At 12:31 AM 4/19/2006, Olivier F5MZN wrote:
>Hi -
>
>I wonder if someone has any experiences on tree attenuation caused by
>jungle trees on short waves below 30 MHz for horizontal antennas. I
>googled a bit but without too much success because most of the
>informations are relative to higher bands.
As you say, the interest these days is in UHF and higher, however, there's
quite a bit of research at lower frequencies, back to the 60s and 70s.
Do you have access to IEEE journals?
You might want to look at the papers by Theodore Tamir.. for instamce:
"On Radio-wave propagation in Forest Environments", IEEE Trans on Ant and
Prop, V AP-15, #6, Nov 1967
covers 1-100 MHz
and a followup paper from 1977:
"Radio Wave Propagation Along Mixed Paths in Forest Environments", IEEE
Trans Ant Prop, V AP-25, #4, Jul 1977
Tamir shows that you can model the forest as a big lossy dielectric slab,
so what you need to know is what the properties of "your" particular forest
are.
Cavalcante and coworkers also studied HF frequencies in Brazil
"Optimization of Radio-Communication in Three Layered Media", IEEE Trans
Ant and Prop, V AP31, #1, Jan 1983
"Mobile Radio Propagation ALong Mixed Paths in Forest Environment", IEEE
MTT-S IMOC proceedings, 1999
and
"Radio Loss in Forest using a Model with Four Layered Media", Radio
Science, V18, #5, 1983
They're looking at the effects of things like clearings and roads, as
opposed to the infinite uniform slabs of Taheri.
There's a newer paper from Tewari, et al., "Radio Wave Propagation Through
Rain Forests of India", IEEE Trans on Ant and Prop, V38, #4, Apr 99
This one starts at 50 MHz, but is useful lower because of the measurments
of dielectric properties.
Googling for all these authors and the word "propagation" might turn up
useful stuff. The "gold mine" would be if some college has put a masters
thesis online that summarizes the data. The typical thesis has a nice
chapter 2 that covers the previous research, and the usual masters thesis
is written at a level that is not too obscure and arcane. Some of the PhD
theses get pretty esoteric and narrowly focused.
>--
There's also some ITU Recommendations/Reports that provide models and
tables for forest attenuation. You can sign up at the ITU website and get 3
reports for free.
http://www.itu.org/ I think..
>Actually, we are planning to add a new tower in our contest QTH which
>might not be located in a clear environment, especially for the Europe
>path where there is a small but dense forest (500-meter long) in the
>near field. Trees are about 15-meter high but the tower and antennas
>will be well above. You can have an idea on the trees with this picture:
>http://www.fy5ke.org/gallery/PICT0255.JPG
>
>How important could be the attenuation?
If the antenna is well above the forest, the effect is probably
minimal. Forest is lossy, but nowhere near as lossy as soil.
Jim
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