Design Of Ka-band Broadband Substrate Integrated Waveguide Circulator

Ferrite circulator as a very important microwave ferrite device is widely applied in microwave communication and microwave measurement now. Sorted by construction of transmission lines, the conventional circulators can be divided into microstrip circulator, stripline circulator and waveguide circulator. Microstrip circulator and stripline circulator are the planar structure devices, which can easily integrate with circuit, while the disadvantage is small power capacity. The waveguide circulator owns the great power capacity, however, the size is bulky which is difficult to achieve integration and realize system miniaturization. These two kinds of transmission lines have the irreconcilable problem between the small size and great power capacity.Substrate integrated waveguide(SIW) is a new kind planar structure transmission line, the circulator fabricated by this transmission has the small size, which not only can easily integrate with circuit but also has a big power capacity. Moreover, SIW circulator with the small processing cost, it can be fabricated by PCB technology or LTCC technology.Based on this merits of SIW, the design and experiment of Ka-band broadband SIW circulators were studied in this paper. Firstly, the structure character and propagate feature of SIW were discussed, the power capacity of SIW and kinds of matching methods of SIW-to-microstrip were also studied. Secondly, the design and measure of SIW circulator has been researched in detail. In this paper, kinds of SIW circulators were proposed, from the aspect of matching method, the circulator designed with microstrip line matching and coplanar waveguide matching; from the aspect of figure, Y-junction and T-junction circulators were both designed. In the figure of T-junction, the curved was applied at the connection between SIW and microstrip line, which realized the excellent impedance matching and dramatically broadened the working bandwidth, the bandwidth of return loss and isolation achieved 6 GHz at the-20 dB points, and the insertion loss less than 0.5 dB. This circulator has an advantage of small size, whose geometric dimensions was 8 mm×8 mm. The circulation was fabricated by PCB technology and measured by vector network analyzer, insertion loss of measure and simulation are the same, while the bandwidths of return loss and isolation narrower than simulation, the differences between experiment and simulation were analyzed. At last, the average power capacity of this T-junction circulator was obtained by calculation and simulation, and the value reached to 37.66 w. High temperature test and low temperature test were carried out by oven heating and refrigerating to circulation, the result indicated that the circulation can still steady operation between-60 °C and 120 °C.