The objective of the design is to make a "travelling wave" structure
with currents in the elements all contributing to the far field in the
forward direction. The contributions are designed to add up in phase
in the forward direction, and to cancel in the reverse direction.
The director elements are cut shorter than the driving element,
which is itself a little shorter than a half wavelength at the design
frequency.
The reflector is cut to be about a half wavelength and it is longer than the driving element, and spaced closer than are the directors. The directors present a capacitative impedance, acting like two lengths of open circuit transmission line each a little
radiated fields all add up in phase.
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