Ultrawideband Compact Microstrip Circulator Loading SSPPs Based on Multiple Azimuthal-Rotating Modes

An ultrawideband (UWB) microstrip circulator loading spoof surface plasmon polaritons (SSPPs) based on multiple azimuthal-rotating modes is presented. To address the limitations of existing ferrite resonator theories in UWB circulators, a design methodology for enhancing the frequency bandwidth is realized through characteristic analysis and mapping mechanism discussions of multiple rotation modes (MRMs). By employing various resonator configurations, the frequency responses of multiple forward azimuthal-rotating modes are effectively regulated. The construction of composite ferrites (CFs) is leveraged to suppress radial radiation, significantly improving the isolation performance. The stable and compact topological circuit is achieved via utilizing a metal conductor loaded with multiple SSPP stubs. The experimental prototype demonstrates an excellent agreement with simulated and measured results. The proposed compact circulator boasts a cross-sectional dimension of merely <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.264{\lambda }_{0} \times 0.264{\lambda }_{0}$ </tex-math></inline-formula>. Within frequency band of 6.83–18 GHz, both return loss and isolation of the circulator are better than 15 dB, and the insertion loss of the circulator remains below 1.2 dB. The proposed circulator possesses potential for future applications in broadband communication and radar components.