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.
>
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