# Design and SAR Analysis of a Dual Band Wearable Antenna for WLAN Applications
## 1 Introduction  
Wearable systems provide an extension in the classical concept of wearable apparel, which is used for the protection of the body from environmental effects. These systems have introduced new services and need to be completely integrable into the wearable apparel without affecting its comfortability and wearability. A wearable antenna is the main component in a wearable system and serves as a crucial bridge between the external electronic devices and the on-body electronic circuitry [1]. These antenna-based systems are being utilized for security, rescue, sports, entertainment and many other applications [2]. Therefore, they are expected to exhibit resilience to the harsh environment and have a small size. Antenna miniaturization is often achieved by the reduction of the ground plane [3]. However, a reduced ground plane results in spurious currents flowing in the ground plane, which can affect the radiation characteristics of that antenna. Some other miniaturization techniques are also used for antenna size reduction, such as loading lumped components, creating slots [4] and bends, and using short circuits and spiral-shaped radiators [5], among many other techniques. However, patch antennas, because of their peculiar properties, arealways favorable for wearable applications [6]. The radiation pattern of a patch antenna is perpendicular to the antenna plane, which is why, in terms of low specific absorption rate (SAR), patch antennas are the best candidates for these applications. Furthermore, the full ground plane, which is a component of a patch antenna, is beneficial in preventing the wearer's body from unwanted radiation [7], and this further helps reduce the SAR value. Whenever an antenna is installed on a wearer's body, its side and back radiation lobes penetrate into the user's body, thereby being absorbed. Absorption of these lobes or waves produce hot spots on the human body, and destruction of tissues may occur [8; 9]. The specific absorption rate (SAR) defined in Equation (1) is the value formulated to calculate the absorption of electromagnetic waves by human tissue:  
\[\text{SAR}=\frac{\sigma E^{2}}{\rho}(\text{W}/\text{kg}) \tag{1}\]  
where \(E\) is the root-mean-square electric field induced in the tissue (V/m), \(\rho\) is the tissue density (kg/m\({}^{3}\)) and \(\sigma\) is the tissue electrical conductivity (s/m), and its value dictates quantity of maximum exposure to the electromagnetic fields. Beginning in the year 1990, the SAR value of the interaction of an antenna with the human body received tremendous attention [10; 11] because of the huge increase in wireless body area network (WBAN) applications. The SAR value should not exceed the recommended exposure thresholds, and it should remain within a predefined SARlimit. Therefore, different techniques [12; 13] were used by researchers to bring the SAR value into the desired limit. The electromagnetic band-gap (EBG) acts as a shield and reduces the SAR value [14; 15] by efficiently radiating the side and back radiation lobes in the forward direction to protect the human body. It has been shown in some publications [16] that a ferrite sheet also acts as a protective shield and provides stability in terms of resonant frequencies, irrespective of the antenna position on the body. It therefore can be used as an alternative to EBG for SAR reduction. In most cases, the antenna performance is degraded when it is in proximity to the wearer's body or when it acquires a bend [17]. This degradation in performance is due to the permittivity and conductivity of human tissues. Besides the effects due to conductivity and permittivity of tissues, bending may produce a change in the effective electrical length of antenna or mismatch the impedance. Consequently, a shift in the resonance frequency or decrease/increase in the return loss may occur [18]. Therefore, an evaluation of the different parameters of a wearable antenna on a body and in different bending conditions is required to be explored.