In this paper, the inter-digital capacitance loaded loop resonators (IDCLLRs) are proposed to design microstrip band-reject filters. The analyzed structures are based on the coupling of IDCLLRs to a conventional 50 Ω microstrip transmission line. We have firstly studied the frequency response of one-stage IDCLLR-loaded microstrip transmission line. The main fratures of the IDCLLRs are small dimensions (much smaller than the wavelength at resonance) and more structural parameters (provide flexibility in design); Then a 6-stage IDCLLR-based microstip band-reject filter was designed and fabricated, it performs relative rejection bandwidth of 8.1% and rejection level of 23 dB. This stop-band is resulted from presence of the artificial medium with negative effective permeability (μeff<0). In addition, it is easy to control the rejection bandwidth by only employing different length of inter-digital fingers (for multiple tuning). A 9-stage band-reject filter with multiple tuned IDCLLRs has been fabricated and tested, its relative rejection bandwidth is extended to 11.5%. Simulated and measured results are presented.
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