Generalized Miniaturization Method for Coupled-Line Bandpass Filters by Reactive Loading

This paper presents a generalized miniaturization method for coupled-line bandpass filters by reactive loading, including the series-inductive loading method that is proposed in this work. It is shown that bandwidth reduction seen in the previous miniaturization method of shunt-capacitive loading is a special case. In fact, one can choose a coupled-line filter to be miniaturized with its bandwidth reduced, expanded, or maintained after miniaturization. The ratio of the bandwidths before and after miniaturization plays an important role in determining the even-/odd-mode impedance, as well as the reactance levels after miniaturization. Therefore, the freedom in choosing the bandwidth ratio provides a great flexibility in miniaturizing coupled-line filters since one can choose an appropriate bandwidth ratio to maintain the impedance and reactance at practical levels after miniaturization. The proposed generalized miniaturization method enables filter designs focused on size reduction, improved stopband response, low-cost fabrication, or a combination of these, which are verified by experimental results.