Above-resonance dielectrically loaded integrated S-band circulators

Microstrip circulators using all-ferrite substrates (ε = 15) become excessively large at lower microwave frequencies (≤ 3 GHz). Typically, at 1.8 GHz, the circulator will have a diameter of 2½ inches. Therefore, they are incompatible with other microwave integrated circuits. This paper describes the development of a dielectrically loaded circulator operating in the above-resonant mode that reduces the size of an 1.8 GHz three-port circulator to 1 × 1 × 3/4 inch without any degradation in performance characteristics. Substrate (NPO Material) with a dielectric constant of 31 was used, with a ferrite puck imbedded in the center of the substrate forming a composite substrate. Both sides of the composite substrate are metallized with chrome-seeded copper. The etched microstrip circuit consists of a resonant disk in the center over the ferrite puck and three λ/4 impedance transformers that transform the impedances of the resonator to 50 ohms at the output of the three ports. Circulation results when two permanent magnets, properly spaced and mechanically supported by rexolite spacers, are placed on both sides of the composite substrate. The electrical performance of the circulator is summarized as follows: fo= 1.8 GHz Isolation 50 dB (max) 30 dB over 3.5% BW 20 dB over 15% BW Insertion Loss 0.25 dB min. ≤ 0.4 dB over 14% BW VSWR ≤ 1.3:1 over 29% BW. Detailed description of the design procedure, fabrication process, tuning and measurement techniques will be presented.