Via-less X-band stacked filters on Duroid substrate

This paper presents the design, fabrication and measurements of 4 and 5 stacked bandpass filters. The center frequencies of these filters are respectively 7875 MHz, 8675 MHz, 9550 MHz, 10600 MHz, and 11850 MHz with a relative bandwidth of around 10%: it results in a total coverage of the [7.5-12.4] GHz band. These filters are fabricated using conventional etching techniques on a low-cost flexible RT/Duroid 6010.2LM substrate (ε_r = 10.2, tan δ = 0.0023) coated with an 9-μm thick laminated copper layer. The filters are stacked in two different configurations (2×2 or 5×1) thanks to a stripline configuration which also allows the reduction of the length of the resonators. Therefore, an overall compact size can be achieved and the dimensions of each filter are less than 4.9 mm × 4.2 mm. The S-parameters of the filters have been measured using optimized via-less CPW-to-stripline transitions: return loss better than 15 dB and insertion loss varying from 3.6 dB to only 1.3 dB have been measured and are in good agreement with the simulated results.