----- Original Message -----
From: "Jim & Velma Woods" <woods@grantspass.com>
> I live in southern Oregon on a level 2.4 acre lot. Have quite a few trees
in the 70 to 80 foot high range. They include ponderosa and sugar pine,
white pine and Douglas firs. Am considering a tower for a yagi or quad and
as a support for wire antennas and am wondering how tall the tower would
need to be to avoid serious absorption of RF.
>
> Soil here is full of fist-sized rocks with clay ... and dries out in the
hot summers.
>
> Also, a few miles away is a mid-sized mountain which is east of my site.
I used a topo map to determine that the top of the mountain forms an angle
about 15 degrees above the horizon. Is this high enough to be a concern?
>
> Thanks for your input.
>
> Jim, W7PUP
> woods@grantspass.com
If it were me, Jim, I would be more worried about the 15 degree
mountain than absorption from the trees. While it's debatable
what effect the trees will have, the +15 degree horizon will effectively
cut off signals arriving at very low angles from that direction. If you
look at the angle of arrival statistics in terrain programs like HFTA
(HF terrain analysis), you will see that a fairly large percentage of
DX signals are arriving below +15 degrees. My experience with
the +14 degree mountain at my QTH bears this out. Anecdotally,
the effect seems to be lessened during periods of high sunspot
activity where AOA's tend to be higher due to the higher MUF.
As far as the trees go, some guys will tell you that it has a
noticeable effect, while others will swear equally vehemently
that it does nothing. I frankly don't know. The experiments needed
to make the determination are hard to do. I think professional
researchers have done some work in this area, but I am not sure
what if any conclusions they reached. My guess is that it's for the
most part a small effect. The terrain and the antenna height above
that terrain will be the dominant factor.
Good luck!
Mike, W4EF.............................................
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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