15–40 GHz and 40–110 GHz Double-Ridge Open-Ended Waveguide Antennas for Ultra-Wideband Medical Imaging Applications

This paper presents the design, fabrication, and measurement of two ultra-wideband doubleridge open-ended waveguide antennas (OEWAs) for emerging high-resolution millimeter-wave (mm-wave) medical imaging applications. The proposed antennas can collectively provide a 95-GHz synthetic imaging bandwidth in the mm-wave regime. The antennas feature the smallest aperture sizes while providing the widest bandwidths amongst all OEWAs reported in the literature. The proposed antennas consist of coaxial to double-ridge waveguide (DRW) transitions and free-space matching sections. Two DRWs with 8:1 single-mode bandwidth in the mm-wave regime are designed. In the transition structures, coaxial flange launchers are connected to the DRWs through customized 50-Ω coaxial lines. To improve the impedance matching while meeting the fabrication requirements, an effective tuning technique is applied to adjust the input impedance of the transitions. Finally, to achieve a smooth transition and a well-matched impedance to the free space while maintaining small aperture sizes, piecewise tapered-ridge matching sections with the same aperture size as that of the designed DRWs are employed. The antennas are fabricated using the computer numerical control (CNC) milling process. Each antenna demonstrates |S
₁₁
| <; -10 dB and stable and symmetric radiation patterns across 15-40 GHz and 40-110 GHz with aperture sizes of 9.47 ×5.79 mm
²
and 3.78 ×2.27 mm
²
, respectively, reporting state-of-the-art results.