Re: [TowerTalk] verticals in woods vs. in a field

["Guy Olinger, K2AV" <olinger@bellsouth.net>]

----- Original Message ----- 
From: "Jim Lux" <jimlux@earthlink.net>
To: "towertalk reflector" <towertalk@contesting.com>
Sent: Wednesday, September 15, 2004 9:57 PM
Subject: Re: [TowerTalk] verticals in woods vs. in a field


> An easier way to measure the EM properties of a tree at HF would be 
> to wind
> a big coil around the tree, and hook it up to your antenna analyzer 
> and
> measure the change in Q, compared to the same measurement in free 
> space.
> You could also just wind a single turn, resonate it with a 
> capacitor, and
> look at the Q that way.

The problem with that is the measurement technique changes what is 
going on. Unless you are in the habit of winding miscellaneous wire 
around your trees in the vicinity of the antenna, what's the point.

> This technique is used to measure water content in various things 
> (wood,
> soil, etc.), since the loss is highly dependent on the water 
> content.  Once
> you've calibrated it against some known standards (typically by 
> taking the
> sample and gradually drying it), it's fast, easy, and repeatable.

The soil, even poor stuff, seems considerably more conductive than 
live trees by some orders of magnitude. The worst situation is when a 
medium conducts some, but not well. As in a 1/4 wave vertical grounded 
to a pipe driven in the ground with 50-100 ohms ground resistance.

> > I'm not sure the dielectric properties of such a poor conductor 
> > will
> > mean much. The dielectric of something touching a conductor is 
> > another
> > matter, but these are not "close" and the field around conductors
> > diminishes very quickly.
>
> Close, in antenna near field terms, would be within a wavelength or 
> so
> (depending on the size of the antenna, etc.), so for a 40m or 80m, 
> that's a
> fair distance.  Whether a lossy tree would have significant loss
> contribution at 80 meters away is another story.  It might happen to 
> be 377
> ohms, and look like free space.

What close means here in terms of significance probably has only to do 
with absolute distances. Volts/meter field diminishment due to 
distance in free space is not modifified by wavelength.

> > I went outside and took the sharp picks of my Fluke multimeter and
> > stuck them through the bark into the wet of several different kinds 
> > of
> > live trees. An oak measured over 1 meg across two feet vertical on 
> > the
> > trunk. One of my maples measured over 100K and interestingly did a
> > very slow charge like an electrolytic capacitor, measuring over 
> > 500K
> > after five minutes.
> What you're seeing is the electrolytic decomposition of the tree 
> with a DC
> bias.   (also called "electrode polarization")

Interesting. Then the AC resistance would be the lowest of the values, 
but still 100K.
> AC resistance and DC resistance will tend to be different
>
> I think that the most conductive layer of the tree is going to be 
> the
> cambium, right under the bark, with the heartwood not being such a 
> great
> conductor (doesn't most of the water transport occur in the 
> cambium?).  A
> two point measurement will read quite high, even for a sheet of a 
> fairly
> good conductor.  Consider a sheet of space cloth with 377 
> ohms/square.  If
> you had a 1 cm wide electrode on each end, and measured over 60 cm 
> distance
> would read 22K.  You've got electrodes that are a lot smaller than 
> 1cm wide
> (maybe a couple mm?). That same sheet resistance with 2mm wide 
> electrodes
> would be 100K.

Your example presumes that the major modifier to the tree resistance 
phenomena is the size of the contact point. I don't see how that 
applies to a situation where the "sheet" resistances are so high 
compared to the resistance of the probe point.

It is clear that the cambium is doing the conducting. Watching the 
value of the resistance as the prods were driven into the wood, at a 
certain depth the resistance suddenly went from off scale high to a 
measurable value that changed very little with additional depth.

> > All you need is a multimeter with sharp enough prods to dig into 
> > the
> > wet layer of the tree.
> >
> > If there isn't something in the tree that will allow a fair amount 
> > of
> > current, there won't be enough current to induce loss by the
> > dielectric properties. 50K or more ohms per foot not exactly 
> > conducive
> > to current.
>
> But a sheet that's 2 feet across and 2 feet long will be only 
> 300-500 ohms.

Well, the tree simply is not. It measures in the 100 kilohms and 
megohms. What the tree IS will determine its behavior.

What would be the detuning or lossy effect of a "close" vertical wire 
broken every two feet with a 1 meg resistor? Doubt it could be 
measured.

Unless you can find something in the tree that is conductive down in 
the hundreds of ohms range, it's like that string of resistors.

73, Guy


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