## 4 Experimental Evaluation
### Measurements Techniques and Methods  
Antenna measurements can be conducted in an anechoic chamber or in a real environment (outdoors [71], and indoors). As previously noted, important parameters that need to be extracted from measurements are reflection coefficient S11, E-field distribution in the near field region, and far-field parameters (radiation efficiency, gain, directivity, and radiation patterns). Prior to the description of WTA measurement techniques, it should be mentioned that simulation models by employing numerical phantoms need to be used depending on the application (e.g., arm, head, whole body, homogenous, layered, and flat phantoms) [61; 84; 85; 86; 87; 88; 89; 90; 91]. On-body antenna simulations are of great importance since they save time in terms of optimizing the antenna performance before proceeding to antenna measurements on a body/phantom. The design procedure presented in Section 3, steps (c) to (e), should be repeated by using a numerical phantom since the first run is for free space operation. Then, the antenna design parameters are re-adjusted to obtain the final optimum CC WTA design and prototype. Alternatively, the proposed design process can be realized by directly employing a numerical phantom in simulations from the beginning of the design process. It should be noted that potential differences from free space can be accepted as long as the operational requirements are at an acceptable range. The critical question for all measurement methodologies that have been applied so far is: what is their contribution to the evaluation of WTA performance, and what is the influence of the body/phantom? It is useful to address one by one all WTA performance parameters and highlight for each one the difficulties, challenges, and advantages that exist to evaluate them. The reflection coefficient (S11) of a WTA expresses the input antenna impedance (Zin), which is highly affected by the presence of the human body. Therefore, it is critical that the S11 parameter should be measured on-body to assess the antenna resonance response under this lossy surrounding medium. A lot of papers have successfully presented on-body/-phantom S11 measurements in various locations and under various postures [92; 93; 94; 95; 96; 97; 98]. Furthermore, it is important to evaluate the repeatability of the S11 results by realizing measurements on different bodies and phantoms, if possible. This will provide an average estimation of the S11 and will allow a more accurate evaluation of antennas which are highly influenced in terms of Zin by the presence of the body/phantom. Such antennas are dipoles or monopoles where there is no ground plane to shield the radiation from the human body [99]. The radiation efficiency, gain, directivity, and radiation patterns are fundamental antenna parameters that can be directly measured in a far-field test site. However, measurement setups with phantoms introduce difficulties in such test sites. Specific equipment (positioner) is required to rotate the phantom in the elevation and azimuth axis to obtain the 3D radiation patterns. If the phantom is liquid or semi-solid (gel) [73], there is a risk of breaking or spilling the liquid of the phantom while rotating (scanning) it on the roll/elevation axis. Therefore, manual rotation is required to effectively capture multiple elevation angles. In case the phantom is solid, the rotation of the phantom on the roll axis may be more effortless. Still, there are two main concerns for the solid phantom: (i) it is quite expensive compared to other types of phantoms and (ii) it is heavy so a firm-duty positioner is required [82]. This concludes to the fact that in far-field test sites, the easiest and most feasible solution is to solely rotate and scan the phantom on the azimuth plane [77; 78; 100; 101; 102]. This leads to measuring far-field antenna parameters only on the 2D plane around the phantom [103]. The above restriction is mostly valid in the case of the torso, or whole-body phantoms [82].