In a message dated 2/22/01 10:45:09 AM Eastern Standard Time,
[email protected] writes:
<< If a sufficiently large L is raised to the top of the vertical to self
resonate a practical top load, this behaves as an infinite load. Think
about a series tuned circuit, at just below resonance the impedance
drops quickly and this looks like a rapidly increasing capacitor, (until
actual resonance which then appears as a short circuit or small
resistance). Hence a fully resonant top load equals an infinite top
load ! And current in the vertical section stays constant. Since the
system is now resonant no loading coil is needed at the bottom. >>
Andy, I believe it might be good to reconsider this theory.
An electrically short vertical antenna, with a top hat and an inductor
_anywhere_ along the vertical length, is a series-resonant circuit whether
tuned predominantly at the base or higher up. Current into the coil
continues to equal current out of the coil wherever one locates it along the
vertical run. The need for higher inductance to achieve resonance when the
coil is near the top is simply because the capacitance to ground on the
high-potential end of the coil is less from "way up there."
The effect of this is that the C to ground from the low side of the coil is
less relevant simply because that's the low-Z end. The voltage on the wire
is also lower there. It has an incidental effect on current distribution as
Mike notes; not in the same way it does on taller antennae that begin with a
non-linear current distribution, but because lower voltage on the C of the
vertical run does not result in current being coupled as readily to the
surroundings.
I call this incidental, though, because the difference in current at the base
and the top of the antenna is fairly small if the radius of the top hat
already measures an appreciable fraction of the height of the antenna.
I base this on measurements under our US no-license regulations at 1750
meters. One of my last attempts at operation from this qth, surrounded by
trees, resulted in 200ma at the base and roughly 150 to 170ma entering the
top hat with base tuning only. (I say "roughly" because I could only get
myself to about half the height of the vertical run and had to sight through
binoculars. Also, my notes have long since gone missing, so I'm having to
depend a lot on my write-only memory. This was all with a vertical run of a
little over 8m and a top radius a little less than 7m.)
Moving a larger value inductor to the top and tuning with a small variable
inductor at the base resulted in a top-load current very slightly less than
150ma and a base current of 150ma. As with Mike, mine was a lossier coil at
the top.
Now, since h(effective) of a short vertical under ideal conditions is
h(actual) times the sum of base and top currents divided by two, one would
expect my signal strength to have been very little different. But in
practice, it was a good S-unit better at my reference test point. I have to
attribute this to a reduction in absorption of reactive power by the
surroundings, as Rik has said. Reduce the amount of antenna capacitance
subject to high voltage, and the loss goes down.
Sadly, it still wasn't enough of an improvement to do nearly as well as any
of the successful US LowFERs, but my landlord is at last removing some
extraneous trees from the property. Perhaps next winter I'll try again. I
don't expect to see the same results as if I could pump a couple hundred
times the power into the same antenna, but I'll be grateful for all I can
legally manage until we finally obtain a ham band.
73,
John
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