> type array.. 1. The 4 square array needs identical currents in each
> vertical to work properly. 2. When driven with equal currents at the
> required 0,90, and 180 degree phasings, the feed point impedances at
> the verticals are NOT identical because of the mutual coupling between
> them. Not only that, when you switch directions, the impedance at each
> tower changes. 3. How do you put equal currents into 4 antennas which
> have different feed point impedances? The answer is to drive them
> through feed lines which are 1/4 wave long or an odd multiple
> (1/4,3/4,5/4 etc.) long. These length lines, when driven with a fixed
> voltage, will force a constant current into a load which varies within
> wide limits. 4. In other words, if you apply equal voltages to the
> transmitter end of the 4 cables to the verticals, they will have the
> same currents in them even though they do not have the same feed point
> impedance. These are sometimes called current forcing lines.
There is another advantage you missed. When the line is 1/4 wl or
a multiple of 1/4wl long, the phase shift in the line is constant
regardless of load impedance. So you not only, if the lines are fed
with equal voltage, get equal currents with an ODD multiple of 1/4
wl in the feeders, you also have equal phase delay in each line.
If the lines are NOT multiples of 1/4 wl, phase shift in the line will
vary with line termination so each element of the four square will
have both incorrect phase and incorrect current.
73, Tom W8JI
W8JI@contesting.com
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