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Re: Topband: Tree conductivity

To: "Tom W8JI" <w8ji@w8ji.com>, "Rudy Severns" <rseverns@gmail.com>, "Topband" <topband@contesting.com>
Subject: Re: Topband: Tree conductivity
From: "Ulrich Weiss" <dj2ya@t-online.de>
Date: Sun, 4 Aug 2013 09:09:01 +0200
List-post: <topband@contesting.com">mailto:topband@contesting.com>
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@w8ji.com>
To: "Rudy Severns" <rseverns@gmail.com>; "Topband" <topband@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



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