Correcting the secondary focus of Fresnel zone plate antennas

The large aperture Fresnel zone plate antenna has been extensively investigated in recent years for application at frequencies from the microwave range to the terahertz region. These zone plate antennas have a focal length (F) and diameter (D) that are comparable (F/D = 0.3 to 2.5). The results of these investigations show that the phase-correcting zone plate gives performance comparable to a true lens. There is one limitation, however, in that most cases a secondary focus occurs on-axis at one-third the focal distance from the zone plate. The intensity is 10 dB or more down from the main focus. A method has now been developed to essentially eliminate this secondary focus by properly adjusting the amount and location of the phase-correcting grooves or rings. A revised positioning of the grooves and degree of correction can reduce the secondary focus, but maintain the integrity of the main focus. Specific examples have been analyzed and will be presented at the conference.