# Loop-Type Field Probe to Measure Human Body Exposure to 5G Millimeter-Wave Base Stations
## 2 Probe Design  
### Probe Structure  
In [6; 7], horn and monopole antennas were, respectively, used to measure PD from the 5G mmWave base station. Those antennas, however, have the disadvantage of a low spatial resolution because of the large antenna size. High spatial resolution is essential, especially for the exposure measurement of a mmWave whose wavelength is very small. A line antenna has been used for a field probe measuring exposure at radio frequencies much smaller than the mmWave. A typical example for the line antenna is the dipole or monopole one in [12; 13]. Those line antennas have a short length much smaller than one wavelength at the measurement frequencies. Such a short-line structure suppresses any resonance, so the field probe has a flat frequency response over measurement frequencies. Also, it is possible to achieve a high spatial resolution for the exposure measurement.  
When the line antenna is used to measure the mmWave exposure, its length is reduced to a few millimeters owing to its small wavelength. In this case, the antenna performance is affected by a feeding line that is connected to the line antenna. E-fields are coupled to a part of the feeding line in addition to the line antenna, so its effective length becomes larger than the original one. Such a length increase becomes critical at mmWave frequencies where the line antenna has is a length of only a few millimeters. The coupling to the feeding line is dependent on the length and the shape of the feeding line, so a different strength is measured depending on the feeding line even for the same E-field. Such unintended coupling increases the measurement uncertainty in an exposure measurement.  
To resolve this problem, we propose a field probe with a loop antenna like in [14]. Figure 1 shows the loop antenna that is proposed in our study. Unlike the line antenna, the loop antenna experiences less coupling with the feeding line. This is because a closed loop is formed by the load resistor, as shown in Figure 1, while the feeding line makes no loop near the loop antenna. The incident H-field is coupled only with the loop antenna and induces a voltage at the load resistor. Such a coupling structure leads to less measurement uncertainty than a line antenna. The load resistor reduces the quality (Q) factor of the loop antenna and, consequently, suppresses any resonance resulting in a wideband operation of the probe.  
As shown in Figure 2, the loop antenna is followed by the low-noise amplifier (LNA). Such a structure makes it easy to enhance the dynamic range of the field probe. The LNA amplifies the dissipating power at the load resistor, \(R_{A}\), so the weak H-field causing low-power dissipation can be measured, resulting in a wide dynamic range. Our study used a commercial off-the-shelf (OTS) LNA for easy implementation [15]. Currently, no OTS LNA operating at mmWave frequencies supports a differential input. A passive-type balun can be used to transform the loop antenna output into the single-ended input for LNA. Such a balun, however, causes an additional signal loss in the signal path after the loop antenna. As shown in Figures 1 and 2, the loop antenna in our study has a single-ended output, so its output can be directly connected to the single-ended input of LNA. Like the LNA, a commercial OTS was used for a root mean square (RMS) power detector. As shown in Figure 2, the LNA is followed by the RMS power detector [16]. The RMS detector outputs a dc voltage, \(V_{PD}\), proportional to PD of an incident wave, which will be explained later in this paper. The loop antenna is not a resonant one, so the operating bandwidth of the probe is determined by the LNA in which the bandwidth is about 13.5 GHz [15].