Absorptive planar bandstop filters

A simple and very effective way to realize narrowband and wideband absorptive band stop filters (ABSFs) is presented. Analytical design equations are established to allow the direct synthesis of ABSFs according to the desired specifications. Good stopband rejection, low input reflection coefficient, and high level of input power dissipation within the stopband are achieved as demonstrated by the design examples. In addition, wide passband with low passband insertion loss and good impedance matching is also obtained. Both the narrowband and wideband ABSFs can be easily extended to higher-order ones for better selectivity. They can be used in applications where the absorption of unwanted signal/interference is required.