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

An inherent zero of a microstrip coupled stage near twice the design frequency (2f₀) 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 |S₂₁| peaks at 3f₀ and 4f₀ of the filter, so that the upper stopband can be greatly enhanced up to 5f₀. 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.