Compact CPW dual-band bandpass filters based on semi-lumped elements and metamaterial concepts

In this work we present the design of a three-pole dual-band bandpass filter implemented in coplanar waveguide technology (CPW) by using semi-lumped elements. The structure is described by a circuit model which is roughly the canonical circuit model of an order-4 composite right/left handed (CRLH) transmission line, which is identical to the circuit model of a dual-band bandpass filter that results from frequency transformation from the lowpass filter prototype. The reported filter is a dual-band bandpass filter with central frequencies at f_c1 = 1.02 GHz and f_c2 = 1.68 GHz, and 20% fractional bandwidth for both bands. Since the filter has been implemented by synthesizing the circuit model, rather than by using distributed elements, the resulting filter dimensions are very small, that is, 20.9 mm × 17.5 mm. Measured in-band insertion and return losses are very satisfactory, and the first spurious band appears at 2.3 times the central frequency of the upper band.