On Mon, 10 Mar 2008 15:15:02 -0400, jim Jarvis wrote:
>an antenna in the air will outperform a model, every time.
>Just put the damned thing up!
I agree, Jim. It's easy to go overboard with the handwringing and
procrastinate while one could be having fun.
One important quibble re: trees though. While I agree that trees
usually don't attenuate HF signals enough to matter, there's one
A big tree trunk VERY close to an HF vertical WILL negatively
impact antenna performance. While I haven't yet done A/B
comparisons with a vertical NOT close to a tree trunk to prove it,
careful on the air observations of a 40M vertical dipole hung next
to one of my tall redwoods indicate that it's 3-6 dB down from
where it would be if the tree were not there. This is based on on-
the-air comparisons with my high horizontal dipoles, and
comparison of those results with NEC models of both antennas. I
suspect the cause is RF coupling into the tree and being
dissipated in the resistance of the trunk. The 40M vertical in
question is a half-wave dipole with the top at about 100 ft.
Photos and computed NEC results are near the end of
Trees DO suck up a lot of RF at VHF, and even more at UHF. At my
QTH (in a dense redwood forest), 10M is still great, 6M works, but
2M is seeing the trees (horizontal or vertical polarization), and
432/440 is a waste of joules. And nobody's cell phone (850 MHz)
works up here.
Jim Brown K9YC
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