Low-Cost Low-Loss Tunable Dual-Band Filter Using Coupled Lines With Variable Coupling Coefficients

A novel low-loss tunable filter using its own coupled lines is proposed in the paper. The filter is based on a substrate-integrated suspended line (SISL) structure with two inner layers, fabricated on a low-cost FR4 substrate. The suspension line and the cavity are completely covered by copper. In the novel structure, the coupled line can be tuned and has a variable coupling coefficient. The performance of the dual-band filter can be flexibly tuned by using the coupled lines. The passband widths can be tuned independently with the range from 0.31 to 0.92 GHz. The center frequencies of the passbands can also be tuned independently, with the tuning range of 1.1 GHz. A low insertion loss of less than 0.54 dB is shown in the proposed filter. The proposed filter shows potential application in future tunable microwave circuit.