Comparison of electromagnetic band gap and split-ring resonator microstrip lines as stop band structures

In this paper, microstrip lines magnetically coupled to split-ring resonators (SRRs) are compared to electromagnetic bandgap (EBG) microstrip lines in terms of their stop-band performance and dimensions. In both types of transmission lines, signal propagation is inhibited in a certain frequency band. For EBG microstrip lines, the central frequency of such a forbidden band is determined by the period of the structure, whereas in SRR-based microstrip lines the position of the frequency gap depends on the quasi-static resonant frequency of the rings. The main relevant contribution of this paper is to provide a tuning procedure to control the gap width in SRR microstrip lines, and to show that by using SRRs, device dimensions are much smaller than those required by EBGs in order to obtain similar stop-band performance. This has been demonstrated by full-wave electromagnetic simulations and experimentally verified from the characterization of two fabricated microstrip lines: one with rectangular SRRs etched on the upper substrate side, and the other with a periodic perturbation of strip width. For similar rejection and 1-GHz gap width centered at 4.5 GHZ, it has been found that the SRR microstrip line is five times shorter. In addition, no ripple is appreciable in the allowed band for the SRR-based structure, whereas due to dispersion, certain mismatch is expected in the EBG prototype. Due to the high-frequency selectivity, controllable gap width, and small dimensions, it is believed that SRR coupled to planar transmission lines can have an actual impact on the design of stop-band filters compatible with planar technology, and can be an alternative to present solutions based on distributed approaches or EBG. (C) 2005 Wiley Periodicals, Inc.