Ultra-wideband bandstop filter with multiple transmission zeros using in-line coupled lines for 4G/5G mobile applications

This paper presents an ultra-wideband bandstop filter (BSF) with extended upper passband. The filter is mainly based on a single section coupled line with amended open circuited stub (OCS). Two parallel connected unit cells of in-line structure are introduced at input and output ports to enhance the filter selectivity and providing structure symmetry. Even and odd mode analysis is presented based on a symmetric line. For demonstration, a prototype is designed having a 3-dB frequency range from 6.5 to 23 GHz with 92.62% stopband fractional bandwidth. The filter is simulated, fabricated, and measured. Good agreements between simulated and measured results are observed.