Topband: Tree conductivity

[Ulrich Weiss]

so far the discussion has only treated the effect of trees in the immediate 
near field area... what about the effect of wood in the Fresnel zone?
especially at low angles the ground reflected ray will surely be more 
attenuated when passing through a relatively long space of dense wood than 
being reflected in an open field... is there a difference between horizontal 
and vertical polarization?
has this problem ever been treated in antenna literature?
73
Uli, DJ2YA

----- Original Message ----- 
From: "Tom W8JI" <w8ji at w8ji.com>
To: "Rudy Severns" <rseverns at gmail.com>; "Topband" <topband at contesting.com>
Sent: Sunday, August 04, 2013 2:42 AM
Subject: Re: Topband: Tree conductivity


>> Some time ago I attempted to make direct measurements of the HF 
>> conductivity of trees, at least for the trunk and limbs.  I simply put a 
>> two rings, with nails to penetrate through the bark, around the trunk 
>> spaced a couple of feet apart.  Basically what I had was a resistor.  I 
>> then measured the impedance of this "resistor" using a network analyzer.
>>
>
> I think there is some confusion caused by  conversations on this reflector 
> about **resonance**,  and conversations about attenuation.
>
> I think there are some people who believe the issue is resonance and think 
> a tree has enough conductivity to exhibit resonance effects. I've never 
> seen any documentation or experiment to support a tree exhibiting resonant 
> effects (at least for HF and VHF).
>
> There isn't much doubt dielectric losses would play a role.
>
> Years ago, because of some "fractal tree antenna" nonsense discussion, I 
> measured a fresh cut pine tree log and it had pretty high RF resistance 
> over a foot of trunk length. The resistance was high enough that a single 
> tree could not show resonance effects. I did a sweetgum later, when I had 
> to remove a sweetgum. It was similar.
>
> This is different than attenuation by having either a strong electric 
> field near a single tree's foliage, or attenuation through thick foliage.
>
> Another place where this comes into play is with seawater. Another goofy 
> thing appeared where someone was claiming a vertical jet of seawater could 
> be used to make a good stealth antenna. We all know seawater has a 
> profound effect and enhancement on patterns and loss, yet the resistivity 
> of sea water is so high it really makes no antenna at all when used as an 
> antenna. As a matter of fact, saltwater makes a pretty good dummy load 
> when current density is high. Current would be high if seawater were used 
> as an antenna conductor.
>
> What we have is an inability to understand the difference between very 
> good conductors, poor conductors,  and or lossy dielectrics. We'd have a 
> difficult time powering something through saltwater conductors, or having 
> resonance effects with saltwater jets at low frequencies. At the same 
> time, even crummy soil has a profound effect on EM fields and other things 
> when cross sectional area is large enough.
>
> There is a danger that people will not understand the big picture, and 
> write more seawater antenna or tree antenna articles.
>
> 73 Tom
>
>
>
> _________________
> Topband Reflector
>