Zi-Jun Zheng

Four-port single-junction circulators are widely employed in in-band full-duplex systems owing to their advantages in size and manufacturing cost. This article presents a broadband synthesis method for miniaturized four-port single-junction microstrip circulators. By applying multiple rotation mode (MRM) analysis to the design of the four-port single-junction configuration, the azimuthal characteristics of the internal modes within the cross resonator are effectively reconstructed. For the central junction with arbitrarily complex conductor geometries, a simple and effective method for extracting input impedance is proposed, and the reactance compensation principle of the central junction is analyzed in combination with the equivalent circuit. A folded transmission circuit is constructed via a compact impedance-matching network to achieve the miniaturization of broadband circulators. The proposed miniaturized four-port single-junction circulator prototype is fabricated and measured, with the measurement results demonstrating an excellent agreement with the expected performance. Measurement results indicate that the proposed circulator achieves a relative bandwidth of over 70% for 1-dB insertion loss, with return loss better than 17 dB. The isolation of S₃₁ and S₄₁ is better than 16 and 17.8 dB, respectively. The cross-sectional circuit dimensions of the miniaturized circulator are only $11\times 11$ mm ( $0.39\lambda _{0} \times 0.39\lambda _{0}$ ).

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 0.264λ₀ × 0.264λ₀. 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.