Abstract
A Cooke triplet antenna used for near-field focusing is designed by a quasi-optical method. To design the Cooke triplet, we propose the quasi-optical method to reduce the aberration spot radius in the design step. The ray tracing method, calculating only the coordinates in the image plane rather than the field intensity, is used without considering the diffraction effect. The diffraction effect is taken into consideration only in the verification step. We get a conclusion that the essence of the aberration optimization is to optimize the phase distribution in the diffraction integral formula. In this case, only optimizing the aberrations without considering the diffraction effect is acceptable in the design step. While in the verification step, the diffraction is used to calculate the resolution of the Cooke triplet antenna. The field of view of the Cooke triplet is −18°∼18° in simulation and −14°∼14° in experiment. The feed sources can be positioned vertically to the optical axis in the image plane without the focal surface. The resolution of the off-axis beam is only 17% lower than the resolution of the central beam whose resolution is less than 0.5 cm at the distance 0.37 m. The experiment agrees well with the simulation.
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