> Not exactly scientific, but the empirical evidence speaks volumes. Check
> out this guy's "tree antenna."
> http://www.qsl.net/kf4bwg/treeant.htm
Ford and all,
The problem is people can say it is a tree radiating, when it isn't. Coax
with a unshielded center conductor exposed hanging on the tree will excite
the shield of the coax quite well with even a very small exposed center.
Antennex has been full of articles about "antennas" that actually use the
shield as an antenna more than the thing called the "antenna". They hang a
small "something" on the end, excite the outside of the braid with current,
and write long articles about how the "thing on the end" radiates!
In order to be resonant or have length-related resonant effects, the
resistance of the tree would have to be small compared to reactance per unit
length. In the early 90's I measured RF resistance of a series of tree
samples by attaching flat plates firmly against the edges of large cross
sections of wet fresh-cut trees, and they were many kilo-ohms per foot. This
included acidic and very sappy Georgia pines, and fast growing trees that
water would actually pour from (we called them Water Oaks) cut branches.
Nails would be even worse, because tree cross section contacted by the nail
would be very small.
Reports of trees "radiating" lower frequency signals (below upper UHF) are
largely due to people not understanding how coaxial cable works, and the
importance of the center conductor hanging out. Trees do reflect RADAR, but
they are still poor. They are mostly absorbers due to water content, a
single tree will attenuate a 2GHz signal many dB. The effect that causes
this rapidly becomes smaller as frequency moves below 2GHz.
I wonder how people have measured the effects they report, or if it is just
a feeling based on skywave signal reports. That would be important to know.
73 Tom
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