A Compact Broadband Transition From Ground Coplanar Waveguide to Folded SIW

In this letter, a compact and broadband ground coplanar waveguide (GCPW) to folded substrate-integrated waveguide (FSIW) transition is proposed by using a metallic blind via and a modified tapered strip line (TSL) in the middle layer. The modified TSL incorporates five U-shaped groove structures of different sizes and a pair of interdigital structures on a conventional tapered quarter-wavelength transformer. The subwavelength U-shaped groove structures provide a significant slow wave effect, reducing the longitudinal size of the quarter-wavelength transformer while enhancing the operating bandwidth. To verify the results, back-to-back transitions are fabricated and measured. The measurement results show that the proposed structure operates at 7.8–22.0 GHz with reflection coefficient more than −10 and −10-dB fractional bandwidth (FBW) of 95%. Compared to the GCPW-to-FSIW transition using the traditional quarter-wavelength transformer, the proposed transition can increase the bandwidth and reduce the physical length of the quarter-wavelength transformer from 4.3 to 2.2 mm, achieving a 49% miniaturization of the longitudinal length.