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It is shown that open complementary split ring resonators (OCSRRs) are useful particles for the design of lowpass filters with very narrow transition bands in microstrip technology. Owing to the small electrical size of OCSRRs, the filters are also very small. These filters are implemented by cascading several OCSRR stages in a microstrip line. The OCSRR stages can be modelled as parallel resonators in series configuration, thus providing a transmission zero at their resonance frequency. By designing the OCSRRs with small inductance and high capacitance, very sharp cutoffs are achieved. To illustrate the possibilities of the approach, a prototype device example is provided. It consists on a five-stage periodic microstrip lowpass filter with 3 dB insertion losses at 1.33 GHz and 40 dB rejection at 1.45 GHz. To improve out-of-band rejection, four additional OCSRR stages tuned at the spurious frequency band have been cascaded. Filter dimensions are 39  7.4 mm (i.e. 0.46l 0.087l, where l is the guided wavelength at the cutoff frequency). These filters are of interest in applications where small size and severe cutoff requirements are required. Introduction: Open complementary split rings resonators (OCSRRs) have been reported recently (e.g. [1]) as useful particles for the synthesis of planar metamaterials and for the miniaturisation of microwave components in coplanar waveguide (CPW) technology. The OCSRR is the complementary counterpart of the open split ring resonator (OSRR) [2]. OCSRRs and OSRRs are open resonators, i.e. both particles can be excited by means of a voltage or current source connected to their terminals. The OCSRR (see Fig. 1) can be modelled as an open parallel resonant tank, where the inductance L o is the inductance of the strip connecting the internal and external metallic regions of the particle, and the capacitance C c is equivalent to the capacitance of a disk of radius r o 2 c/2 surrounded by a metallic plane at a distance c of its edge [1]. Such element values can be roughly inferred from the geometry of the particle (following the model reported in [3] for CSRRs).

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