Extremely Miniaturized Bandpass Filters Based on Asymmetric Coupled Lines With Equal Reactance

This paper demonstrates extremely miniaturized bandpass filters based on asymmetric coupled lines and equal reactance. By reactively loading asymmetric coupled lines while the first and the last inverters are eliminated, the proposed method overcomes the limitation of previous methods to achieve extreme miniaturization. Furthermore, the utilization of asymmetric coupled lines allows the method to miniaturize a filter with equal reactance, which provides a great advantage of low-cost fabrication. A complete set of design equations for an arbitrary number of filter order is provided for both shunt-capacitive and series-inductive loading methods. Experimental results for various miniaturized filters at 0.9 GHz are provided. An unprecedented miniaturization rate of more than 91% is demonstrated for a third-order parallel coupled-line filter.