Substrate integrated waveguide (SIW) is widely used in filter design due to its advantages of high Q value, high power capacity, small size and easy integration. In this paper, a symmetric folded substrate integrated waveguide (SFSIW) miniaturization method is proposed. Through the comparison of the miniaturization degree of the resonant cavity before and after folding, the feasibility of this method is verified, and the miniaturization theory of SIW filter is further improved. Using a symmetrically folded SIW resonator, a two-cavity filter and a three-cascaded cross-coupling filter were designed. This structure achieves better miniaturization of the filter. The high Q value of the SFSIW resonator makes the filter's insertion loss smaller, the transmission characteristics better, and the simulation and measurement results are consistent.
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