“…In the effort to reduce the presence of spurious modes-which can often become too close to the passband-studies on the improvement of filter's out-of-band rejection by works such as [5], [6], [7], [8], and [9] provide effective solutions to suppress spurious modes. However, a method to achieve a well-balanced tradeoff, while effectively miniaturizing a given structure, is the use of metasurfaces inside the resonators [10], [11]. This method has been shown to achieve a miniaturization factor of up to 60% [10], [11].…”