Assessment of mmWave Exposure From Antenna Based on Transformation of Spherical Wave Expansion to Plane Wave Expansion

To protect human beings from overexposure to mmWave electromagnetic fields, international guidelines and standard specified the (exposure) reference level in terms of the peak spatial-average incident power density (psPD). In this study, we proposed a method to effectively assess the psPD from a mmWave antenna using its spherical near-field measurement data. This method employed the transformation of spherical wave expansion to plane wave expansion to reconstruct the electric and magnetic field components on an evaluation plane in front of the antenna aperture. The psPD was subsequently obtained based on the time-average Poynting vector. The accuracy of the proposed method was validated by comparing it with full-wave simulation results using 12 array antennas at frequencies ranging from 10 to 90 GHz. It was revealed that at 30 GHz, the reconstruction errors for psPD(4cm2) were less than 0.8 dB and 0.2 dB on evaluation planes 2 and 5 mm, respectively, in front of the antennas. The robustness of the proposed method against the noise, scan range, and number of spherical modes were evaluated. The proposal may eliminate conventional compliance assessment procedures once antenna performance is evaluated in spherical near-field measurement.