Hi all,
The discussion has brought out some interesting points on what goes on in a
practical loading coil. I would like to investigate (measure) some vector
relationships of current and voltage on a practical setup, but in the
meantime I am occupied with preparing for an LF DX weekend at Quartz Hill,
so it will be a while till I can find time to do quantitative testing. I'm
interested to find out if maximum current in to the "cold end" of the
loading coil corresponds with maximum radiation (far field, not near field)
from the system (system being not only the wire connected to the "hot end"
of the loading coil).
One can expect a big difference in relationships between tuned and untuned.
When tuned (the usual situation for LF transmitting) one can expect an
approximate linear voltage rise (similar volts per turn) and at the top "hot
end" it will be some Q times the voltage applied at the "cold end". If
there is similar stray capacitance per turn, then the current "lost" has an
approximate linear distribution up the coil, as "lost current" is
proportional to driving voltage, which increases along the coil.
There will also be a phase shift due to physical length of wire in the
inductor, with 360 degrees corresponding to a wavelength (even for applying
DC, there is a finite time from applying excitation at one end till current
comes out the other end, it is the "propagation delay" that forms the phase
shift for steady state sinewave excitation). A coil winding length of 100
metres of wire is about 4.5% of a wavelength, or some 16 degrees for 136
kHz. The stray capacitance will of course modify the ideal situation of the
16 degrees being proportional to length along the winding, especially around
the upper part of a resonant coil where the voltage is highest.
73, Bob ZL2CA
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