[TowerTalk] Loss Due to Trees

[Cqtestk4xs at aol.com]

...and some food for thought.  For those of us who live in  wooded areas 
where all the trees lose their leaves for the winter and go  dormant, is this 
less of a problem in the winter than the summer?  Less  moisture in the tree 
when it's winter? Just asking, any biologists out  there?
 
Bill KH7XS/K4XS
 
In a message dated 12/26/2011 6:38:34 P.M. Greenwich Standard Time,  
jim at audiosystemsgroup.com writes:


Since my QTH here in Northern California is in a dense redwood  forest 
and a lot of those trees are on my property, and because I'm an EE,  I've 
thought about that a LOT, and I've also searched for technical  writing 
on the subject.  What I think I've learned can be summarized  as follows.

1) Some trees DO absorb RF, and the absorption (loss)  increases with 
frequency.

2) The absorption (loss) seems to be  greatest for straight vertical 
trees (pines, firs, redwoods)

3) The  absorption (loss) seems to be greater for vertically polarized  
signals

4) The closer the antenna is to the tree(s) the greater the  loss

When I say "dense redwood forest," I'm talking about trees that  are 
typically 3-6 ft in diameter at their base, 100 - 175 ft tall, and  often 
within 10-15 ft of each other. it's quite common for a very old  (tall) 
tree to be surrounded by three or four younger (shorter) trees only  a 
few feet away.  It took me four years to find a place I could turn  a 
3-el SteppIr.

After living here nearly six years, I have six wire  antennas suspended 
by trees, all of them up about 110 ft.  I also  have a SteppIR at 120 ft, 
monobanders for 20M at 45 ft, 15M at 35 ft, and  10M at 20 ft. The 
horizontal antennas all work well.

The only  vertical that works well is an 86 ft Tee vertical for 160M with 
about 70  radials that vary in length from 70 ft to about 120 ft. I've 
tried  resonant quarter-wave verticals on 80 and 40 over the same set of 
radials,  and they don't work well.  The 160 vertical is near the middle 
of my  clearing (roughly an acre), and it seems to work OK. My tower is 
not in  that clearing, surrounded by the dense forest, and I've tried 
loading  wires hanging on it on 160M. NEC says it should have a few dB of 
gain  relative to the Tee vertical, but the Tee Vertical outperforms it 
on  transmit, and by more than a few dB.  I have a 40M end-fed vertical  
dipole hanging in a redwood from a pulley at the 110 ft level, perhaps  
10 ft away from the tree at the low end of the antenna. It works, sort  
of, but it's close to being a dummy load compared to the two horizontal  
dipoles at 110 ft.

I have a loaded vertical for 2M and 440 sitting  on a 20 ft mast just 
outside my shack, also near the center of my  clearing, that I use to 
work repeaters. It's got about 3dBi on 2M and 6dBi  on 440. I can't work 
much on 440, due to a lot of absorption from the  trees. I can work a 
little more on 2M.  The local DX club has a  repeater that's about 17 air 
miles from me that I can bring up with 150W,  but my signal strength is 
far too low to talk it. So I bought a 22ft long  Yagi for 2M about 15 
dBi, mounted it vertically on the side of my short  tower that is also 
surrounded  by close-in trees, and pointed it to  that repeater. It can't 
even bring up that repeater.  Next I stuck a  short 5-el Yagi designed 
for vertical mounting (about 9dBi) on a 20 ft  mast near the shack, in 
the clearing. It can work the repeater, and while  I'm still noisy into 
it, I can talk it.

Virtually NO cell phones  work up here in the forest.  Several years ago, 
I read anecdotal  reports from a tech who was responsible for installing 
cell systems in the  pine forests of the American southeast, and he also 
reported very strong  absorption (high loss) for his systems.  My 
neighbor WA6NMF, an  engineer who has considerable experience with UHF 
and microwave systems,  says "Jim, at cell phone frequencies, think -3dB 
per tree."

My  neighbor, NI6T, lives deep in a ravine whose steep walls rise 300-500 
ft  above his antennas, and also by a dense redwood forest. Garry has 
made the  DX honor roll using a big tri-bander that's up about 100 ft in 
a big  redwood that he topped off.  He's also done well with a 160M wire. 
It  helps, of course, that Garry is a superb operator.

When I was thinking  about that sort of mounting here Garry strongly 
advised me against it. The  reason is that when (not if) something goes 
wrong, you're stuck with a  tree climber to fix it.  It's relatively easy 
to find a tower climber  who can fix antennas, but I don't know of any 
tree climbers who can do  anything more technically complex than rig 
pulleys to trees and string  support ropes through them. He's got exactly 
that problem now -- his coax  at the top of the tree has been attacked by 
rodents.

As to WHY  there's loss?  Think of the trees as big masses of wet wood 
that act  like resistive conductors, and foliage that contain water.  
When an  EM field hits them it induces current, and the resistance 
creates  heat.  That heat (power) is subtracted from the EM field.

73, Jim  Brown  K9YC

_______________________________________________



_______________________________________________
TowerTalk  mailing  list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk