[TowerTalk] Loss Due to Trees

[Jim Brown]

On 12/26/2011 5:49 AM, Eddy Swynar wrote:
> People will go to such great lengths like that to get their wire creations supported on the wings of one of Mother Nature's "organic towers", whilst guys on other Reflectors condemn the use of trees as antenna supports due to their "lossiness"...!
>
> I never could quite wrap my head around the claim that having a wire supported in a tree is a "...lossy" affair.

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