Designing cross-coupled bandpass filters with transmission zeros in lossy microstrip

Bandpass filters play a significant role in many wireless communication systems. The filters pass desired signals and reject unwanted one. In this paper, we design a four-pole bandpass filter using a rigorous computer simulation. The resonators used are square open-loop resonators. To get a sharper selectivity around the pass band, transmission zeros though cross-coupling between resonators are introduced. In simulation, we compare the results for relative permittivity 4.4 and 4.9, and for tangent loss 0.025 (lossy) and 0.0 (lossless). The simulation shows, the required specifications are fulfilled for lossless case, whereas for lossy case, an insertion loss of 8.3 dB observed. For verification, the bandpass filter is built in FR4. In this lossy microstrip, a measurement shows, we get an insertion loss of about 6.64 dB and a bandwidth of about 120 MHz.