"A quick google on tree antennas gave me this:
http://w5jgv.com/tree_antenna/index.htm which has links to some stuff"
I don't buy that that guy is actually loading a tree.... that is, I doubt
he's coupling significant power in and out vs. using the leakage of his air
toroid as a magloop.
As far as I can tell, you can actually model a tree trunk, at least, just
fine in EZNEC.
Resistivity of the material in use can be chosen by the user. The
segmentation guidelines
allow you to build ~100 foot "wires" with diameters of a foot or two no
problem.
Resistivity = 1/conductivity. Let's assume the conductivity of a tree is
roughly the conductivity of
"good" soil, often quoted as 0.03S/m. The reciprocal of that is about 33
ohm meters (about 20 billion times worse than copper)
For a 90 foot "trunk" two feet in diameter with a resistivity of 33 ohm*m
installed over *perfect* ground, maximum gain is about -27dBi. The current
distribution is obviously highly modified (tapers much faster)
And I think trees aren't even that conductive based on anything I've found.
Maybe a factor of ten less.
A tree would have to clock in around saltwater conductivity to start to
become reasonable as an antenna. This makes it
unlikely to develop much current as a parasitic element. However, I'd like
to look at details to make sure, because it's not exactly an insulator
either.
I understand that most of the time other issues are going to trump tree loss
even if it exists and is important, because it's certainly *small* But it's
probably worth thinking about occasionally. Caging trees is a good idea.
Is it necessary if you're trying to squeeze out the last dB?
As far as I can tell the answer is "maybe," so it seems like a discussion
worth having.
73
Dan
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160 meters is a serious band, it should be treated with respect. - TF4M
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