Recent advances in microstrip filters for communications and other applications

Modern microwave communication systems require, especially in satellite and mobile communications, high-performance narrow-band bandpass filters having low insertion loss and high selectivity together with linear phase or flat group delay in the passband. Usually, these criteria are fulfilled using waveguide cavity or dielectric resonator loaded cavity filters because of their low loss. However, in order to reduce size, weight and cost, there has been a growing interest in planar structures. The disadvantage of high conductor loss of the planar filters using conventional conducting thin films can be overcome by replacing them with high-temperature superconducting (HTS) thin films. These can have a very low conductor loss. An alternative is by combining with active MMIC devices to compensate the loss. Under those driving forces many novel types of microstrip filters have recently been developed. This paper is intended to address this development. Different types of newly developed microstrip filters, particularly those developed at the University of Birmingham, such as slow-wave microstrip filters, cross-coupled microstrip filters and miniature dual-mode microstrip filters are presented.