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Current Distribution in the A.L.C.

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ANTENTOP- 03- 2003, # 004

Current Distribution in the A.L.C.


In summary:
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  -

"...there is 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 antenna systems.  


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. 




Comments from the REC.RADIO.AMATEUR.ANTENNA News Group



Here are some comments relating to the subject of current distribution through loading coils as rehashed on 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

current minimum point on the other side.

That same principle holds true for standing wave antennas which are
antennas with (surprise!) standing waves. The current is NOT the same
at each end of the coil (unless a current maximum or current minimum
occurs in the middle of the coil). However, for traveling wave antennas,
the current at each end of a loading coil would be close to equal.

73, Cecil"

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