In a message dated 2/23/01 3:30:21 AM Eastern Standard Time,
[email protected] writes:
<< I believe that the 'basic rule' with an elevated loading coil is that the
current remains constant from the bottom end (feeding point) up to the coil
and will drop linear from the coil toward the end of the antenna.
So if you have the coil at the top of the vertical section you will get a
constant current over this section, the same that you would get with a
infinite topload (that is Andy's theory in other words ..) >>
That's a true enough rule of thumb, as far as it goes, but I'm doubtful of
the "infinite topload" part. We're talking about electrically very short
antennae with top loading by means of inverted-L or T configurations, or
other tophat capacitance sections. The taper of the current distribution
need not be all that significant to begin with, depending on the size of the
top load section. The key point is, with the systems many experimenters
appear to be using, it's not enough by itself to account for the dramatic
apparent difference in performance when one elevates the loading coil.
(In addition, even though you can resonate a system that has very small top
capacitance and achieve a more constant current, the L/C ratio precludes the
current from being the same as if it were a physically large top load. No
free lunch, in other words.)
You were perceptive in catching the current discrepancy. I neglected to
caution that my current readings were relative, not calibrated. The
"instrument" (it's a bit grandiose to call it that) was a toroid core
transformer with a rectifier and DC microammeter that had at originally been
calibrated at 50kHz. I believe it had to be overstating the current
significantly, for I am certainly not one of the fortunate LowFERs who lives
atop open patches of 30milliSiemens/m soil either. My best grounding efforts
here have barely approached 90 ohms, so far as I can measure with a
semi-accurate homebuilt impedance bridge. With 12v and 83ma into the final,
100ma or less would be the maximum likely current if the meter had been
correct for absolute values. It did track well with changes to the collector
voltage of the final, however, so I believe it was reasonably linear over the
part of the scale where it was being used.
Still, I'd love to see a reading in the multiple ampere range and to know it
was real!
73,
John
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