[TowerTalk] verticals in woods vs. in a field

[Jim Lux]

At 02:36 PM 9/16/2004 -0400, Tom Rauch wrote:
> > > I actually measured the resistance of a freshly cut pine
> > > tree by attaching metal plates to each end of a one foot
> > > piece, and it as in the many hundreds of ohms per foot
>at
> > > radio frequencies.
> >
> > Interesting measurement.  What was the diameter of the
>tree?  Did the metal
> > plate cover all the end of the tree, and did you use any
>sort of conductive
> > gel to make a good contact with the tree (assuming that
>the metal was
> > flatter than the saw cut).
>
>If you ever saw (pun intended) a Georgia Pine, it is thick
>with sap. I used multiple small screws to hold the plates
>against the ends, and made sure the cut was flat.
>Effectively the plates became glued to the wood.
>
>The "log" was roughly one foot in diameter. This was as
>close to living as you can get, it was fresh cut less than
>an hour before the measurement. Pines are also very acidic
>and very wet inside, so I doubt it was on the less
>conductive side of how trees go.

So, we have a sheet that is 3 feet wide and 1 foot long with a resistance 
of, say, 500 ohms, measured in the 1 foot direction.  This implies a area 
resistivity of about 1500 ohms/square.

That's a useful ballpark number.



> > It would be nice to get some decent HF measurements of
>living trees.
>
>It would indeed. For all the interest in this, I'm really
>surprised no one has made any meaningful measurements. It is
>a very common question, and all we have for answers are wild
>guesses. Whoever does this and does this correctly would
>really be contributing a great deal towards answering a
>common question.


It's an interesting problem. A tree is a tiny fraction of a wavelength in 
one direction and a significant fraction in the other direction, so it 
would be kind of like a wire.  One might be able to put a loop or dipole 
near it and carefully measure the Z.  With your 500 ohm/ft wild estimate, 
we can figure out how much difference it might make (using something like 
NEC) and see how hard the measurement will be to make. (is it antenna 
analyzer territory or precision impedance bridge territory)