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Old October 24th 04, 07:09 PM
Cecil Moore
 
Posts: n/a
Default An easy experiment with a coil

The results of the measurement of current at the bottom and top of the coil
in an off-center-fed dipole configuration should leave no room for doubt.

------1/4WL------FP------1/4WL------

Start out with a 1/2WL dipole. Measure the resonant fundamental frequency.

Then, to one end, add a coil and another 1/4WL of wire

------1/4WL------FP------1/4WL------//////------1/4WL------

Adjust the coil inductance so the antenna is resonant on three times
the fundamental frequency of the original dipole.

Measure the current at the left(bottom) of the coil and measure the
current at the right(top) of the coil.

The current at the left of the coil will be nearly zero. The current
at the right of the coil will be approximately the same as the feedpoint
current, somewhere around 1-2 amps for 100w input. This can probably be
demonstrated using the helix feature of EZNEC.
--
73, Cecil http://www.qsl.net/w5dxp
"The current and voltage distributions on open-ended wire antennas are
similar to the standing wave patterns on open-ended transmission lines ...
Standing wave antennas, such as the dipole, can be analyzed as traveling
wave antennas with waves propagating in opposite directions (forward and
backward) and represented by traveling wave currents If and Ib ..."
_Antenna_Theory_, Balanis, Second Edition, Chapter 10, page 488 & 489


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