The loblolly pines on my property have the least resistance measured
on the Fluke. If the ratio of 1/5 the linear resistance or 1/100 the
linear resistance of holds for the other trees, only the loblollies
around here would have even the slightest effect.
Thanks for the figures.
73, Guy.
----- 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
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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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