Olivier:
I would also suggest you search on NVIS or Near Vertical Incidence Skywave
and Vietnam. As Jim writes (below) much of the propagation research through
forests was done in the 60s and 70s in southeast Asia during that conflict.
NVIS was especially valuable to the US Special Forces who used HF to
communicate to their HQs in the days before cheap and compact stacom radios.
73 de
Gene Smar AD3F
-------------- Original message ----------------------
From: Jim Lux <jimlux@earthlink.net>
> 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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>
>
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