On 2/1/18 10:25 AM, Jim Brown wrote:
There's a piece in QST this month about trees absorbing RF
from VERTICAL antennas. They have much less effect on horizontal
antennas. I suggest you take a look at my applications note on Antenna
Planning.
The modeling in QST (which was a nice article) only modeled the trunk
of the tree, so it wouldn't find any H-pol effect.
You'd really need to model the branches on the tree as well.
A nice "further work for the student" would be to take the QST model and
add horizontal branches, then run it with some horizontal dipoles.
They've already done the hard work of figuring out the dielectric
properties of the tree wood, and the equations for 4nec2 to convert that
to RLC loads.
There is a reference in the article to Tamir's 1977 paper, but that's
more of "modeling the forest as a dielectric slab" for propagation, not
the near field effects. Cavalcante's 1982 and 1983 papers extend on
Tamir's work, but still ignore the near field effects (well, he
describes it, gives the equation, and then blithely goes on with: "Since
the interest here is the calculation of the fields in the far zone, the
Hankel function which appears in (16) may be replaced by its asymptotic
expression.In addition, one over the expression that appears in the
denominator of (18) may be expressed in an infinite series form
(binomial expansion)."
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