Wideband suppression of spurious response for microstrip filters based on coupled lines of incommensurate lengths

This work demonstrates design methods that can achieve remarkable enhancement in the stopband response of microstrip coupled-line filters. The frequencies of resonance and/or the transmission zeros of a coupled-line section can be controlled by its length. Therefore, by constructing a filter with coupled lines of incommensurate lengths such that these frequencies do not overlap, substantial improvement in the stopband response can be obtained. Besides its compact structure and simple design procedure, an unprecedented performance is demonstrated experimentally in which the transmission level is maintained below-20 dB up to as high as 11.9 f0 for a third-order filter, where f0 is the center frequency.