# A Circularly Polarized mmWave Dielectric-Resonator-Antenna Array for Off-Body Communications
## 3 Measurements  
At mmWave frequencies, the precise alignment and assembly of the DRA represent significant challenges that were addressed by outlining the DRA positions on the ground plane. The fabricated feed network is presented in Figure 17a,b, along with the outlined DRA positions. Once the DRA positions were outlined, the antennas were bound to the ground plane using an extremely thin double-sided adhesive copper tape with a thickness of 0.08 mm. The assembled DRA-array prototype is presented in Figure 17c; it was measured without any alignment or bonding concerns. As with the previous investigation, the human body was observed to have a negligible impact on the performance; therefore, all the measurements were conducted for the DRA array in free space. The NSI-MI Technologies system was utilized for the far-field measurements, and the N5245B vector network analyzer (VNA) was used to quantify the return losses [31]. The measurement system is presented in Figure 17d. Figure 18 demonstrates the simulations and measurements for the array's reflection coefficient, where it can be noted that the measured \(-\)10 dB bandwidth was 8.16 GHz. In addition, a good agreement was achieved between the measured and simulated impedance bandwidths of 29% and 28.4%, respectively. However, there is a slight difference in the curves, which may have been the result of marginal fabrication and experimental errors. The axial ratio and gain of the DRA array are presented in Figures 19 and 20, with close agreement between the measurements and simulations. The measured gain, however, was marginally reduced to 13.7 dBic compared to the maximum simulated gain of 13.9 dBic at 28 GHz. This may be attributed to experimental errors, as well as any slight variations in the dielectric constant of the utilized materials. On the other hand, the measured AR bandwidth was \(\sim\)13%, which is similar to the simulated counterpart. It should be noted that the achieved AR bandwidth was at least 160% wider than those reported in the literature for DRA arrays operating in the same frequency range [5,20]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Standard** & **Input Power (dBm)** & **SAR(W/kg)** & **Distance (mm)** \\ \hline FCC/ANSI & 15 & 0.4495 & 5 \\ \hline ICNIRP & 15 & 0.18123 & 5 \\ \hline FCC/ANSI & 18 & 0.8975 & 5 \\ \hline ICNIRP & 18 & 0.3619 & 5 \\ \hline FCC/ANSI & 20 & 1.4225 & 5 \\ \hline ICNIRP & 20 & 0.5735 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The average SAR for 1-gram and 10-gram tissues using input power of 15 dBm, 18 dBm and 20 dBm. Figure 16: The SAR of the DRA array for a 10-gram tissue and input power of (**a**) 15 dBm, (**b**) 18 dBm, (**c**) 20 dBm. Figure 17: The array prototype: (**a**) back view of the feed network, (**b**) top view of the feed network, (**c**) assembled DRA array, (**d**) measurements system. Figure 18: The simulated and measured \(\left.\right|\)S\({}_{11}\)\(\left.\right|\) of the DRA array. Figure 21 presents the simulated and measured E- and H-plane radiation patterns at 28 GHz, 29 GHz and 30 GHz for the proposed antenna array in free space. Overall, a good level of agreement can be observed between the measurements and simulations at these frequencies. For example, the respective measured and simulated gains are 13.7 dBic, 13.9 dBic at 28 GHz, 13.68 dBic, 14 dBic at 29 GHz and 12.45 dBic, 12.6 dBic at 30 GHz. The presented 2 \(\times\) 2 DRA array's performance parameters, gain, AR, size and bandwidth, are listed in Table 5 and compared to those of various mmWave DRA-array configurations that have been reported in the literature. It can be observed that the proposed array, with only four DRA elements, provides a higher gain than that reported the literature; it even outperforms that achieved using eight elements [19] and offers only 2 dB less than that achieved using sixteen elements [21], which illustrates its higher performance with reduced  
Figure 19: The simulated and measured axial ratio of the DRA array.