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Author Topic: Do trees attenuate HF signals?  (Read 6667 times)
KW4CQ
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Posts: 132




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« on: June 09, 2003, 05:47:54 PM »

Has anyone out there conducted any serious measurement efforts to determine if trees (e.g., oaks, poplers, pines, etc. with or without leaves) in the near field of an HF yagi's antenna radiation pattern introduces any significant amount of RF signal attenuation?  Does anyone know of any research papers or publications in existance on this subject?
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WB2WIK
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« Reply #1 on: June 11, 2003, 11:38:44 AM »

The U.S. Army has, although the most recent work I've seen on the subject is from the early 1950's, and was published by RCA who researched "Signal attenuation from mid-latitude foliage" and included HF as well as VHF in their studies.  It was quite a thorough study, as I recall, although I don't have a copy of it that I can find -- I read this originally when I was working at Bell Labs in the 1970's.

More recent studies have included only the UHF-microwave bands, which of course are more influenced by foliage than HF and can cause real problems setting up satellite links in the tropics and jungles.

As I recall, RCA found that dense tropical foliage caused a surprising amount of attenuation even at HF, and I'm guessing this was important to know since at the time of the study we were engaged in the Korean conflict and already setting our sights on N. Vietnam for possible military engagement.

Personally, I've never noted a great deal of influence from foliage on HF.  When I crank my tower up above the local tree canopy, everything works better, but I'll bet that mostly from height above ground and not because I'm clearing the trees....

WB2WIK/6
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KW4CQ
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Posts: 132




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« Reply #2 on: June 12, 2003, 04:36:11 PM »

Thanks for the come back on this subject Steve (WB2WIK).  I seem to recall some work performed by the Army (Sig Corps?) in the 60s or 70s on this subject now that you  remind me.  They were also experimenting in RVN around that time loading up a tree and descovering that it made a pretty decent emergency radiator, which sort of leads me to believe that tree trunks and leaves may in fact have some RF properties at HF that we are not fully aware of.  Surely there must be some absorbtion (or attenuation?) and possibly reflection by wet leaves that would affect an antenna pattern.  A novel and intriguing subject isn't it?  If I find any references I will be sure to pass them on to you.

Thanks again and 73,

Bob, KW4CQ
Reston, VA
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WA4PTZ
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Posts: 528




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« Reply #3 on: June 15, 2003, 08:03:35 AM »

YES !  The mathmatical data is not available. Trees
also effect your antenna SWR. Consult the "Proximity
versus Reflectivity" or the "RF Relativity" theorems.
73 - Tim
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VE6ADD
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Posts: 20




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« Reply #4 on: June 19, 2003, 03:46:23 AM »

Here is a foolproof method to check if items absorb RF energy: put a sample in your microwave. If it gets warm, it absorbs RF.

Herman, VE6ADD
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WB2WIK
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Posts: 20613




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« Reply #5 on: June 19, 2003, 04:09:52 PM »

The "microwave oven test" is interesting but yields no useful data regarding HF absorption, since the ovens operate above 2 GHz, where lots of things absorb RF.

I can get PVC insulator blocks to get warm in the microwave oven, but the same ones are commonly used as HV insulators in log periodic HF antenna installations, at KW++ power levels.

Note: When trying the microwave oven experiment, always place a glass of water or some known load in the chamber along with the material you're trying to test.  Otherwise, if your test material absorbs no energy, the generator has no load at all and this isn't good for them....

WB2WIK/6
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AA8RF
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« Reply #6 on: September 16, 2003, 11:31:29 PM »

In particular microwaves operate at 2.4GHz, the resonant frequency of water molecules. Anything with water in it will heat in a microwave.

-Jim
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N2CTZ
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Posts: 8


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« Reply #7 on: September 17, 2003, 07:28:32 PM »

trees do not attenuate hf signals.

I put my antennaes on top of the trees with a cherry picker and then there is not problem.


n2ctz
http://bigal@thebarbecuemaster.com
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alan
ON4MGY
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Posts: 214


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« Reply #8 on: October 21, 2003, 01:19:10 PM »

Yes and no!
Trees and plants do attenuate HF signals.
I found out that maïs does attenuate a lot on 10m. Just use your 10m HT or mobile setup near a maïs-field and you'll notice the attenuation of the received signals.
When you're using a 80m antenna near trees it will have (almost) none influence.
This is not a scientific study, but just the things I personally discovered.

73 de ON4MGY Nic
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KC8VWM
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Posts: 3121




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« Reply #9 on: October 27, 2003, 10:29:38 AM »


The amount of RF energy absorbed depends on the type of tree or plant in question.

Also, trees and plants would absorb more RF energy in the summer months. The exact mathematical data for a specific absorption rate for a tree would vary considerably under different seasonal or weather conditions.

For example, if there is a drought vs. high groundwater conditions. This would affect the ability for a given tree to "reflect" or "absorb" RF energy.

73

KC8VWM
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