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Despite the low cost and easy fabrication afforded by 3D printing technology, high dielectric loss of the 3D printing filament degrades the performance of 3D printed radio frequency electronics at high frequencies. In this paper, we propose a 3D-printed substrate-integrated waveguide (SIW) using a lossy polylactic acid or polylactide (PLA) filament. To minimize the insertion loss of the SIW, a non-uniform honeycomb is 3D-printed as the dielectric material of the SIW. The non-uniform honeycomb-shaped substrate is composed of large unit cells, which achieve low insertion loss (0.01 dB/mm) because of the low volume of the dielectric material. The performance characteristics of the proposed SIW is compared with those of SIWs made of solid and uniform honeycomb-shaped structures. The average insertion loss of the microstrip line-fed SIW with the proposed non-uniform honeycomb substrate is 0.92 dB in the frequency range of 1.97 to 3.35 GHz and those of the solid PLA and uniform honeycomb substrates are 2.49 dB and 1.38 dB, respectively. The proposed 3D-printed SIW additionally has the advantages of light weight and low cost.