## 3 On-Body Antenna Performances  
In this section the antenna on-body performances are investigated for two different body locations: the footwear toe-cap and the wrist. The body model consists of voxel files representing the wrist and the foot. In the wrist model case the distance between the antenna and the body is equal to 5 mm. For the footwear case the antenna is located next to the skin voxels. The voxel model of the foot is non-planar, so the distance between the antenna and the foot varies from 1 mm to 8.5 mm. The foot and wrist models include different body tissues, such as skin, fat, muscle and bone. The electrical properties of these materials are frequency dependent [19].  
The antenna located on the simulated voxel model and the electrical properties of the tissues are shown in Figure 2.  
The voxel file resembles the shape of the human body parts and it reproduces the internal geometry. The voxel foot model is mostly composed of bones, characterized by small values of permittivity and losses. The voxel wrist model is mostly composed of high water-content material such as muscle, although there is a thin layer of skin and a thicker layer of fat closer to the antenna. A comparison between simulation and measurement of the antenna located on the footwear and on the wrist are shown in Figure 3. The shoe materials are not considered in the simulation model. The thin toe cap layer is negligible compared to the close proximity to the high dielectric constant and lossy body tissues.  
The measured and simulated S\({}_{11}\) of the wrist and toe-cap mounted antennas is less than \(-8.5\) dB across the band of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Antenna [red points]**} & \multicolumn{2}{c}{**Ground [green points]**} \\ \hline
**Parameter** & **Dimension [mm]** & **Parameter** & **Dimension [mm]** \\ \hline \(P_{I}\) & 12 & \(S_{I}\) & 5 \\ \(P_{2}\) & 11 & \(S_{2}\) & 5 \\ \(P_{3}\) & 8 & \(S_{3}\) & 4 \\ \(P_{4}\) & 6 & \(S_{4}\) & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters value of the spline.  
Figure 2: **(a)** On-body model; **(b)** Electrical properties of the human body materials (continuous line \(\varepsilon_{1}\), dotted line \(\varepsilon_{2}\)).  
Small disagreements between simulation and measurement are due to differences between the voxel model shape and the actual human case. The antenna was perfectly centered on the wrist with human body tissue equally distributed below the antenna. The loading of the toes in the footwear case is not symmetrical due to the presence of air-spaces and the non-constant antenna-foot separation. The simulated radiation patterns are shown in Figures 4 and 5. The realized gain as a function of frequency is summarized in Table 2.  
Figure 4: Realized gain patterns for monopole antenna on footwear for \(\varphi=0^{\circ}\) and \(\varphi=90^{\circ}\).  
Figure 5: Realized gain patterns for monopole antenna on wrist for \(\varphi=0^{\circ}\) and \(\varphi=90^{\circ}\).  
Figure 3: Comparison of measured and simulated S\({}_{11}\) for wrist (**a**) and footwear (**b**).  
The antenna radiation efficiency increases with frequency because the electrical distance between the antenna and the body increases. The difference in the shapes of the radiation pattern for the footwear and the wrist locations is due to dissimilar body geometries and to the different positions of the antenna respect to the body. The antenna is characterized by the maximum realized gain for the 3.95 GHz frequency for the wrist case. Considering a walking subject, the 3 dB beamwidth for \(\varphi=90^{\circ}\) for the toe-cap mounted antenna is wide enough to cover the arm swing of the walking subject.