## 5 Challenges of Wearable Textile Antennas  
Once the wearable textile antenna is manufactured then the step of characterizing it is the next in line. Characterize how the wearable textile antenna operates in Free Space and when mounted on a human body. This section addresses some of the challenges of measuring wearable antennas. Initially it is useful to measure the antenna performance (gain, impedance matching (S11), directivity, efficiency, _etc._) in free space [92; 93; 94]. Then the wearable textile antenna will generally operate under the movement of human body, some specific deformations on the antenna such as bending (on cylinder or on a human arm) [13; 95; 96; 97; 98] and crumpling [99, 100] must be considered. The antenna should be measured under different environmental conditions such as high humidity to represent perspiration or exposure to water [101; 102]. Additionally, the embroidered textile antennas are supposed to be part of the garment. So testing the durability of the antenna in terms of performance after washing is required [103]. Also the repeatability in terms of performance of a specific textile antenna should always be examined [104]. Measurements on a real human body [105; 106] or with a human phantom [107; 108; 109] should take place. The antenna on-body or on-phantom should be measured in terms of S11, near-field, far-field [110; 111; 112]. The antenna-body interactions need to be fully characterized. Measurements can take place in an anechoic chamber or in a real environment (outdoors [113] and indoors). Measurements are the final and the most important step to characterize an antenna system. All practical applications rely on measurements results rather than purely simulations. Measurements of wearable antennas are particularly challenging. The conductivity of the flexible materials is often lower than copper. Dielectric properties of the substrate are generally not well known and can vary from different suppliers or even different batches of the same supplier. As the permittivity is related to the density of fibers it can even change within the same substrate. The thickness of the substrate may changes depending on the method of attaching the different elements which is often an arbitrary non-repeatable process such as adding the same amount of glue across the substrate. The point where the connector meets the antennas is critical in all antenna designs. However, in wearable antennas, this sensitive point depends on the position and orientation of the antenna. These parameters increase the difficulty of wearable textile antenna measurements and make it a real research challenge. Additionally, when antennas are placed in close proximity to the human body, the specific absorption rate (SAR) must comply with International Commission on Non-Ionizing Radiation Protection (ICNIRP) and IEEE limits [114]. This means that the body can only be exposed to limited levels of electromagnetic fields. There is still public concern about possible health issues and EM radiation. Hence, while the majority of people are happy using mobile phones a new type of antenna embedded in clothing is likely to cause some level of public debate. Wearable antennas are clearly close to the body and the SAR must be considered via simulations and measurements. The emitted power of a device is related to the communication distance. Therefore, Bluetooth wearable devices that connect to a mobile phone will use low power levels and hence have very low SAR values. Whereas devices communicating with mobile phone base stations or even satellites will transmit more power and hence have higher SAR values. The connection between the wearable antenna and the electronics is arguably the greater weakness in wearable electronics. The majorities of published papers do not consider this and use an SMA connector and a coaxial cable. Having a rigid connector negates some of the advantages of having a textile antenna.