Material Fabrication Technology And The Application For Ka-band Self-biased Circulators

The ferrites used for the ferrite circulator substrates are classified as spinel,garnet and hexagonal ferrites.Substrates of traditional circulators are spinel and garnet ferrites,which require permanent magnets to provide a biased magnetic field,therefore,it is not benefit to miniaturize microwave integrated circuit systems.The hexagonal ferrite material can provide a self-bias field for the circulator substrate due to its high anisotropy,high coercivity and high remanence ratio（M_r/M_s）,so the external magnet of the circulator can be reduced or even completely removed,which is of great significance for the miniaturization and chip demonstrition of the circulator.Herein,BaM ferrite materials are investigated based on the application requirements of Ka-band self-biased circulators.Furthermore,the self-biased circulator was designed and fabricated.BaM ferrite materials were prepared by a traditional oxide ceramic method,and effects of three kinds of additives on the properties of M-type hexagonal ferrite were investigated.The results show that（1）The H₃BO₃ additive can promote liquid phase sintering,but excessive doping will lead to grain growth,coercivity（H_c）decrease,the moderate doping amount is 0.2 wt%;（2）The V₂O₅ additive can promote solid-phase reaction and increase the density of sintered sample.With the increase of V₂O₅ addition,the remanence（4πM_r）and saturation magnetization（4πM_s）of the samples first increase and then decrease,the remanence ratio remaines basically unchangless,and the coercivity declines.The moderate doping amount was 0.15 wt%;（3）The Ca CO₃ additive can improve the densification of M-type barium ferrite,and the remanence and coercivity of the sample first increase and then decrease.Finally,the sintered sample has excellent comprehensive performance when the doping amount of Ca CO₃ additive is 0.6 wt%.Based on the self-developed hexagonal barium ferrite material,the Ka band self biased circulator was designed and studied.Firstly,the influence of the thickness of the circulator substrate on the remanence and remanence ratio of the substrate was studied,respectively.The results show that when the substrate thickness increases from 0.4 mm to 0.8 mm,4πMr is increased by 23%,and the degree of magnetic moment orientation of the sample is also improved significantly,which may help reduce the magnetic loss of ferrite substrate.Secondly,the simulation model of Ka band composite ferrite substrate self-biased circulator is established and optimized using HFSS software,and the optimization model was obtained.The results show that the insertion loss S₂₁ of the circulator is less than 1 d B,the minimum insertion loss is low to be 0.85 d B,the isolation S₁₂ and the return loss S₁₁ are both greater than 20 d B,the maximum isolation can reach53 d B,the bandwidth is about 1.6 GHz when the,and the overall size of the device is 7mm×7 mm×0.4 mm.The circulator presents good circular performance.According to the simulation results,microstrip biased-circulators were fabricated by a semi-conductor process,and then the circulators were measured.The results show that the bandwidth is 0.7 GHz when isolation degree is more than 15 d B,and the isolation degree reaches its maximum of about 35d B at about 23.18 GHz.In the bandwidth range of isolation degree more than 15 d B,the insertion loss S₂₁≤3.7 d B,and the standing wave ratio SWR≤1.464.