On 12/27/11 12:17 PM, K8RI wrote:
> On 12/27/2011 2:22 PM, Jim Lux wrote:
>> There's a difference between H and V pol as one might expect (thinking
>> of trees as lossy conductors).
>> Lower frequencies are more affected by wood, not leaves.
>> Rain/humidity/water has an effect (more wet = more loss)
>> Single trees have an attenuation around 5-20 dB at UHF (860 MHz) CP
> Don't forget the conductivity of deciduous trees changes dramatically
> from winter to spring. It's *probably* highest in early spring than it
> is in summer for HF and the sap flow is highest in early spring and the
> content is different. However I can only speak for the trees that grow
> around here in the frozen North.
You're almost certainly correct. They've done work with trees in an
anechoic chamber where they measured them with leaves, and after
removing the leaves, and on live/dead trees, etc.
There is huge interest in measuring the moisture content (and carbon
content) of trees from orbiting and aerial platforms. Those
measurements tend to be done at UHF or L band, because you get a "big"
effect, low enough frequency that it's not much affected by atmospheric
effects, and high enough that you can build a practical sized antenna
and do SAR processing to get resonable resolution.
Mahta Moghaddam at University of Michigan
has done a bunch of work with this over the last 10-15 years (at least).
She has a big (we could only drool about it at the JPL radio club..it's
the size of a semi trailer) tower trailer that can put an antenna up
100-150 ft to look down at a forest for "ground truth" measurements.
There isn't a whole lot of data I'm aware of for HF. It's an
interesting propagation problem because the tree dimensions
(particularly height) are comparable to a wavelength, so there's both
diffraction and transmission effects. There are modern modeling codes
that could be used to good effect, if you had access to the
computational horsepower. For antennas in the woods, too, there's the
whole "near field" issue as well. I was looking into this back in 2004
(I think) and was accumulating information on "forest tree size/density"
properties, and doing some quick calculations to bound the problem. You
could probably do an OK model with NEC4 and vertical wires to represent
Unfortunately, it's not a sexy topic for researchers these days, so I
don't think there's much funding available to do work on it. But gosh,
if you could get someone interested in it, it would be a great area for
a Masters thesis or PhD dissertation.
There might be some data from the folks doing HF sounding radars,
although I think they're more interested in deep subsurface structures
(particularly in ice covered areas), and to them, forest effects are
"clutter" And besides the propagation direction is straight up and down
for the most part. (Maybe applicable to NVIS)
TowerTalk mailing list