# Loop-Type Field Probe to Measure Human Body Exposure to 5G Millimeter-Wave Base Stations
## 1 Introduction  
Many studies are being carried out to investigate effects of the fifth generation (5G) wireless communication on the human body since the 5G commercial service has started [1; 2; 3; 4; 5; 6; 7]. Especially for the 5G service using mmWave, human body exposure to electromagnetic fields (EMFs) from a base station is one of major concerns because base stations generate sharp beams and steer beams directly to 5G service users. The sharp beam results in a high power density (PD) to be exposed to the human body while the beam steering increases the possibility of such high-PD exposure. In [5], the mmWave exposure is studied in a cell level by exposing the cultured cells to the mmWave EMF. Refs. [6; 7] investigated the EMF exposure in real environments by measuring PD from mmWave base stations.  
5G mmWave base stations use millimeter waves in which the wavelength is very short. Due to this reason, the volume of the near field region is wide compared with that of the other base stations under 6 GHz. When the PD is measured in the near-field region, the E- and H-fields should be measured simultaneously because the two fields have different phase, which is a typical property of the near field. In this condition, the PD can be measured only when the E- and H-fields are measure simultaneously.  
The PD is limited to be less than 20 W/m\({}^{2}\) over the mmWave band to protect the general public from EMF exposure [8]. The PD to be compared with the limitation is an average PD that is obtained by averaging PD samples within a 4 cm\({}^{2}\) area. Such spatial averaging is reasonable considering that a mmWave base station generates a sharp beam, and consequently, the PD can change sharply within a 4 cm\({}^{2}\) area. Previous studies used a horn or monopole antenna to measure PD generated from mmWave base stations [6; 7]. Those studies measured the PD at a long distance of several dozens meters from base stations. Such a long distance makes the antenna beam spread widely, so the PD does not change much at adjacent measurement points in a 4 cm\({}^{2}\) area. General antennas are suitable to measure PD at a long distance from a base station. Within a short distance,however, the antenna beam spreads less, and PD changes sharply within a 4 cm\({}^{2}\) area. In mmWaves, the field intensity changes significantly even with a small distance difference of several millimeters, so an array-type antenna is needed to measure PD [9; 10]. Instead of antennas, a small-sized field probe having high spatial resolution is required for the exposure measurement of a 5G mmWave base station.  
This paper proposes a small-sized probe to measure the mmWave exposure from a 5G base station. It provides high spatial resolution to measure PDs at a short distance from the base station. A probe structure and a processing circuit are designed and fabricated in a single printed circuit board (PCB). The fabricated probe is evaluated using a reference mmWave source that is defined in [11]. The proposed probe includes a loop antenna to measure PD. The utilization of the loop antenna is advantageous in that PD measurement is not affected by a feeding line connected to the antenna. The proposed probes can be arrayed within a 4 cm\({}^{2}\) area, so PDs can be measured at multiple points with a single measurement. Finally, the proposed probe is advantageous in measuring E- and H-fields at the same time and, consequently, can measure PD even in the near-field region.