From G3PLX:
Dex said..
I initially used this antenna with a single vertical wire and the coil
grounded to the base of the tower ground system. I think this is what you
described above. There was some discussion about the capacitance between
the vertical wire and tower shunting the antenna current to ground so I
went looking for a way to feed a short grounded tower. So I found the
balanced feed method on Rik's site and give it a try.
The antenna current goes to ground one way or the other, it doesn't matter
which. The only thing that matters is that you have a nice big current
flowing between the top hat and the ground. With the coil at the bottom
rather than the top, you will indeed need to pump more current into the
bottom of the wire than flows into the top hat, but it's all capacitative
current so it's not lost and it doesn't cost anything.
What I wondered was that this capacitance, effectively across the coil,
might be responsible for increased losses in the coil. Removing the ground
wire would reduce this capacitance. You might need a few more turns on the
coil but it might run a bit cooler. If the coil was a good one anyway, it
might not make any difference.
But one thing is certain. Running a 'hot' wire up the side of a grounded
tower like this is perfectly valid, whether the coil is at the top or the
bottom. At first it looks like the tower will shield the wire and
short-circuit the RF, but current flowing down a tower is as good as any
other kind so long as the other end of the current flows into the sky.
73
Peter
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