Compact dual-band bandstop filter by using open-loop resonators

In this study, a second order dual-band bandstop filter design method is developed. The two LTE frequency bands 2110 MHz-2200 MHz and 2500 MHz – 2690 MHz are chosen as stopbands since wideband receivers suffers from interference in these frequency range. Direct coupling matrix synthesis technique is utilized for implementing dual-band bandstop filter. The design process of a dual-band bandstop filter is reduced by implementing the filter with a coupling matrix size of a single-band bandstop filter. In order to reduce the coupling matrix size, dual mode resonators are produced by grouping Square Open-Loop resonators (SOLR). Since the resonators have been used in groups, the size and insertion loss have been reduced while the rejection level is kept constant. The identical coupling matrix is implemented as a filter on two distinct substrates, RT/Duroid 6010 and FR4. The substrate RT/Duroid 6010 is utilized for simulations for its high RF performance potential. However, for implementation of designed filter, cost-effective FR-4 is utilized as the substrate. The measurement results of the implemented filter demonstrate that it complies with the design specifications.