Compact bandpass filter for RFID reader applications

A novel filter prototype composed of a slotline resonator and back-to-back microstrip-slotline transition is an eligible candidate for radio-frequency identification (RFID) reader applications. Two microstrip feedlines are placed on the opposite plane, perpendicular to the slotline resonator, which is further twisted to an M shape to reduce the overall filter size, thus assigning two transmission zeros towards the out-of-band signal, thereby suppressing the spurious passband. The filter has been fabricated in microstrip line and simulated by CST Microwave Studio software. The dimensions of the module can be controlled within 16 × 15 mm, and the results exhibit a passband from 5.3 to 5.9 GHz with mid-band insertion loss of 0.5 dB and group delay variation <;0.5 ns.