High performance parallel coupled microstrip filters

A construction technique is described that allows microstrip parallel-coupled filters to have greater passband symmetry while largely removing the parasitic passband at twice the center frequency. The filter design extends the odd mode phase length by allowing the coupled lines to overlap with lines outside the resonator proper, i.e. the coupling gap of the resonator is extended out onto the 50- Omega lines. The even-mode length has been set by the distance between the 50- Omega lines. Thus, the reference plane for the even mode has been moved into the resonator. This configuration makes the odd-mode length longer than the even mode and thus compensates for the phase velocity difference between modes.<>