Grant Saviers grants2 at pacbell.net Mon Jun 15 14:09:50 EDT 2020 wrote:
> It's a tough game to achieve low angles from horizontal antennas on 80m
> until over 1/2 wl high.
My 80m dipole is attached to the tower at the 120' level, with the ends
drooping down to 100'. With that small amount of slope I consider it closer to
a true horizontal dipole than an inverted vee. The feedline is open wire
line, two #10 conductors spaced 2" apart, running up through the interior of
the tower, spaced at the geometric centre of the cross-section triangle of the
Rohn 25G.
The 127' tower is insulated at the base, with 120 uniformly spaced buried
radials, each 133' 4" long. The base-insulated tower is used as a quarter-wave
vertical tee for 160, with the dipole serving as the horizontal portion of the
tee. Even though neither the dipole nor the feeders are electrically in
contact with the tower at any point, the close proximity of the feedline to
the tower over its entire length causes the dipole to be closely enough coupled
to the tower to add substantial top loading; as a result, the base impedance of
the tower on 160 is in the range of hundreds of ohms with significant reactive
component, rather than ~38-40Ω + j0 as it would be expected with just the tower
with no dipole attached.
A few years ago I tried loading the tower as a half-wave vertical on 75. As
expected, the performance was mediocre at best at less than 600-800 miles, but
I had interesting results working DX. I contacted the same French station on
75m AM phone several times, as well as stations in the pacific northwest, and
asked for signal comparisons between the half-wave vertical and the (nearly)
half-wave high horizontal dipole. In every case, at both distant locations,
the reports were that the signal strength peaked about the same with both
antennas, but with the vertical tee, the fades were much deeper. The signal
was decidedly more solid with the dipole.
I had expected the half-wave vertical tee with 120 half-wave radials on 80m to
be a killer antenna out beyond 2000 miles or so. I was a little disappointed
that it didn't perform quite so well as the dipole. I never tried the
comparison at KH6, VK/ZL or Asia; maybe that's where the half-wave vertical
would have dominated.
I rarely work DX, so since then I have stuck with using the dipole on 75-80m.
The 80m dipole will load on 160 as a quarter-wave dipole (using the appropriate
ATU). Normally on 160 the vertical is about 10 dB stronger at points beyond a
couple hundred miles, but at locations 50-100 miles away, in the evening the
80m dipole may be as much as 30 dB stronger than the vertical tee.
I also use the dipole as a double-zepp aka two-halfwaves in phase on 40m with
good results.
Don k4kyv
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