Low-temperature cofired ceramic planar helical filter with single-sided transmission zeros

A planar low-temperature cofired ceramic helical bandpass filter with a compact size is presented. The proposed filter is constructed from short-circuited λ_g/4 helical resonators, while the size is reduced significantly by winding resonators vertically. Two single-sided transmission zeros can be created by the proposed mixed electric and magnetic coupling structure. A prototype with size of 6.2 × 4 mm² (0.049 × 0.032λ_g²) has been fabricated for demonstration. The measured fractional bandwidth is 5.9% at 0.845 GHz, and two transmission zeros are observed at 0.90 and 1.07 GHz.