Parallel-coupled microstrip filters with over-coupled stages for multispurious suppression

An inherent zero of a microstrip coupled stage near twice the design frequency (2f/sub 0/) is found tunable by varying its coupling length. This zero is used to suppress the unwanted response of parallel-coupled line filters at this frequency by using over-coupled end stages. The above idea is extended to design over-coupled middle stages for suppressing the spurious |S/sub 21/| peaks at 3f/sub 0/ and 4f/sub 0/ of the filter, so that the upper stopband can be greatly enhanced up to 5f/sub 0/. The passband preserves a response as good as the traditional design. Measured results have a good agreement with simulation data and show that the idea works very well.