## 4 Discussion
For EM wave directed toward the back of the head (\(\varphi=180^{\circ}\)), the average value of \(SAR\) averaged in 10 g of tissue was reduced by the metal surface shield from 0.082 to 0.038 W/kg. The data presented in Table 2 show that shielding effectiveness also depends on the incident angle in the vertical plane, but the shield increased the maximum value of \(SAR\) and reduced approximately by 50% the average value of \(SAR\) for all the cases considered. The \(SAR\) simulation results for 3600 MHz frequency are presented in Tables 3 and 3. In this case, data are given for the unshielded model and for the shieldmade of metal surface and wires. The maximum values of \(SAR\) in the case are also slightly greater for the metal surface shield compared to the unshielded case, but this effect is less significant than for 750 MHz. The simple shield made of vertical wires reduces the energy absorbed by the head for a vertically polarized wave that illuminates the head with an incidence angle \(\theta\) that has its value close to \(90^{\circ}\), that is, the case of a distant base station radiating in an almost horizontal plane. For \(\theta=0^{\circ}\), shield does not influence the \(SAR\) parameter. This shield does not cause a noticeable increase of the maximum value. Both shielding structures (metal surface and wire) reduce the average value of \(SAR\); however, this effect depends on the angle of exposure. For an EM wave directed toward the left side of the head (\(\varphi=180^{\circ}\), \(\theta=90^{\circ}\)), the average value of \(SAR\) averaged in 10 g of tissue was reduced from 0.04 to 0.019 W/kg in the case of the metal surface shield and to 0.023 W/kg for the shield made of wires. Summarizing these considerations, the following effects can be noticed: the use of a metal surface shield reduces the average \(SAR\) value but increases the maximum value of this parameter. In addition, locally, it can increase the power density. To investigate this phenomenon, I simulated the current distribution in the horizontal cross-section of the head shielded with the metal surface. Results obtained for 750 MHz are presented in Figure 17 and those for 3600 MHz are given in Figure 18. It can be noted that at the edges of the shield that are close to the face, there is a local increase in current density. The current in the shield is the secondary source of the wave that, together with external excitation (illuminating wave), can at some points increase the power density and \(SAR\) compared to the unshielded case. Figure 17: Current distribution for 750 MHz. The complex distribution of current in the conducting shield is presented in Figure 19 for the 3600 MHz case. It can be seen that there are many local maxima of current that can increase power density in the body tissues close to the shield. To analyze the reason for the limited level of shielding, I performed a simulation of the electric field in time domain. For this purpose, instead of sinusoidal excitation that was  
Figure 19: Current distribution in the shield structure for 3600 MHz with the head (**a**) visible and (**b**) not visible. Figure 18: Current distribution for 3600 MHz. used for fixed frequency analysis (750 and 3600 MHz), I used a short pulse, as presented in Figure 20. This short pulse makes possible the analysis of energy propagation in the model. I recorded subsequent distributions of the electric field, which are presented in Table 4. The figures in the table present the electric field distribution on the horizontal cross-section of the human head covered with the metal shield. The wave was propagating from the right side to the left side of the figures. For time \(t=0.5\) ns, the wave reflected from the face is visible. For \(t=0.9\) ns, both the wave traveling outside the head cover and inside the cover are visible. When \(t=1.5\) ns, the wave that was outside the shield is radiated to the left side of the figure, while the part of the energy that was propagating between the head and conducting shield is still in the model.