Low-Loss Lowpass and Bandpass Filters on FR4 Substrate

This paper presents the design of high-performance, low-loss planar microwave filters by using an inverted shielded transmission line topology with an air gap. By suspending inverted conductive traces above a longitudinal air cavity and enclosing the propagating fields within multi-row vertical shielding via arrays, the architecture effectively mitigates the severe dielectric dissipation inherent in low-cost substrates. To demonstrate the efficacy of this design framework, two distinct planar filter components are synthesized, fabricated, and experimentally characterized: a 7th-order Stepped-Impedance Resonator (SIR) lowpass filter and a 3rd-order Parallel-Coupled Line (PCL) bandpass filter. Both implemented devices demonstrate high in-band signal integrity, achieving minimal insertion losses of 0.31 dB and 1.1 dB, respectively. Ultimately, this design approach successfully resolves the traditional trade-off between high electrical efficiency and manufacturing cost for advanced microwave filtering applications.