Positioning-robust broadband microwave transmission as a minimal-complexity approach to thoracic dielectric sensing: simulation and phantom analysis.

Microwave-based dielectric sensing of pulmonary fluid has developed along two main paths: complex multi-antenna torso-scanning systems with reconstruction algorithms, and simplified body-mounted transmission devices such as the FDA-cleared ReDS system. The engineering trade-off between measurement complexity, dielectric sensitivity, and positioning robustness remains insufficiently quantified. Here we study a low-power broadband microwave transmission system (1-2 GHz, 1 mW) using two cavity-backed helical antennas mounted at a mechanically constrained 6 cm distance from the chest. Full-wave simulations were performed in two voxel torso models, Gustav and Emma. Lung permittivity was varied over ε' = {20, 30, 45, 55, 75}, and antenna mispositioning was tested using a 3 × 3 grid of ± 10 mm lateral and cranio-caudal shifts. In the 1.15-1.35 GHz band, the dielectric-to-positioning dominance ratios in Gustav were 4.4, 2.6, and 1.2 for ε' = 20-75, ε' = 30-75, and ε' = 45-75, respectively. In Emma, the corresponding ratios were 5.1, 1.6, and 2.8. Thus, the same predefined band remained informative in both anatomies, but the exact spectral behavior and robustness margin were anatomy-dependent. A simplified multilayer phantom confirmed the qualitative attenuation trend during progressive water loading, but was not used to derive quantitative physiological estimates. The study demonstrates methodological feasibility rather than clinical readiness. Its main contribution is a quantitative dielectric-to-positioning framework for evaluating a minimal-complexity thoracic microwave transmission geometry and identifying a fixed operating band for future validation.