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

[Jim Lux <jimlux@earthlink.net>]

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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