Attenuation due to tress at HF

I sure do know the effects of Trees / Vegetation at Microwave Frequencies but I really do not know much about the effects at UHF/VHF and HF below 30 MHz.   I know that there would be some affect at VHF/UHF and that it would significantly decrease as you get down to the HF band.  So I googled the topic and here is what I found......

Ora E. Smith:
There is book called "HF Communications: A Systems Approach" by Nicholas Maslin (1987, I think) that addresses the issue briefly.  My interpretation of what he says is that vegetative attenuation negligible below 2 MHz, but can be a major factor at 30MHz.   A curious result of his analysis is that for ground wave propagation through vegetation, the loss below 30MHz for a given frequency is essentially constant and independent of distance -not sure I follow his thinking, but that seems to be his conclusion. The only way I can make sense of that is to conclude that wave propagation extends above the tree line and that the attenuation occurs mostly in the antenna near field, but that is not his explanation. If my version of this is true, it would seem that unless you are operating in a very tall rain forest, getting the antenna a few wavelengths away from trees so none of them are in the nearfield would reduce attenuation effects markedly.  Also, objects in the near field are able to distort the radiation pattern of the antenna considerably, unlike far field objects.

If you're talking about skywave attenuation, then it seems that you would want the antenna to have a clear view out to a few wavelengths, according to my theory anyway.

At 30MHz he states that the standard deviation of loss (so this must be based on either some kind of data or lots of simulation runs) is on the order of 4 db through bare trees, and 7 db through leafed trees.  Loss increases exponentially with frequency, he says, and is negligible by the time you are down to 2MHz.  I generally try to stay away from trees except as antenna supports, and I have found that operating portable in the woods is difficult, though it's hard to separate out all the other portable antenna shortcomings from the tree influence.

Do trees attenuate HF signals? « Reply #8 on: October 21, 2003, 01:19:10 PM »

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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

Trees have fairly minimal effect on HF propagation.

Trees touching your HF antenna will tend to ground out the antenna.  The more touching points, and the wetter, the more signal you will lose.   Foliage touching at high voltage points can and has caught fire!  There are a number of posts here and on the other board in the last couple years, some with photos, to attest to this.

An insulated wire vertical could be hauled up alongside a trunk and avoid the grounding problems.  Stake out some radials at the base.  Very stealthy and neighbor friendly.



"Trees
As potential hazards these are usually much overrated. Horizontally polarized antennas can usually be more or less buried in trees with little or no effect"

Here is my added advice, Use INSULATED wire, do not use bare wire....

For HF, any attenuation will be essentially unmeasurable by any equipment the average ham is likely to have. Wires in the trees are common and work quite well.

As the frequency goes up past 50 mhz, the attenuation will be more and more of a factor and will be measurable. For 2m/440, higher is not only better, the antennas really need to be above the trees, unless all you are doing is hitting local repeaters. For weak-signal work height and tree clearance are everything.