Hi Dick,
The feedlines to each vertical need to be equal lengths. They do not have
to be 3/4 wavelength, however. You can save on coax by using two lines that
are equal length and long enough to connect to each other at the center of
the array. You do not have to account for velocity factor so each cable can
be just over 1/4 wavelength. You do have to consider velocity factor for the
1/2 wavelength line that is switched in/out to change directions. The
verticals don't have to be exactly 1/2 wave apart either to achieve 90
degree switching.
More info can be found in The ARRL Antenna Compendium volume 2, page 10.
73, Bob, W5AH
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