[TowerTalk] verticals in woods vs. in a field

Wed Sep 15 21:06:41 EDT 2004

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 at dellroy.com>
To: "Tom Rauch" <W8JI at contesting.com>; "RICHARD BOYD" <ke3q at msn.com>; 
"towertalk reflector" <towertalk at contesting.com>; "Jim Lux" 
<jimlux at 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 at earthlink.net>
> To: "Tom Rauch" <W8JI at contesting.com>; "RICHARD BOYD" 
> <ke3q at msn.com>;
> "towertalk reflector" <towertalk at 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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