A simple method for improving the stopband response for unequal-length microstrip coupled-line filters with stubs

This work demonstrates a simple method for improving the stopband response of microstrip coupled-line bandpass filters in an extremely wide bandwidth. Substantial enhancement in the stopband response is achieved by constituting a filter with reduced-length coupled lines of unequal length, which are loaded with open stubs. The lengths of these stubs are optimized to compensate for the reduction the reduction in the lengths of the coupled lines, and at the same time, to provide transmission zeros for further suppression of spurious responses. A rigorous mathematical analysis demonstrates a systematic method is verified experimentally with the experimental results obtained with third-order Chebyshevy-type bandpass filters that maintain the transmission level below −30 dB up to 17.8f₀, where f₀ is the center frequency. Even further enhancement can be achieved simply by increasing the filter order.