The current in a typical loading coil in the shortened antennas
drops across the coil roughly corresponding to the segment of
the radiator it replaces.
... and that's the
way it IS, hardly W8JI's -
an immeasurable reduction in current in the coil."
I hope this will help
to better understand the loaded antennas, to incorporate the effect
into the modeling software and to develop more efficient shortened
Below is the
result of plotting current in the G5RV antenna using inductors
in the form of loading stubs as done by W5DXP in Eznec. It can
be seen that the current entering the stub is greater than current
exiting the stub. When simple inductance in Eznec is inserted
in place of the stubs, the current erroneously is shown as the
same at the both ends of the inductor.
Here are some comments
relating to the subject of current distribution through loading
coils as rehashed on rec.radio.amateur.antennas news group:
Posting by Cecil, W5DXP shedding some light on the
"theoretical" (Kirchoff and Ohm laws) arguments and
their propriety to the case:
Assume a transmission
line with an SWR of 10:1. Put a series inductor in series with
the transmission line. Assuming negligible losses, the forward
current is the same at each end of the coil and the reflected
current is the same at each end of the coil. The question is:
Do the superposed currents, Ifwd+Iref, remain constant? Of course
not, because of phase shifts. With a large enough coil, one could
cause a current maximum point on one side of the coil and a
minimum point on the other side.
That same principle holds true for standing wave antennas
antennas with (surprise!) standing waves. The current is NOT the
at each end of the coil (unless a current maximum or current minimum
occurs in the middle of the coil). However, for traveling wave
the current at each end of a loading coil would be close to equal.
73, Cecil http://www.qsl.net/w5dxp"