# Loop-Type Field Probe to Measure Human Body Exposure to 5G Millimeter-Wave Base Stations
## 3 Probe Fabrication and Measurement
### Fabricated Probe and Measurement Setup  
Figure 4 shows the fabricated probe for the 5G mmWave exposure measurement. The loop antenna has a size of 5.1 \(\times\) 1.9 mm\({}^{2}\). The maximum length of the loop antenna is about half of a wavelength at 28 GHz; hence, the probe provides the spatial resolution of two points within one wavelength in the free space. The antenna substrate is composed of three layers, the total thickness of which is 800 \(\upmu\)m. Each layer of the antenna substrate has a low tangent loss of 0.0024 at 10 GHz. The OTS LNA and the RMS power detector were attached on the substrate surface with resistor and capacitor components. As shown in Figure 4, the fabricated probe is terminated with the connector. This connector is to connect the probe to an interface board that reads the output voltage of the RMS power detector and controls the probe operation. Figure 5 shows the measurement setup to measure the performance of the fabricated field probe. The measurement setup includes the EMF radiator operating at 28 GHz. Our study used the reference antenna in [11] as the EMF radiator. As shown in Figure 5, the EMF radiator is the horn antenna in which the aperture has a slot array. EMFs radiated from the horn antenna with the slots are well analyzed in [11]. Numerical simulations can be performed to obtain PD in the free space, and the simulated PD is then compared with the measured one to evaluate the probe performance. The horn antenna with the slots in [11] is an EMF radiator to test the performances of a field probe that are used to measure PD on a surface of a mobile unit operating at 28 GHz. Such a test locates a field probe close to a mobile unit within several millimeters. In our study, however, the proposed field probe is used to measure PD from a base station at 28 GHz and, consequently, located at a longer distance from the EMF radiator, i.e., a 28 GHz base station. The horn antenna with the slots represents a 28 GHz base station well because it includes an array antenna which is composed of many dipoles or slots. Considering this point, the utilization of the horn antenna with the slot array is suitable to evaluate the probe for a 5G mmWave base station. Figure 6 shows the measurement setup to measure the coefficients in (2) and probe performance, respectively. The coefficients include the effects of the radiation pattern of the loop antenna in the probe. The probe was horizontally aligned with the center of the horn aperture. As shown in Figure 5, the horn antenna was moved toward or away from the probe with the alignment condition maintained. The dc power supply provides dc voltages to operate the LNA and the RMS power detector composing the field probe. Figure 5 also shows the columns to place the field probe and the horn antenna, both of which are 50 cm above the bottom. The column for the horn antenna was clamped by the rail on the bottom and moved forward or backward along the rail to control the distance with the probe. The mmWave absorbers were placed at the bottom and all around to reduce any unintended reflections. Figure 4: Fabricated 5G mmWave field probe. The product of the coefficients in (2) should be determined prior to the PD measurement. The product \(G_{\text{LNA}}C_{PD}\) can be measured directly by measuring a response of the active-circuit path. For this, another PCB is fabricated by removing only the loop antenna and leaving the LNA and the RMS power detector. The active-circuit response can be measured by measuring the circuit output, i.e., the output voltage of the RMS power detector, by driving the circuit input with a 28 GHz signal with the power known. The active-circuit response is obtained by calculating the ratio between the input power and the output voltage. The calculated ratio corresponds to the product \(G_{\text{LNA}}C_{PD}\). The calculated \(G_{\text{LNA}}C_{PD}\) also includes any signal loss that occurs between the LNA and the RMS power detector. Another product \(C_{E}C_{H}\) in (2) can be measured by applying a radiating field to the probe.