Slot-Coupled Directional Filters in Multilayer LCP Substrates at 95 GHz

In this paper, traveling-wave directional filters (DFs) in multilayer liquid crystal polymer substrates were proposed at 95 GHz, consisting of two terminating microstrip lines (MSLs) in the top circuit layer, a shared ground and coupling slots in the second layer, and MSL loop resonators in the bottom layer. By using asymmetric phase topology for the loop, the proposed DFs address higher directivity and Q-factor over the traditional symmetric traveling-wave DFs. The single-loop DF was demonstrated with a 3-dB passband width of 4.8% and an insertion loss of 4.6 dB at 94 GHz. To improve the directivity, particularly the insertion loss, two identical DFs were cascaded in series in the direction of the terminating lines. A bandwidth of 8% and a low insertion loss of 2.6 dB can be obtained with a phase delay of 360° between the two DFs. Limited by the practical application, the proposed DFs were fabricated and demonstrated in hybrid substrates, which slightly increases the insertion loss. The measured insertion loss of the single-loop and double-loop DFs is 5.2 and 3.1 dB at 95 GHz, respectively, showing good agreement with the simulated data.