[TowerTalk] verticals in woods vs. in a field

[Michael Tope]

----- Original Message -----
From: "Guy Olinger, K2AV" <olinger at bellsouth.net>

> > 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 very relevant. In fact it is relevant in the same way
that the wire diameter in a two-wire parallel transmission
line is relevant to the impedance of the line (the
mathematical expressions are similar). In fact, you can
use space cloth (377 ohms/square sheet resistance) as
an analog computer to calculate the impedance of a two
wire transmission line by inserting conductive
probes into the cloth that are same diameter as the
wires in the transmission line and the same distance
apart as the spacing of the two wires in the line. You
can find a more lucid explanation of all this in W8JK's
"Electromagnetics". In any case, the expression for the
resistance between to probes in a conducting sheet
works out to be roughly proportional to the log(D/d)
where "D" is the distance between the probes and
"d" is their diameter. When the diameter of the probes
gets really small, this number blows up in the same
way that the impedance of a two-wire line blows up
when the conductor diameters get really small. If you
are probing a good conductor, you won't see this
effect, because you aren't going to notice the
difference between 1 milliohms and 100 milliohms.

> 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.
>

Sounds like a thin cylindrical resistive sheet would
be a good model for the electrical properties of a
live tree.


> > 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.
>

You are right, what the tree IS will determine its behavior,
but (see my above comments) your measurement with
small probes won't necessarily be a good predictor of
the resistance/unit length of the tree. Its analogous to
the capacitance between two electrodes. The value you
measure depends on both the properties of the medium,
the distance between the electrodes, and the size of the
electrodes. Same thing for resistance between two probes
in a resistive medium (the fields map out the same way).
The bigger the probe diameters, the larger the capacitance
between them. Same thing for conductivity. The bigger
the probe diameters, the higher the conductivity between
them.

> 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.


I don't think the effect from dense trees is real strong
at HF. There are plenty of guys with antennas in these
kinds of environments that seem to get out reasonably
well. If there is any degradation at all, I would bet that it
is only a few dB compared to open land.

73 de Mike, W4EF.................