This paper presents the design of a high-gain, wideband planar horn antenna array fed by a multilayer full-corporate air-filled waveguide network with stable response across K-band (18–26.5 GHz). The primary goals are to showcase the feasibility of 3D printing in the fabrication process of complex structures and to conduct a comprehensive performance analysis for validation purposes. The overall configuration of the proposed planar horn antenna array, featuring 8x8 waveguide-fed small-aperture horn antennas, is depicted in Figure 1. A multilayer full-corporate air-filled waveguide feed network structure, detailed in [12], incorporating X-Junctions and Y-Junctions 1-to-4 in-phase waveguide power dividers has been implemented. Vertical short rectangular waveguide sections were utilized to interconnect adjacent layers of the feed network. The design incorporates small-aperture waveguide-fed horn antennas as radiating elements, coupled directly to the outputs of the Y-junction power dividers. A waveguide tapering section was implemented at the input port of the feed network to match the waveguide dimensions of the structure with standard WR-42 specifications.