A Near-Field Coupling Efficiency Optimized Antenna for High-Accuracy Axillary Thermometry through Clothing

This paper presents a novel antenna design methodology and a fabricated prototype for accurate axillary temperature measurement through common clothing. Unlike infrared thermometers, microwaves can penetrate low-loss fabrics. However, existing antenna designs optimized solely for S₁₁ minimization fail to ensure that the received thermal noise originates predominantly from the body rather than from other interfering sources. To overcome the problem of mixed noise origin, we introduce the near-field coupling efficiency as a complementary design objective. This leads to a new dual-objective design strategy where the antenna must be co-optimized for both low S₁₁ and high near-field coupling efficiency. We implement this strategy through a systematic 'body-clothing-antenna' co-design methodology, which treats the unknown garment as a configurable dielectric layer within the matching network for joint optimization. A 32-36 GHz antenna based on this methodology is designed, fabricated, and rigorously characterized through simulation and measurement. Measurements confirm its robust performance, maintaining S₁₁ below -15 dB across various clothing types. Integrated radiometric measurements confirm the antenna's capability for accurate axillary thermometry through polyester Tshirts, with a mean bias below 0.2° and no clothingspecific calibration required. This work therefore delivers a complete design methodology and a validated prototype for high-accuracy, through-clothing axillary thermometry.