On 7/16/2020 9:57 PM, Ken WA8JXM wrote:
So pruning a horizontal dipole will never provide a 1:1 SWR except at 0.18
wavelength above ground (the ONLY elevation where the radiation resistance
is 50 ohms)? About 97 ft for 160m, 24 ft for 40m, 6 ft for 10m? A dipole
a half wave above ground has a 70 ohm radiation resistance and therefore a
1.4:1 SWR is the absolute best?
No, it's FAR more complex than that. Horizontal antennas see the
reflection from the earth as a parasitic element, which adds a complex
impedance to the equivalent circuit. The reflection, and thus the
parasitic impedance, will depend on height above ground, the skin depth
of the earth, and the electrical qualities of the soil.
The ON4UN book, "Low Band DXing," includes a set of very interesting
plots of antenna impedance as a function of height and soil quality. In
general, the feedpoint Z of low dipoles oscillates as height varies,
with the average near 50 ohms, while high dipoles oscillate around 70-80
ohms. It's easy to see this in the very simple NEC model of a center-fed
dipole (the model is a wire with a generator in the center of it) by
varying the height and the soil parameters.
My dipoles for 40 and 80M are up about 125 ft in tall trees, and I have
lousy soil. The model predicts Z at resonance in the range of 85 ohms,
and I've measured 88 ohms. I feed them with RG11. Some years ago, I was
planning antennas for a CQP county expedition in a spot that has scrub
trees (max rigging height ~40 ft) and lousy soil, but is on a small
knoll, so the takeoff angle is great. NEC predicted 75 ohms for one of
them and 50 ohms for the other, so we carried RG8 for one and RG11 for
the other. When we measured them, they turned out as the NEC model
predicted.
There are computational tools and instrumentation that one can learn a
lot from if we use them to study stuff like this. NEC is one of them.
73, Jim K9YC
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