What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase?
Why With 1/4-wavelength spacing, the wave from one element takes 90 degrees of extra travel time to reach the other. Feeding the elements 90 degrees out of phase makes those two 90-degree shifts cancel in one direction (fields add, 2x voltage) and add to 180 degrees in the opposite direction (fields cancel, deep null). The result is a heart-shaped unidirectional pattern with a single null off the back, favoring the element that is fed with the lagging phase.
Watch out The figure-eight answers describe other common combinations: two verticals fed in phase give a broadside figure eight, and 1/2-wavelength spacing with 180-degree phasing gives an end-fire figure eight. A single vertical, not a pair, is omnidirectional.
Quarter-wave spacing plus quarter-wave (90 degree) phasing equals cardioid: one front lobe, one back null.