Harmonic suppression for planar ultrawideband bandpass filters employing broadside-coupled microstrip patches

The design of a planar bandpass filter with ultra-wideband behavior and suppressed second harmonic is presented. The proposed filter utilizes broadside coupling between elliptical microstrip patches at the top and bottom layers of the filter´s structure via an elliptical slot located on the ground plane on the mid layer. Short stubs are introduced in the mid layer to suppress the higher harmonics. The design procedure for a two-section broadside-coupled filter with suppressed harmonics is explained. The results of full-wave electromagnetic simulations show that the designed device has a 3 dB insertion loss bandwidth from 3.1 to 10.6 GHz, a sharp cutoff stopband, and more than 20 dB insertion loss in the stopband. The designed filter has a compact size of 1.5 cm × 2.5 cm.