## 4 Experimental Evaluation
### Measurements Techniques and Methods
In the case of applying an arm or head phantom, it is easier to rotate the phantom on the roll/elevation axis to acquire a second plane and, finally, obtain the desired 3D radiation pattern. Regarding the radiation efficiency, it is proven that it can be measured with satisfactory accuracy in a reverberation chamber (RC), while the antenna is mounted onto the body [76; 104; 105; 106; 107; 108]. Therefore, the reverberation chamber is an efficient, though expensive, solution for wearable antenna efficiency measurements. This solves the issue of producing 3D radiation patterns to calculate the efficiency on a body/phantom. Still, this measurement technique lacks in providing images of the E-field distribution in the near field region, gain, directivity, and the radiation patterns of wearable antenna. The wearable antenna radiation efficiency has, also, been successfully measured by a modified Wheeler cap [83]. All antenna parameters described above can be measured by using near-field measurements (spherical and cylindrical). In the last couple of decades, the spherical near-field measurement technique (STARGATE SG-62 [109] and RAMS [110]) has been successfully used as an alternative method for assessing the far-field radiation of antennas in close proximity to the human body (handset antennas). This indicates the growing interest in using the near-field measurement technique for wearable antenna performance evaluation [111; 112; 113]. Furthermore, the cylindrical near field technique (CNF) (Figure 12) has been successfully proposed for effective WTA antenna measurements [114]. CNF is considered to be one of the best and most competing techniques in terms of measuring antenna parameters and facilitation of measurements. The advantages of this technique can be listed as follows: less physical space is needed compared to the far field measurement; the phantom needs to be mounted on a turntable which rotates only on the azimuth axis, excluding rotation on the roll axis (as it will be conducted when using the Spherical Near Field measurement technique); information about the image of the E-field distribution in the near field region of the phantom by the antenna is acquired by the near field scan which could be important for the best position selection of the wearable antenna leading to a better design of a wearable antenna system and far-field results (gain, directivity, efficiency, and radiation patterns) can be produced by transforming the near field ones. The measurement potential of the CNF technique for evaluating wearable antennas is that it can capture several possible antenna positions with a single scan due to the approximate cylindrical shape of the body/phantom (the CNF measurement technique lacks in capturing sampling data at the top and bottom of the AUTs elevation plane). This can reduce the measurement time and the required volume of the test site. Another important parameter that should be evaluated is the specific absorption rate (SAR) so that the exposure of the human body to the EM-fields produced by the WTA is kept at limited levels defined by ICNIRP and IEEE standards [115]. The assessment of SAR is critical for safety reasons and constitutes a sufficient electromagnetic dosimetry measure for wearable applications. International guidelines determine the maximum permitted values of the SAR to guarantee the user's safety. For example, the IEEE C95.1-1999 international standard limits the SAR averaged over 1 g of tissue in the shape of a cube (SAR-1g) to values lower than 1.6 W/kg for general public exposure [116]. Furthermore, the IEEE C95.1-2005 standard limits the SAR averaged over 10 g of tissue (SAR-10g) to values lower than 2 W/kg [117]. Therefore, compliance with international guidelines needs to be examined. The above-described measurements follow the principle of evaluating the effect of the lossy body/phantom on antenna performance. SAR follows the crucial evaluation of the impact of the antenna operation on the body. This is the reason for reporting this separately.