On Mon, 13 Jul 2009 11:16:49 -0500, wb0m@flashdog.us wrote:
>Does anyone have any guess on what would be the minimum distance from
>the branches and leaves and from the trunk of a tree to avoid major
>disruption of a signal from a beam antenna up about 30 feet? The tree
>is a silver maple about 50 feet high.
I live in a dense redwood forest, so I've searched the technical literature
attempting to learn how trees affect radio signals. There isn't much
published on HF, a bit more on VHF and UHF. Here's a summary of what I've
learned.
1) Absorption (loss) increases with frequency. Trees absorb a lot of RF at
VHF and UHF, to the extent that a dense forest like mine pretty well kills
440 MHz and above, and puts a pretty good dent into 2M.
2) Tall straight trees like pine and redwoods seem to absorb vertically
polarized RF a lot more than horizontally polarized RF.
3) It seems to be mostly the tree trunks that absorb RF.
4) At various Field Day operations (Chicago, Ohio, WV) I've worked with HF
antennas in trees like yours and they worked very well. My first antennas
in WV ran between two maples about the size of yours.
5) Here in my redwoods, I started with a 160M dipole that ended close
enough to a redwood that the end of the antenna arced to a big limb and
scorched the insulation (THHN wire). This is with max legal power. That
antenna worked great.
6) I just put together a 3-el SteppIR that's temporarily at 20 ft. I'm
building a tower that will put it at 120 ft, where it will still be well
below the tops of the redwoods that surround it on all sides (so close that
there's only a few feet of turning radius remaining). I expect it to work
well. I currently have dipoles for 160-10M hanging between trees at that
height, and they work well (and the 80/40 fans fed with low loss RG11 work
well on 6M).
Bottom line -- put your antenna up and don't worry about anything but a
tree falling on it.
73,
Jim Brown K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|