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Re: [TowerTalk] verticals in woods vs. in a field

To: Tom Rauch <W8JI@contesting.com>, "RICHARD BOYD" <ke3q@msn.com>,"towertalk reflector" <towertalk@contesting.com>
Subject: Re: [TowerTalk] verticals in woods vs. in a field
From: Jim Lux <jimlux@earthlink.net>
Date: Wed, 15 Sep 2004 14:58:29 -0700
List-post: <mailto:towertalk@contesting.com>
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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