Four-port circulators are pivotal components in modern multi-channel radio-frequency (RF) front-ends because they enable compact routing and protection among multiple transceiver chains. However, practical four-port ferrite circulators often suffer from a pronounced trade-off among operating bandwidth, isolation, and miniaturization, and their design is further complicated by the inevitable nonuniformity of the bias field in realistic permanent-magnet packages. To address these challenges, this paper proposes a broadband and high-isolation four-port microstrip ferrite circulator together with a design methodology that tightly couples magnetostatic physics, junction modal behavior, and circuit-level matching.
The proposed methodology starts from a magnetostatic field–coupled resonant mode analysis, where the computed static bias-field distribution is directly embedded into the central junction’s mode analysis. In contrast to conventional approaches that assume a spatially uniform bias field, the developed framework provides a more rigorous description of magnetostatic–mode coupling and its impact on the frequency separation between the desired circulating modes and competing spurious modes. As a result, the biasing arrangement and the junction geometry can be co-optimized with significantly reduced trial-and-error iterations, yielding a clearer physical guideline for simultaneously enlarging the usable band and enhancing multi-port isolation.
Guided by the above analysis, a novel slow-wave central junction is further developed. Starting from a traditional disk-shaped configuration, the junction is evolved by integrating curved-edge conductors and slow-wave stubs, which strengthens electromagnetic field confinement in the junction region and effectively reduces the phase velocity of the involved modes. This slow-wave characteristic is crucial because it increases the modal separation margin and suppresses undesired reciprocal leakage paths, thereby improving isolation while supporting broadband operation. In addition, a compact broadband transmission matching network is carefully designed to transform the modified junction impedance to standard 50 Ω microstrip ports, ensuring stable matching across the target band without sacrificing the nonreciprocal transmission behavior.
A prototype is fabricated and experimentally characterized to validate the proposed design and methodology. The measured results demonstrate that the circulator operates from 8 to 12 GHz, achieving a 40% relative bandwidth with insertion loss below 1.6 dB and return loss better than 15 dB. Importantly, high isolation is obtained, with S31 better than 20 dB and S41 better than 25 dB over the operating band. Meanwhile, the physical footprint remains highly compact, only 12 mm × 12 mm (0.4λ₀ × 0.4λ₀). These results confirm that the proposed magnetostatic-aware mode-analysis-driven design together with slow-wave junction engineering provides a practical and effective route to broadband, miniaturized, and high-isolation four-port microstrip ferrite circulators for next-generation RF and microwave systems.