## 4 Discussion  
The measurements of antennas in portable radios are often challenging. Due to the small size (with respect to the wavelength) of the antenna being tested and the finite ground plane of the PCB, the impedance measurements suffer from the influence of the attached RF cable [41]. Therefore, a sleeve balun with a diameter of 8 mm placed on the RG405 coaxial cable close to the antenna was used (Figure 5). This suppresses unwanted surface currents propagating onto the outer shield of the feeding cable. The length of the sleeve balun choke was 0.23 \(\lambda\), which is the optimal length to achieve maximum insulation [42].  
According to ETSI EN 300 328 V2.1.1:2017-058 for transmission systems operating in the 2.4 GHz ISM band, the limit of EIRP is equal to 100 mW (Wi-Fi 2G, Bluetooth class 1) [18]. In the case of an RF module continuously using the maximum EIRP, located on headband, the values of SAR10g may be 32 times and 6 times higher than the limits for the general public (2 W/kg) and occupational exposure (10 W/kg), respectively. In fact, these values are unlikely to occur due to the technically limited input power to the analysed MIFA (built in the ESP32-WROOM-32 RF module).  
The analysed frequency band (2.4 GHz--ISM) is very close to the frequency bands used by LTE: 2100 (1920-1980 MHz uplink; 2110-2170 MHz downlink), 2300 (2305-2315 MHz uplink; 2350-2360 downlink) and 2600 (2500-2570 MHz uplink; 2620-2690 MHz downlink) MHz. The limit of unlicensed use of EIRP is the highest for LTE devices and equal to 200 mW (ETSI TS 136 101 V15.9.0 (2020-02) [43]. Consequently, the obtained results of SAR values near the modelled MIFA at 2.45 GHz may also be used to roughly estimate the SAR values for the use of similar RF transmitters in these LTE frequency bands, which may have an EIRP up to twice as high as it is allowed following the technical specifications for Wi-Fi 2G and Bluetooth devices. Consequently the worst-case SAR values caused by the use of MIFA in the LTE system are also twice as high as in analysed Wi-Fi/Bluetooth systems. The most realistic time-pattern of EMF emission in analysed WSN application is not continuous, but the worst-case exposure level as discussed in this section also needs attention. Continuous emission from MIFA is allowed in the software controlling the considered RF module; it may be used intentionally, e.g., when communication within the WSN is tested, but also unintentionally because of programming errors or technical malfunctions of the system.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Exposure Scenario** & **WHSAR, (1) W/kg** & **SAR10g, (2) W/kg** & **EIRP, (3) mW** \\ \hline RF module on the headband & 0.035 & 4.0 & 6.1 \\ RF module on the helmet & 0.005 & 0.4 & 350 \\ \hline \end{tabular} \({}^{(1)}\) WHSAR—SAR evaluated as averaged over the entire exposed head; (2) SAR10g—maximum local SAR averaged over 10g of mass of any continuous tissue; (3) EIRP—equivalent isotropically radiated power.  
\end{table}
Table 2: Specific energy absorption rate (SAR) values in the head of the user of an ESP32-WROOM-32 RF module equipped with a MIFA, for an input power to the antenna of 100 mW @ 2.45 GHz.  
Only wearable IoT devices (similar in construction to the investigated device) using Wi-Fi/Bluetooth communications technologies emitting at below 3 mW EIRP may be considered as environmentally insignificant EMF sources (i.e., their direct biophysical influence on the body of their user seems to be weaker than the limits of SAR caused by EMF influence provided by international safety guidelines). When higher EIRP is required for sufficient use of particular applications of wearable IoT devices (e.g., when WSN is improving safety in the work environment), the workers using such wearables need to be counted as EMF-exposed workers, together with application of an EMF safety program, as labour law defines.