The travelling wave structure supports a wave in the forward direction,
and the currents in the directors are all about the same, although with a progressive phase delay.
Gain is approximately proportional to the number of elements for a moderate number of elements
The reflector has an induced current that contributes a wave in the backwards direction
which cancels the backward wave from the driven element _ little power is radiated backwards.
The net power radiated by the reflector current appears as a contribution in the forward direction.
The length and spacing of the reflector have a strong influence on the residual
backward radiation from the Yagi-Uda. Typically the reflector will be spaced by 1/8 to 1/4
wavelength, and the directors by about 1/3 wavelength each.
The array factor gain of a Yagi-Uda is therefore limited to the number of elements, and the
element gain is that of a half wavelength dipole, 1.66.
Therefore the maximum gain over isotropic from the Yagi-Uda antenna is 1.66 times
the number of elements, (the number of elements times the gain of a single half-wave dipole).
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