Some of the recent postings on TT lead me to believe that some of the
facts about the 4 square are not well understood. Here is my
understanding and I believe it agrees with info in ON4UN's Low Band
DXing handbook and the writings of W7EL and K2BT who pioneered this 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.
5. How can you split your transmitter output power into 4 equal voltages
with 0, 90, and 180 degree phasings to apply to the 4 fed lines? There
are several different methods used.
W7EL and K2BT used LC networks to do the job. These are very frequency
sensitive but can do a precise job and their designs provide for
impedance matching to the transmitter feed line.
The most widely used system is the quadrature coupler for the 90
degree source and a wide band transformer for the 180 degree source or a
1/2 wave coax line. The coupler works very well if it has 50 ohm loads
on all of its outputs. Unfortunately, the current forcing lines do not
usually provide 50 ohm loads on the couplers ports, particularly when
you move off the design frequency but it works reasonably well for
getting the 90 degree drive voltage.
I first learned about the 4 square array when I read a copy of a rough
draft for a book written by Roy Lewallen, W7EL on Understanding Phased
Arrays. That was in 1982. He thanked Forrest Gehrke,K2BT, Walt Maxwell,
W2DU, Dana Atchley, W1CF, and Wes Hayward, W7ZOI for reviewing the
manuscript. This is a virtual Hall Of Fame of antenna experts. I built
my first 4 square after reading that and used it to work 39 zones from
Dallas and the last one from here in Spokane. Nearly all of the
information is available in ON4UN's Low Band DXing HB and this book is
the best buy in ham radio, in my opinion. 73, Dan, N5AR
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