Very nice design, Don. Everything you report makes sense. A horizontal
dipole at 110 ft is a VERY respectable antenna on 80M (and 40M too). By
contrast, it's a low antenna on 160M, so it will have significant
radiation at higher angles, compared to a vertical which has very little.
You didn't say how tall the tower is. Looking at 80M, a half-wave
vertical is a pretty low angle antenna -- peak radiation is at 18
degrees, and it's 6 dB down at 45 degrees and 4 degrees. As we make it
30-40 ft taller, the main lobe peaks at 15 deg and a second lobe
develops. At 170 ft (5/8-wave), The higher lobe is only 0.6 dB down from
the main lobe, and is 6 dB down from the main lobe at 76 degrees. (These
calcs for Rohn 25). The deep fading we have long called "selective
fading," is caused by cancellation of the same signal arriving from
different paths; the taller vertical will have more selective fading
simply because it adds high radiation to the low angle radiation.
When I first moved to NorCal, I had both a 160M dipole (with loading
coils at about the 70 ft point on both legs) with about 15 ft beyond the
coil to resonate it. It was at about 120 ft. I also had a Tee vertical,
about 85 ft tall, with about 70 on-ground radials varying in length from
about 60 ft to about 125 ft, but a few shorter ones that ran into a
building and concrete. I often switched between them, being careful
that AGC didn't cloud my observations. The vertical usually yielded the
best signal, only occasionally the dipole. 160M contests start at 2 PM
local here; with the vertical, I can reliably work big stations out to
at least 800 miles running legal limit; with the dipole, not even a QRZ?
When a storm broke something in that 160M antenna (which, with the
loading coils also worked well on 80M), I rebuilt it only for 80M.
My soil conductivity is pretty poor (rocky). That doesn't affect
horizontally polarized antennas, but it reduces radiation strength of
vertically polarized antennas. What I'm getting at here is that a decent
vertical with a decent counterpoise or radial system is going to beat
almost any horizontal antenna on 160M that most hams can rig.
Another thing I learned along the way. Wires on the ground have a much
lower VF. I learned that by measuring (with an MFJ259B) a dipole laying
on the ground and noting where it resonant. I also did a model, which
arrived in the same range, with VF on the order of 0.7 - 0.75, so
roughly 95-100 ft for a quarter wave on 160M. Although I didn't model
it, I suspect that it also varies with soil conductivity.
The model said I didn't have to get the radials very high for VF to
approach 1. Certainly any radial high enough to be effectively
"elevated" (N6BT says at least 20 ft on 160M) will have VF slightly less
than 1. I'm using the NEC2 engine; NEC4 may yield better answers.
73, Jim K9YC
On 6/28/2020 8:07 PM, Donald Chester wrote:
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.
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