The length of coax for the phasing lines depend on the (fractional)
wavelength spacing of the antennas and the pattern you desire (figure 8,
broadside, carotid, endfire, etc.) so correct answers to your question
could be 1/8 wavelength, quarter wave, half, full, or even 1+ wavelength.
Also remember that the velocity factor of the coax shortens the physical
length of the phasing lines. The velocity factor varies from one type of
coax to another. The ARRL Antenna Book has some information on antenna
patterns and spacing and phasing lines.
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