I'm not sure the dielectric properties of such a poor conductor will
mean much. The dielectric of something touching a conductor is another
matter, but these are not "close" and the field around conductors
diminishes very quickly.
I went outside and took the sharp picks of my Fluke multimeter and
stuck them through the bark into the wet of several different kinds of
live trees. An oak measured over 1 meg across two feet vertical on the
trunk. One of my maples measured over 100K and interestingly did a
very slow charge like an electrolytic capacitor, measuring over 500K
after five minutes.
All you need is a multimeter with sharp enough prods to dig into the
wet layer of the tree.
If there isn't something in the tree that will allow a fair amount of
current, there won't be enough current to induce loss by the
dielectric properties. 50K or more ohms per foot not exactly conducive
to current.
73, Guy.
----- Original Message -----
From: "Michael Tope" <W4EF@dellroy.com>
To: "Tom Rauch" <W8JI@contesting.com>; "RICHARD BOYD" <ke3q@msn.com>;
"towertalk reflector" <towertalk@contesting.com>; "Jim Lux"
<jimlux@earthlink.net>
Sent: Wednesday, September 15, 2004 7:38 PM
Subject: Re: [TowerTalk] verticals in woods vs. in a field
It might be interesting to take a freshly cut try stump (so 8 to 10
feet),
wrap it with copper sheet, bore the center out for a copper pipe,
and the do some impedance measurements of the resulting
coaxial transmission line. With some careful measurements, you
might be able to get a handle on the dielectric properties of the
green wood, which in turn could be used to predict the losses
associated with a stand of similar trees if you had the right EM
modeling tools. Do any of the NEC programs do lossy dielectric
cylinders?
73 de Mike, W4EF
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Tom Rauch" <W8JI@contesting.com>; "RICHARD BOYD"
<ke3q@msn.com>;
"towertalk reflector" <towertalk@contesting.com>
Sent: Wednesday, September 15, 2004 2:58 PM
Subject: Re: [TowerTalk] verticals in woods vs. in a field
At 03:45 PM 9/15/2004 -0400, Tom Rauch wrote:
> > Anyone have experience, anecdotal or otherwise, on the
> > performance of
a
> > vertical "in the woods" versus in an open field? I potentially
> > have
both
> > options. Especially with lots of radials, "in the woods" would
> > not
"use
>up"
> > my open fields as much -- I can keep them for livestock, crops,
> > or
towers
> > with other antennas. 73 - Rich, KE3Q
>
>
>I've been looking for measured data for years. The closest I've
>found is
>from Roy Lewallen, and even it is somewhat suspect although it
>does
indicate
>dense woods cause very noticeable loss.What I wanted to do here
>was
actually
>install a vertical and measure FS before and after trees were
>removed.
>Someday I might do that.
You're probably looking for near field effects, right?
Seems that there should be some data from the late 60's early 70's.
There
was a fair amount of propagation data being measured through the
jungle,
etc. at HF and VHF frequencies to support various modeling and
antenna
design efforts. Hagn's open wire line soil properties measurement
technique
was developed to replace earlier measurements where they took
dipoles and
monopoles that had been calibrated in free space (or in a precision
environment, like a large metal ground plane, etc.), then put them
in the
test environment and measured terminal impedance, and from that,
attempted
to estimate EM properties.
There's a paper from Vogel and Hagn, presented at ISART '99 in
Boulder, CO
"Effects of Trees on Slant Propagation Paths"
It looks at various paths (horizontal, medium, short) and modeling
the
forest as either a homogenous mixture or as discrete units.
It gives some results for VHF (50MHz) as an attenuation constant of
0.031-0.1 dB/m for horizontally polarized, and 0.045-0.12 dB/m for
vertically polarized. They propose a model of
A(f2) = A(f1)*exp(1.173*(sqrt(1/f1)-sqrt(1/f2)), (f1,f2 in GHz) but
I have
to say that the measurement points don't follow the model all that
well.
One might get a feel for how important things like soil
conductivity vs
tree properties are by putting together a NEC model, representing
the
trees
as vertical wires touching the ground. You could come up with some
wild
guesses for the resistive loading of the trees. Then fool with
changing
the loading and soil properties to see what happens to the monopole
radiation efficiency. You might find that the actual tree
properties
don't
have much effect, or that the soil properties dominate.
You'd pay no attention to the actual numbers (the modeling codes
are not
well suited to this), but things that result in big changes are
probably
worth looking at.
No promises, but I know someone who occasionally gives out problems
like
this as class assignments, so if you can give some tree density
statistics
and tree sizes, maybe someone will take it on. (for instance, it's
10
feet
between trees, they're randomly placed, and range from 20-50 feet
tall and
from 3" to 12" in diameter, and you're interested in 7MHz...) As a
practical matter, there is a fair amount of interest these days in
FOLPEN
(foliage penetrating) sensors, but I suspect they're looking at UHF
and
up.
>The problem of not having good measurements is we all tend to go
>by
>feelings. It's pretty tough to notice several dB change by
>impression
alone.
>Look at the variation between antennas, such as the GAP, to a good
>trap
>vertical. It can be as much as 5 or more dB, yet many people will
>swear
by
>the GAP. That's because we usually can't see several dB change
>unless we
do
>a direct A-B comparison. Another example are the little
>mini-things that
>claim 6dBd gain. Bad measurements or opinions are everywhere,
>that's how
all
>these magical patent-pending antennas get started and why
>notoriously
poor
>antennas have a market.
>
>Maybe you can put two identical antennas up with one in the woods
>and one
in
>the clear and A-B them. That would be a good service to the
>community.
>Myself, I don't like to take chances so I keep my verticals in the
>clear.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless Weather Stations", and lot's more. Call Toll Free,
1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
_______________________________________________
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