Compact dual-band microstrip absorptive (reflectionless) bandstop filter design

Abstract

In this study, designs of dual -band microstrip absorptive (reflectionless) bandstop filters are represented by simulation and experimental studies. Absorptive bandstop filter responses are obtained by adding resistors to the third order bandstop filter designed by loading open circuit meandered stubs to the transmission line connected to the input and output ports. The designed absorptive bandstop filters have low reflection loss at the input port. Reflectionless bandstop filter design can be realized by adding a resistor to the open circuit stub near the input port. In this type of filter, depending on the position of the resistor, bandstop filter with only first or only second absorptive stopband can be designed. The dual -band bandstop filter response having two absorptive stopbands can be obtained by adding two resistors to the stub located near the input port and to the open circuit stub located in the middle. Three dual -band bandstop filters having first absorptive stopband, second absorptive stopband and two absorptive stopbands were fabricated and measured. The center frequencies of the stopbands were measured at 1.18 GHz and 3.5 GHz. The measured results have been obtained in a very good agreement with the simulations.