A multilevel Frequency Selective Surface (FSS) formed by a non-uniform distribution of circular holes on a metallic sheet is presented. The FSS is aimed at working as a planar lens to increase the gain of the feeding aperture. The unit cell of the FSS acts as a high-pass filter that provides the necessary phase shift to focus the beam on the far-field region. The phase shift is synthesized by suitably configuring the size and position of each unit cell in the FSS. The planar lens is placed next to the feeding aperture to confer the structure a low-profile shape. A prototype has been designed, fabricated and measured. Results show an increment of 7.32 dB in the gain compared to the feeding aperture within a 7.33% bandwidth around 20.45 GHz, and a side-lobe level below −12 dB, with an aperture efficiency of 65% and a radiation efficiency above 90%.

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