[TowerTalk] antennas and trees

[Jim Lux]

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