# Flexible Antennas for a Sub-6 GHz 5G Band:
## 3 Parameters to Be Considered for the Design and Analysis of a Flexible Antenna
### Specific Absorption Rate (SAR)  
The absorption of electromagnetic radiation with the use of wearable devices, including antennas, is governed by the specific absorption rate (SAR) limits. The amount of electromagnetic radiation absorbed per unit mass of the human body is known as the specific absorption rate. As far as a WBAN is considered, to guarantee its safety it must be ensured that the SAR value must be as per the recommended value. The highest permissible limit of the SAR for 1 gm of tissue, according to US guidelines, is 1.6 W/kg and the SAR maximum value for any 10 g of tissue has been set at 2 W/kg by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) for Europe [43]. Therefore, one of the main considerations in the use of a wearable antenna is to limit its SAR following international standards. An AMC-backed (artificial magnetic conductor) antenna on different parts of the human body is proposed in [44] for a ISM 5.8 GHz band, as shown in Figure 5a-c show the reflection coefficient behavior and the SAR distribution rate of the antenna on the various parts of the human body, respectively [44]. Figure 4: Effect of bending on the gain: (**a**) Bending of a slotted dipole antenna; (**b**) Gain analysis of the antenna under flat and bent conditions. Reprinted from ref. [42]. Figure 5: Relevance of the SAR on the design of a flexible antenna: (**a**) AMC-backed antenna on various parts of the body; (**b**) Reflection coefficient behavior on the human body; (**c**) SAR of the antenna on different parts of the human body—hand, chest, and head. Reprinted from ref. [44]. To analyze the SAR values of the different body parts at a particular frequency, the dielectric properties, such as the dielectric constant and conductivity of the different tissues, should be considered. Moreover, the SAR values differ depending upon the body part where the antenna is placed, and the thickness of the tissues associated with the specific body part. Table 5 depicts the various tissue layers and their thickness in various parts of the body [43]. When the antenna is placed close to the human body, its performance suffers because of the lossy human body tissue interactions. When placed near the human body, having a high conductivity and dielectric constant, the wearable antenna shifts the frequency of resonance. The dielectric constant and conductivity of the muscle tissue are related to the resonant frequency. The dielectric constant of the tissue is inversely proportional to the resonance frequency and directly proportional to the conductivity of the tissue [44]. Moreover, with the variation in body parts, the conductivity of the tissue is varied, and with the variation in type (skin, fat, muscle, etc. ), the permittivity of the human tissue also varies. The reflection coefficient is affected by the difference in the permittivity and conductivity levels, which in turn affects the antenna's performance. The dielectric characteristics of different tissue layers for different frequencies are shown in Table 6[45]. It is to be ensured that the SAR should be within the permissible limit to be used for wearable antenna applications. The placement of the antenna in various human body parts \begin{table}
\begin{tabular}{c c c} \hline \hline
**Tissue Layers** & **Dielectric Constant \(\varepsilon_{r}\)** & **Conductivity \(\sigma\) (S/m)** \\ \hline \multicolumn{2}{c}{2.4 GHz} \\ \hline Skin & 42.85 & 1.59 \\ \hline Fat & 10.82 & 0.27 \\ \hline Muscle & 53.6 & 1.81 \\ \hline
3.5 GHz & & \\ \hline Skin & 41.41 & 2.35 \\ \hline Fat & 10.5 & 0.42 \\ \hline Muscle & 52.12 & 2.72 \\ \hline
5 GHz & & \\ \hline Skin & 35.78 & 3.06 \\ \hline Fat & 5.03 & 0.24 \\ \hline Muscle & 49.54 & 4.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Dielectric characteristics of tissue layers for different frequencies. Reprinted with permission from ref. [45]. Copyright 2021 ETE Journal of Research.