Topband: the effect of trees

[Carl K9LA]

The Technical Correpsondence column in the November 1991 issue of QST
had some interesting observations by KF4IX and K4OQK.

They put up two top-loaded 60ft verticals for a 160m array.  One was
near large trees and one wasn't.  The one that was near trees had an
input impedance of around 75 ohms, and the one not near any trees had an
input impedance of 37 ohms.

To try to understand what was going on, they did some experiments with
an 80m vertical, and discovered a higher feedpoint impedance (extra
loss) when the top part of the vertical was very near the tree trunk.

Using this information, they moved the 75 ohm 160m vertical farther away
from the trees, and it's feedpoint impedance dropped to 45 ohms.

So it appears that trees do affect our low band verticals.  From KF4IX's
and K4OQK's work, it looks like 6 feet or so away from the trunk is the
minimum distance one should strive for.  The vertical portion of my
elevated radial inverted-L runs parallel (about 4 feet away) up a nice
big tree, and I see a higher input impedance (about 50 ohms compared to
23 ohms per NEC) than what NEC-2 predicts.  I suspect it's mostly caused
by what KF4IX and K4OQK reported, although I can't totally rule out
ground losses.

If anyone would like a copy of this article (it's less than one page),
drop me an e-mail at k9la@gte.net and I'll send you a scanned version of
it.

Carl K9LA