Synthesis of 3-D Polarization-Rotating Surface Based on Orthogonally Inserted Parallel-Coupled Slotlines

In this article, a second-order 3-D bandpass polarization-rotating surface (PRS) that can rotate vertically polarized incident waves by 90° and facilitate the output of horizontally polarized waves is presented. First, each PRS element is constructed using a pair of parallel-coupled slotline sections with open-short-ends, which are etched on vertically and horizontally inserted single-layer printed circuit boards (PCBs), respectively. Then, under vertically polarized incident waves, one of the slotline sections can convert the incident spatial waves into guided waves through its open end. After which, the guided waves are coupled to the other slotline section and the electric field direction will be rotated by 90° through the coupling path, thus leading to a 90° polarization rotation. Subsequently, a direct coupling matrix is used to synthesize and design the proposed PRS based on the microwave filter theory. Investigations reveal that a desirable bandpass filtering response can be obtained by properly designing the internal coupling of the parallel-coupled slotline sections and the external coupling between the incident spatial waves and the PRS. To validate the design concept, two examples with different fractional bandwidths at the same center frequency of 5 GHz and the same return loss of 20 dB were designed, fabricated, and measured. Consequently, a good agreement was attained among the results from coupling matrices, full-wave electromagnetic (EM) simulations, and measurements. All the results demonstrate that the proposed PRSs feature simple topologies, easy fabrications, low insertion losses (ILs), high polarization-rotating ratios, and stable filtering performances under large oblique incidences. In addition, our proposed method can be applied to design higher-order PRSs with sharpened out-of-band roll-off skirts.