Jim Brown wrote:
> This antenna would still work well with poor ground
The bobtail virtually eliminates loss due to feedpoint ground current.
And it minimizes ground current loss near the antenna since high wire
currents are well away from ground. But it can't escape the ground
reflection losses at low angles that all vertically polarized antennas
suffer over poor ground.
With NEC-2 I modeled a bobtail at 3.7 MHz. I fed it at the top of the
center wire since NEC-2 can't accurately feed wires against ground. This
should be essentially equivalent. I also modeled a dipole. Both antennas
used #12 copper wire. I used permittivity = 9.5 and conductivity = 0.61
mS/m. These are the generic Hagn values for "mountains; rocky, steep
hills" at 3.7 MHz. See http://ham-radio.com/k6sti/hfgc.htm.
I had to put the dipole at 140 feet to match the bobtail gain at 5
degrees elevation angle. Patterns:
https://i.postimg.cc/c4dXQnPq/az.png
https://i.postimg.cc/9F111fjD/el.png
There are many trade-offs between these two designs, but I think this is
a fair comparison. Someone with a NEC version that can feed a wire
against ground might want to check my equivalency assumption.
Brian
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