That's a good observation. Although both antennas (an Optibeam OB2-40
and an OB16-3) have all their elements isolated from the boom, I do have
the shields of all my coax runs grounded to both the top and the bottom
of the tower. I suppose it is possible that the combination of the
tribander and the tower is not acting with horizontal polarization, and
that would certainly make a big difference compared with horizontal
polarization of the 40m yagi.
Eventually I want to install two identical non-resonant horizontal
dipoles insulated from the tower (spaced about 10 or 12 feet apart) and
fed with equal lengths of identical coax. That should be a much better
test vehicle.
Still, the significant variability within such a brief period of time
that I'm observing suggests that how we hams typically compare antennas
is not very reliable.
73,
Dave AB7E
On 9/16/2010 12:07 PM, K0DAN wrote:
> Your 40M monitoring using the tribander may have your tower
> acting as a top-loaded vertical.
>
> I have not played much at 40M, however when I listen on 160M
> using my 10 el. log periodic on top of a 75' tower/mast, I get
> better signals (better S/N, less fade, better DX) than with a 250' Zepp
> fed with 600 ohm line.
>
> Am in the midst of tower repairs right now, so that configuration is
> temporarily QRT, however I plan to do more experimenting in that arena.
>
> YMMV
>
> 73
> Dan
> K0DAN
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