A Broadband Inline Transition From On-PCB Microstrip to Hybrid Stack-Up Integrated Additively Fabricated Air-Filled Waveguide

A novel type of a highly integrated low-loss stack-up is explored that leverages additive manufacturing for the integration of a 3D printed and metal-coated air-filled waveguide with a Printed Circuit Board (PCB) by sharing a common ground plane. A complementing microstrip to waveguide transition is proposed exploiting the flexibility of the 3D printing technology to realize an in-line E-field and smooth profile continuous impedance transformation to ensure low insertion losses, wideband operation, and high return loss. Field mode matching and monotonic impedance profile are ensured using various intermediate guides along with the transition. A fast design procedure relies on analytical expressions for guides‘ impedance that was proven not to require further full-wave model optimization. The concept was experimentally validated by the realization of demonstrators operating within the cm- and mm-wave frequency range and fabricated employing a subtractive PCB and additive vat photopolymerization 3D printing and copper plating technologies. A hybrid WR-90-sized waveguide with RMS metal coating roughness of
$1.5 ~\mu \text{m}$
yields power loss below 0.026 dB/cm being comparable to an all-metal counterpart. Measured back-to-back half-wavelength long at lower cutoff transitions to either WR-90 or WR-28 hybrid waveguides operate with bandwidth of ~1.9 and ~1.4 (return loss better than 10 dB) with 0.63 dB and 0.86 dB average loss per transition within 8.2 – 12.6 GHz and 26.5 – 40 GHz bands, respectively.