A Sequential Rotation Filtering Network for High Circular Polarization Purity Antenna Array

This letter introduces a sequential rotation filtering network (SRFN), enabling a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16\times 16$ </tex-math></inline-formula> antenna array to achieve high circular polarization (CP) purity and enhanced frequency selectivity. The proposed SRFN integrates filtering, power dividing, and phase shifting functions, providing phase states of 0°, 90°, 180°, and 270° at the four output ports with 7th-order filtering response. Moreover, four sets of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8\times 8$ </tex-math></inline-formula> subarrays are arranged orthogonally and connected to the outputs of the SRFN. Finally, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16\times 16$ </tex-math></inline-formula> antenna array is monolithically fabricated using the 3-D printing technology. Measurement results demonstrate an operating bandwidth (return loss >10 dB) of 27.7–29.6 GHz, with the radiation gain ranging from 29.5 to 30.5 dBic. Due to the proposed SRFN, the in-band axial ratio (AR) remains lower than 0.5 dB, while the out-of-band realized gain suppression exceeds 30 dB/GHz. The proposed SRFN methodology serves as an integrated feeding network for antenna arrays requiring high CP purity and frequency selectivity.