Hi Jim,
How does this change if a 100'tall 12" diameter tree is less than
2 feet from the vertical or inverted L leg?
Then what happens when the 2' foot away tree is modeled with a
series of trees about 6' - 10' apart in most directions?
This is the forest situation that I am dealing with.
I strongly suspect that the trees are adversely impacting the performance
of my 160 M phased Inv L's, but is not the only
factor affecting their performance.
73
Bob KQ2M
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Thursday, September 16, 2004 6:52 PM
Subject: [TowerTalk] antennas and trees
> Some quick modeling using NEC2
>
> Built a 80 m vertical, 2" in diameter, Sommerfield-Norton ground at 3.8
MHz.
> Put a 100 ft 12" diameter tree (20 segments, 5' long) some distance away.
> loaded the tree as either wire, or with lumped loads.
>
> Ran the model and recorded the "structure loss"
>
> First, the lumped load numbers.. tree is 40 feet away from the antenna,
and
> the same resistance was used for each 5 foot segment. Power input was 100W
> Rload Ploss
> 100 1.726
> 500 .4474
> 1000 .2314
> 2000 .1178
> 5000 .0477 (<<< this might be closest to the number Tom measured for
> his chunk of pine)
>
> For reasonably high resistances, it looks roughly linear.
>
>
> Using conductivity loading (wire) and various distances (with 10 segments
> on the 100 ft tree)
> mho/m 20 ft 30ft 40ft
> .01 .2553 .1592 .1134
> .005 .1285 .0802 .0571
> .001 .0258 .0161 .0115
> .0001 .0026
>
>
> As before, appears to be linear with conductivity, and roughly inversely
> proportional to distance. I have no idea if 0.001 mho/m is a good model
> for a tree (I'll have to do some conversions and make sure we're in the
> right order of magnitude)
>
>
> In any event, it looks like a single tree isn't going to be a big
hassle...
> a few tenths of a watt loss for 100W excitation. Dozens of trees in close
> proximity might be a problem, but..
>
> Say the trees are 100 ft tall and 30 feet apart in a hexagonal grid (dense
> packing of circles). Your antenna is right in the middle of a grid, so
> it's 30 feet from the closest 6 trees. You'd lose, say, a watt for each
> tree. Then, the next tier out is 60 feet away, and so has half a watt per
> tree (6 trees), the next are 80 feet away, .38 W/tree, but there's 12 of
> those..
>
> So far, we're up to 6+6*.5+12*.38 = 13.6 W.... That's about 0.6 dB so
far.
>
> That's assuming we're in the 1W for 30 ft away category. In reality,
we're
> probably more like 0.05-0.10 W.. so our loss is just barely a watt in the
> trees within 80 ft.
>
>
> There's probably all kinds of modeling errors, but I think it's probably
in
> the right ballpark.
>
> The loss in the ground is probably more than the loss due to the trees.
>
>
>
>
> Jim, W6RMK
>
> Here's the model:
> SY S = 40
> GW 1 10 -S 0 0 -S 0 65 1in/ft
> GW 2 20 0 0 0 0 0 100 6in/ft
> GS 0 0 ft
> GE 1
> SY I1 = 1
> EX 0 1 1 00 1.0 0.0
> '
> GN 2 0 0 0 13 .005
> LD 0 2 0 0 100 0 0
> FR 0 1 0 0 3.8
> EN
>
>
> the
>
>
>
> _______________________________________________
>
> 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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>
_______________________________________________
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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