[TowerTalk] antennas and trees

[Guy Olinger, K2AV]

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 at earthlink.net>
To: "TowerTalk" <towertalk at 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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