Antenna pattern characteristics of phase-correcting Fresnel zone plates

The far-field antenna pattern of phase-correcting Fresnel zone plates is analyzed, based primarily on work done in the optical regime. Measured characteristics of zone-plates in the microwave-millimeter-wave region are also presented. Since the zone plate accomplishes focusing through diffraction and interference, rather than refraction, the overall efficiency for collecting the radiation flux that passes through the aperture is an important consideration. This efficiency has been determined analytically to be approximately 40.5% for a phase-reversal plate and 81% for a quarter-period plate. Thus, the intensity at the focus, relative to that produced by a hyperboloidal lens, is 3.9 dB down or 0.9 dB down respectively, for the phase-reversal or quarter-period cases. Early investigations in the microwave-to-millimeter-wave range developed formulas for calculating the radii and depth of annular cuts, and measured the axial intensity variation near the focus and the antenna patterns at 10, 140, and 210 GHz. Measurements have been made of off-axis image intensity and image position errors at 140 GHz, and the results are excellent.<>