I read an article in 73 Magazine about 17+ years ago by a guy who loaded a
tree, about 25' (7.62m) high. 3 to 4 meter long and 15 cm (6") diameter shaft
and ovoid shaped leafy top. With a screw driven a couple of inches he made the
connection point for his gamma rod. He shunt-fed the tree, and reported mostly
vertical polarization even when measuring around the leafy top. I dont recall
if he used radials. The man said that he made a few contacts with 100W on 15 or
20m.
I have a group of four palm trees in the backyard, not in a 4-square
configuration :o( , abt 18 m (60') high the tallest and 60 cm (24") diameter at
the base. With nice and natural capacitive hat. A couple of months ago, I tried
to drive one self tapping screw while at the top of a 4m ladder, but the
shaft's skin is very hard and the force I had to exert was making the excercise
a dangerous stunt. I had to climb down. Palmtree, 1 - Me, nada.
I forgot about it until now, maybe I should try a less dangerous and shorter
omega match!
73
David
HK1KXA
EC5KXA
> Date: Tue, 3 Nov 2009 23:48:06 -0500
> From: n3ox@n3ox.net
> To: topband@contesting.com
> Subject: Re: Topband: Trees And RF
>
> "A quick google on tree antennas gave me this:
> http://w5jgv.com/tree_antenna/index.htm which has links to some stuff"
>
> I don't buy that that guy is actually loading a tree.... that is, I doubt
> he's coupling significant power in and out vs. using the leakage of his air
> toroid as a magloop.
>
> As far as I can tell, you can actually model a tree trunk, at least, just
> fine in EZNEC.
>
> Resistivity of the material in use can be chosen by the user. The
> segmentation guidelines
> allow you to build ~100 foot "wires" with diameters of a foot or two no
> problem.
>
> Resistivity = 1/conductivity. Let's assume the conductivity of a tree is
> roughly the conductivity of
> "good" soil, often quoted as 0.03S/m. The reciprocal of that is about 33
> ohm meters (about 20 billion times worse than copper)
>
> For a 90 foot "trunk" two feet in diameter with a resistivity of 33 ohm*m
> installed over *perfect* ground, maximum gain is about -27dBi. The current
> distribution is obviously highly modified (tapers much faster)
>
> And I think trees aren't even that conductive based on anything I've found.
> Maybe a factor of ten less.
>
> A tree would have to clock in around saltwater conductivity to start to
> become reasonable as an antenna. This makes it
> unlikely to develop much current as a parasitic element. However, I'd like
> to look at details to make sure, because it's not exactly an insulator
> either.
>
> I understand that most of the time other issues are going to trump tree loss
> even if it exists and is important, because it's certainly *small* But it's
> probably worth thinking about occasionally. Caging trees is a good idea.
> Is it necessary if you're trying to squeeze out the last dB?
> As far as I can tell the answer is "maybe," so it seems like a discussion
> worth having.
>
> 73
> Dan
> _______________________________________________
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