# Brain Implantable End-Fire Antenna with Enhanced Gain and Bandwidth
## 4 Specific Absorption Rate and Link Budget Analysis  
### Specific Absorption Rate  
It is required to examine the specific absorption rate (SAR)--the amount of non-ionizing radiated power absorbed by the surrounding biological tissues--for an implantable wireless device. SAR standards differ by countries or regulatory agencies; however, two SAR standards are mainly considered: IEEE C95.1-1999 [29] and IEEE C95.1-2005 [30]. The maximum allowable SAR values are 1.6 W/kg averaged over 1 g of tissue and 2 W/kg over 10 g of tissue for IEEE C95.1-1999 and IEEE C95.1-2005, respectively. SAR values were computed and analyzed using the same simulation setup as the antenna performance analysis (see Section 2). Figure 14a,b show the simulated SAR-1g and SAR-10g for various frequencies after supplying an input power of 1 W to the antenna embedded in the bone layer. The SAR-1g in Figure 14a continuously increased with increasing frequency, while the SAR-10g in Figure 14b shows a peak and null at 3.6 GHz and 5.1 GHz, respectively. The SAR values were very high because of the high input power of 1 W, e.g., 240 W/kg and 59.4 W/kg at 4 GHz for SAR-1g and SAR-10g, respectively. Figure 12: Radiation patterns of the antenna prototype inside the phantom at 4 GHz: (**a**) measured 3D radiation pattern, (**b**) measured and simulated 2D E-plane radiation pattern (normalized by the peak), and (**c**) H-plane radiation pattern (normalized by the peak). Figure 13: A comparison of measured realized gain of the conventional and proposed Vivaldi antennas. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Antenna** & **Frequency (GHz)** & **Size (mm3)** & **Gain (dBi)** \\ \hline
[11] & 2.40–2.48 & 39.9 & \(-\)20.75 \\
[12] & 2.42–2.50 & 50 & \(-\)25 \\
[14] & 2.40–2.48 & 101.6 & \(-\)17.3 \\ Conventional Vivaldi & 3–5 & 42 & \(-\)18.3 \\ Proposed Vivaldi & 3–5 & 42 & \(-\)15.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of brain-implanted antennas. However, the actual SAR is expected to have a much lower value because the power consumed by the RF-front-end of implantable devices generally lies within 100 \(\upmu\)W to a few mW [31]. Hence, we calculated the maximum allowable input power to the antenna that satisfied the SAR-1g (1.6 W/kg) and SAR-10g (2 W/kg) criteria. They are marked by the red dashed lines. The SAR-1g criterion can be fulfilled as long as the input power is less than 5.9 mW for the frequency range of 3-5 GHz as in Figure 13(a). The maximum allowable input power for the SAR-10g was 33.6 mW as in Figure 13(b), which is less stringent than SAR-1g. We measured SAR using a SAR robot in an accredited test facility [32]. Figure 14(a) shows the SAR robot and probe and Figure 14(b) shows the actual test set-up with the phantom and signal generator as the source. Figure 14(c) provides a zoomed-in view of the phantom placements. The seven-layer phantom with the antenna inside was attached at the bottom of a SAR flat phantom. A SAR probe was scanned by the robot arm at the opposite side of the empty flat phantom, and the E-field magnitudes radiated from the antenna were collected. The antenna was fed by a coaxial cable connected to a signal generator. The SAR values at a wireless local area network (WLAN) of 2.45 GHz and 5.8 GHz were measured because of the lack of SAR measurement procedure for 3-5 GHz IR-UWB. The output power from the signal generator was set to 1 mW. Figure 15(d) shows the measured SAR distribution. The total scan area was 150 \(\times\) 150 mm\({}^{2}\). The zoomed scan volume after identifying the hot spot was 40 \(\times\) 40 \(\times\) 35 mm\({}^{3}\). The hot spot locations for 2.45 GHz and 5.8 GHz were comparable. The higher frequency (5.8 GHz) showed a higher averaged SAR-1g value of 0.42 W/kg than that of the lower frequency (2.45 GHz) (i.e., 0.11 W/kg). Both of them were much lower than the 1.6 W/kg limit. These measurement trials provide a reasonable postulation that SAR is not a problematic issue despite that the measured frequencies were not exactly matched to the target frequencies.