Microstrip ring bandpass filters with new interdigital side-coupling structure

In this paper, dual-mode microstrip ring bandpass filters with new interdigital side-coupling structure are presented. The interdigital side-coupling scheme has much lower insertion and return losses than any other side-coupling structure. Dual-mode microstrip ring bandpass filters with one or two local resonant split (LRS) circuits centered at 2.2 and 2.05 GHz show 3-dB bandwidths of only 70 and 75 MHz, the insertion losses of -2.75 and -2.12 dB, and return losses of -19.48 and -19.65 dB, respectively.