[CQ-Contest] forest and trees RF absorbtion

Leigh S. Jones, KR6X kr6x at kr6x.com
Sun Feb 11 00:23:58 EST 2007

I saw that this done with NEC2 once years ago -- broadcast industry studies 
(probably intended to support higher power levels for a limited number of 
radio and television stations) with detailed modeling of tree trunks and 
limbs as conductors.  The conductivity of the limbs and trunks for old 
growth pine forest were painstakingly estimated based on Northeastern US 
timber industry data, but no other types of trees or localities were 
discussed.  The data might not be useful for estimating the effect of 
placing a tribander in a forest.

The results of the simulations varied with recent rainfall, etc. 
Conductivity apparently changes with temperature, also.  In pine forests 
where the "conductors" are largely the trunks of the trees and branches are 
short, the effect on horizontally polarized signals is small, but 
attenuation of vertically polarized "ground waves" is marked.

Unfortunately, the simulations I got to look at focused on frequencies in 
the 550-1600 KHz range, although there were also some television frequencies 
discussed.  The conclusions in text were readable, and the actual simulation 
data was useless in that "pre-computer" age. The differences between 
horizontally polarized signals and vertically polarized signals diminished 
at VHF, apparently due to wavelengths of branches.  Most interesting to me 
at the time was that VHF signals passing well over the tops of the trees 
were shown to suffer attenuation similar to signals passing right through 
dense growth.  Please don't misunderstand me on this, increasing antenna 
height did help, but increasing antenna height also helps over terrains 
without trees.

The conclusion I drew from the data at hand was that in California's 
footlhill redwood forests (with significantly longer horizontal branches 
than pine forest) the absorbtion of horizontally polarized signals on 
10-15-20M could be very significant, especially after recent rainfalls.

And unfortunately, I cannot cite sources or provide better information.  I 
only read a small part of the reports.  My original source of the documents, 
was Ted Gillett, W6HX, who once had been my employer, and who is now a 
silent key.  I believe with uncertainty that his source may have been a 
gentleman named Alan Emerald, with whom I have never been in touch.

----- Original Message ----- 
From: "Gilles RENUCCI" <ve2tzt at arrl.net>
To: <CQ-Contest at contesting.com>
Sent: Saturday, February 10, 2007 2:12 PM
Subject: [CQ-Contest] forest and trees RF absorbtion

> Hi,
> Does anybody has serious data about trees and forest absorption on top 
> band ?
> 73's , Gilles / VE2TZT
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