I. Introduction
Microwave lenses were first proposed in 1960's to focus the electromagnetic energy in the near-field (Fresnel) region [1], and their properties were studied in detail in the 1980's [2]–[4]. The first designs were based on dielectric lenses and metallic reflectors, which are externally illuminated by a space plane-wave which is focused at the desired focal point due to the three-dimensional shape of the focusing body (normally parabolic or hyperbolic geometries) by virtue of Geometric Optics [5]. Phased arrays of horns [3], [6], patches [7]–[10], and printed dipole antennas [11], have also been proposed in order to focus the electromagnetic fields in the near-field zone. In these cases, the array feeding network is responsible to excite each element of the array with the requested quadratic-phase and amplitude illumination to synthesize the desired focusing pattern. Similarly, an externally-fed printed reflectarray has been recently proposed to focus in the near-field [12]. In any case, printed-circuit array configurations [7]–[12] offer lightweight, compact, low-profile, cheap solutions when compared to bulky three-dimensional shaped bodies used for dielectric and metallic microwave lenses.