A Near-Field Measurement and Calibration Technique: Radio-Frequency Electromagnetic Field Exposure Assessment of Millimeter-Wave 5G Devices

Accurate and efficient measurement techniques are needed for an exposure assessment of 5G portable devices, which are expected to utilize frequencies beyond 6 GHz with respect to radio-frequency (RF) electromagnetic field (EMF) exposure limits. At above 6 GHz, these limits are expressed in terms of the incident power density, thus requiring the EMFs to be evaluated with high precision in close vicinity to the device under test (DUT), i.e., in the near-field region of the radiating antenna. This article presents a cutting-edge near-field measurement technique suited for these needs. The technique, based on source reconstruction on a predefined surface representing the radiating aperture of the antenna, requires two sets of measurements: one of the DUT and one of a small aperture. This second measurement functions as a calibration of both the measurement probe impact on the received signal and the experimental setup in terms of the relative distance between the probe and the DUT. Results are presented for a 28- and a 60-GHz antenna array, both of which were developed for 5G applications. The computed power density is compared with simulations at evaluation planes residing as close as one fifth of a wavelength (l/5) away from the DUT.