Design and development of high-Q microwave filters-past, present and future

Dielectric resonator filters are in high demand today in very narrow-band and low-loss applications for mobile telephone systems. They are individually tuned to specifications and due to their frequency-sensitivity are hardly amenable to mass fabrication. Therefore they are expensive. Future filter requirements will be dictated by a shift in operating frequency of communications systems towards shorter wavelengths, mainly the millimeter-wave regime to avoid the frequency congestion at lower frequencies. While at lower frequencies the lack of bandwidth requires narrow-band applications and thus extremely high-Q filter structures, at millimeter-wavelengths channels are not only wideband but so far can also be spaced farther apart. System specifications can therefore be satisfied with filters having lower Q-factor. The key question in those applications is, however, whether or not the filter structure is mass produceable. In this paper a brief review of the last 20 years of filter development is presented. Present day design procedures are discussed and an outlook with regard to future filter developments is given