In this letter, we present a diplexer implemented on a substrate integrated waveguide (SIW) with stepped impedance complementary S-shaped resonators (CSSRs). The variable frequency response of the stepped impedance concept adjoining SIW technology leads to improved device performance in terms of matching and isolation. Simulated and measured results show input matching, |S11|, better than -15 dB and output isolation, |S32|, below -30 dB for the frequency range 1-4 GHz. Furthermore, CSSRs offer a degree of freedom to design fundamental and higher order frequencies by selectively tuning the geometrical parameters. This simple yet effective approach eliminates the complexity to design diplexers based on complementary split ring resonator (CSRRs).
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