A low loss 87-97-GHz bandpass filter with ARCL-to-SCL GSG transition using 3D-micromachined air core recta-coax line

This letter presents a millimeter-wave bandpass filter using 3D-micromachined air core recta-coax line (ARCL). The ARCL structure was fabricated through a high-precision micromachining technique which has advantages of low dielectric and radiation loss, high power capacity, and easy integration. The grounded and open stub lines, as well as impedance transformation lines contribute to the controllable transmission poles (TPs) and transmission zeros (TZs), is finally proposed to realize 87-97-GHz bandpass characteristics. Excellent in-band performance and out-of-band rejection can be obtained by control six TPs and two TZs. Detailed design method is given to guide the ARCL-based filter design. The 87-97-GHz passband filter with ARCL-to-semi-rectangular coaxial transmission line (SCL) ground-signal-ground (GSG) transition is fabricated and measured, which has a low insertion loss (IL) of 1dB, and return loss (RL) is better than -20dB in the passband.