Rapid development of wireless communications present extraordinary demand for narrow-band RF/microwave bandpass filters with high selectivity and low insertion loss. One filter with these attractive characteristics is that of quasi-elliptic function response filters with a pair of attenuation poles at finite frequencies. The capability of placing attenuation poles near the cutoff

frequencies of the pass band improves the selectivity using fewer resonators. This type of filter is usually realized using waveguide cavities or dielectric-resonator-loaded cavities. However, with the advent of high-temperature superconducting (HTS) and micromachined circuit technologies, there is an increasing interest in microstrip filter structures. Two technical approaches are normally used to realize this type of filter. The first is to extract poles from both ends of a filter prototype by using shunt resonators. The size of the microstrip filter resulting from this approach may, however, be large. The second approach is to introduce a cross coupling between a pair of nonadjacent resonators. The filter employing the cross coupling generally results in a compact topology. This is obviously more attractive for those systems where size is important. It has been known that the cross coupling is more difficult to be arranged and controlled in a microstrip filter owing to its semiopen structure. It is obvious, however, that a higher degree is required for a

more selective filter. In this paper I present in detail the design of highly selective microstrip bandpass filters that consist of microstrip open-loop resonators with a cross coupling that exhibit a single pair of attenuation poles at finite frequencies. A practical design technique for this class of filters, which is also different from that reported, is introduced, including tables and formulas for accurate and fast filter synthesis. The design approach enables one to use advanced full-wave EM simulators to complete the filter design, namely, to determine the physical dimensions of the filter. Two design examples of six- and eight-pole filters are demonstrated together with theoretical and experimental results.