[TowerTalk] trees and verticals

[Jim Lux]

On 12/27/11 10:53 AM, Rik van Riel wrote:
> On 12/27/2011 01:22 PM, Mike Ryan wrote:
>> Rather than have to guy wire one of my two verticals at my qth, I stood one
>> up within the branches of a 20ft or so tall tree in front of my house. It
>> helps disguise it as well. It is an HF2V that kicks but. The last thing I
>> ever worried about was a little absorption on HF. If this were a VHF/UHF
>> vertical, that would be another story. On HF? I doubt there is a lot of
>> difference.  - Mike
>
> Some people apparently have noticed a lot of difference.
>
> However, in this thread the person who noticed the most
> difference, also had the tallest&  widest trees...
>
> I imagine a wider tree not only captures more radiation,
> it also has a lower resistance to ground.
>
> Likewise, a smaller tree will probably have a higher
> resistance to ground, while radiation of even shorter
> wavelengths will go around it easily.
>
> I do not know if my thoughts correspond to reality, but
> this thread seems to have caught enough people's attention
> that we may be able to figure this out together...
>


there is a fair amount of literature on absorption/propagation in 
forests. Googling with the right search terms finds a lot of it.

Most of the HF data was acquired back in the 60s in connection with 
jungle comms in Southeast Asia.  George Hagn, for one, published some data.

More recently (90s), Cavalacante and Tamir each have multiple papers. 
They model as a multi layer slab dielectric, and look at scale effects. 
They're particularly interested in things like how do the effects change 
as the forest height goes from "< 1 wavelength" to "many wavelengths, 
and what effect do things like clearings/roads have.

Hagn and Vogel did a nice presentation on tree loss at ISART 99 in 
Boulder, but mostly VHF (starting at 25MHz) and UHF.


Some observations..
At 50 MHz, loss in Jungle is about 0.03 to 0.1 dB/m.  In Eucalyptus 
forest (from pictures, I'm guessing California in the Bay area) about 
half that, even lower in H pol.

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