so far the discussion has only treated the effect of trees in the immediate
near field area... what about the effect of wood in the Fresnel zone?
especially at low angles the ground reflected ray will surely be more
attenuated when passing through a relatively long space of dense wood than
being reflected in an open field... is there a difference between horizontal
and vertical polarization?
has this problem ever been treated in antenna literature?
73
Uli, DJ2YA
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Rudy Severns" <rseverns@gmail.com>; "Topband" <topband@contesting.com>
Sent: Sunday, August 04, 2013 2:42 AM
Subject: Re: Topband: Tree conductivity
Some time ago I attempted to make direct measurements of the HF
conductivity of trees, at least for the trunk and limbs. I simply put a
two rings, with nails to penetrate through the bark, around the trunk
spaced a couple of feet apart. Basically what I had was a resistor. I
then measured the impedance of this "resistor" using a network analyzer.
I think there is some confusion caused by conversations on this reflector
about **resonance**, and conversations about attenuation.
I think there are some people who believe the issue is resonance and think
a tree has enough conductivity to exhibit resonance effects. I've never
seen any documentation or experiment to support a tree exhibiting resonant
effects (at least for HF and VHF).
There isn't much doubt dielectric losses would play a role.
Years ago, because of some "fractal tree antenna" nonsense discussion, I
measured a fresh cut pine tree log and it had pretty high RF resistance
over a foot of trunk length. The resistance was high enough that a single
tree could not show resonance effects. I did a sweetgum later, when I had
to remove a sweetgum. It was similar.
This is different than attenuation by having either a strong electric
field near a single tree's foliage, or attenuation through thick foliage.
Another place where this comes into play is with seawater. Another goofy
thing appeared where someone was claiming a vertical jet of seawater could
be used to make a good stealth antenna. We all know seawater has a
profound effect and enhancement on patterns and loss, yet the resistivity
of sea water is so high it really makes no antenna at all when used as an
antenna. As a matter of fact, saltwater makes a pretty good dummy load
when current density is high. Current would be high if seawater were used
as an antenna conductor.
What we have is an inability to understand the difference between very
good conductors, poor conductors, and or lossy dielectrics. We'd have a
difficult time powering something through saltwater conductors, or having
resonance effects with saltwater jets at low frequencies. At the same
time, even crummy soil has a profound effect on EM fields and other things
when cross sectional area is large enough.
There is a danger that people will not understand the big picture, and
write more seawater antenna or tree antenna articles.
73 Tom
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Topband Reflector
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