To: | Tom Rauch <W8JI@contesting.com>, "RICHARD BOYD" <ke3q@msn.com>,"towertalk reflector" <towertalk@contesting.com> |
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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 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. _______________________________________________ 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. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk |
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