A Six-Zone 3D-Printed Luneburg Lens Using Variable Infill for Gain Enhancement of a WR-28 Open-Ended Waveguide Antenna

This paper presents a practical implementation of a 3D-printed spherical Luneburg lens for gain enhancement of a WR-28 open-ended waveguide antenna operating in the Ka-band. The lens is designed based on Luneburg theory and realized using a six-zone discretized gradient-index structure, providing a balance between theoretical performance and fabrication feasibility. The proposed design enables the realization of the required permittivity distribution using a single dielectric material, where the effective permittivity of each zone is controlled through infill variation in a fused deposition modeling (FDM) process. To facilitate fabrication, the lens is divided into two hemispherical parts, enabling reliable manufacturing and assembly while maintaining the intended dielectric profile. The antenna performance is experimentally evaluated through reflection coefficient (S11) measurements and radiation pattern characterization in both the XZ and YZ planes over the frequency range of 26.5–40 GHz, including co-polarized and cross-polarized responses. The proposed antenna achieves a simulated realized gain ranging from 17.6 dBi to 19.83 dBi, while the measured realized gain ranges from 16.42 dBi to 18.43 dBi, with a maximum deviation of 1.47 dB. In comparison, the standalone WR-28 open-ended waveguide exhibits a measured realized gain of 7.22–8.01 dBi. The integration of the six-zone Luneburg lens results in a realized gain enhancement of 9.20–10.97 dB across the operating band. These results confirm that the proposed approach provides a simple, low-cost, and experimentally validated solution for high-gain millimeter-wave antenna applications, while maintaining good agreement between simulation and measurement.