the condition for total reflection from the interface...
this is pretty significant, guys.. the wave at low angles is going to
propagate within the forest, like in a waveguide...)
Note that if your antenna is anywhere close to the forest/air interface,
this also applies. Just getting your antenna 10 feet above the tree tops
doesn't remove the effect of the forest (and, actually, might make it
worse..). It's much like antennas over ground, where the properties of the
ground affects the pattern.
A lateral wave develops, propagating along the interface between the tops
of the trees and the air above (similar to a "ground wave" familiar to AM
MW listeners), so the foliage attentuation for short paths (<100km) is
independent of distance (in other words, the path is not through the
trees), and you just add in a "foliage factor" for loss. There are paths
contributing to this lateral wave that reflect from the forest/air boundary
as well as the forest/ground boundary (obviously, the hop distance is quite
short, and this occurs even if the wavelength of the signal is greater than
the height of the forest.. the math just gets more complex, but, in this
case (long wavelength) the effect is much less.)
For skywave paths with low elevation angles, the lateral wave contributes
significantly to the total signal transmitted or received.
Forested areas have much less effect from the roughness of terrain. The EM
properties of the forest dominate. (This is relevant for users of programs
like HFTA, which assume a uniform earth EM properties and do ray tracing)
There is an interesting effect where the optimum orientation of the antenna
is somewhere between vertical and horizontal, but knowing what it is
depends a LOT on the exact EM properties of the forest. The effect is on
the order of 0.5 to 3 dB.
Forest depolarizes the transmitted wave.
Here's an observation which DX'ers might be interested in:
Forest might reduce fading depth on skywave paths, because it makes the
antenna effectively "larger", so it averages more paths. (of course, you'll
also have some additional loss from the forest, so the peaks will be lower,
too)
Anyway, it looks like forests and trees DO have a significant effect on HF
propagation.
None of this, by the way, addresses the effects of trees in the "near
field" (say, within 2*L^2/lambda of the antenna.. ), but those effects are
probably more in the nature of added loss.
Jim, W6RMK
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