Novel coupled microstrip wideband filters with spurious response suppression

Wideband coupled line filters with spurious response suppression (2/0) in the millimeter wave range are reported. The first filter exhibits a relative bandwidth of 62% with a return loss better than 20 dB and a suppression of the second spurious passband of more than 40 dB. The very high necessary couplings to achieve that large bandwidth in combination with the spurious suppression are realized by removing partially the ground metallization beneath the proposed coupling sections. The filters are directly compatible to microstrip circuits without the need for complicated transitions. They do not require techniques like substrate suspension or dielectric overlay to match the even- and odd-mode phase velocities, but are realizable on a classical substrate with ground. Further, simulation and measurement results of a second, high selective filter (11th order) with an even higher bandwidth of 78% are shown. Measurement and simulation results are fitting very well.