[TowerTalk] antenna height & trees & fairy tales

[Jim Brown]

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 
dramatic exception. 

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 

http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf

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. 

73,

Jim Brown K9YC