# Eco-Friendly Metadome-Antenna Innovations for Wearable Millimeter Wave Radar Sensing
## 4 Discussion  
Most 24 GHz radars aim at avoiding collision at \(>\)2 m distance and so high gain (G \(>\) 15 dBi) and pencil-beam pattern are pursued for the antennas [16, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42], in contrast to the current target application. Moreover, recent antennas in literature operating at 24 GHz are mostly based on substrates with relative dielectric permittivity higher than 2.9 [43, 44, 45, 46, 47, 48]. In general, using substrates with higher permittivities helps to miniaturize the devices, but it also reduces the bandwidth in microstrip technology and favors surface waves (more so, the higher the thickness). In addition, at millimeter wave frequencies the devices are already physically quite small, so this is not a plus, and in fact may present more manufacturing challenges. Furthermore, when a fair comparison is to be made, it seems more logical to conduct it considering antennas on substrates with dielectric permittivities as close as possible to the one proposed. Confronted to antennas on dielectrics with \(\varepsilon_{\rm r}\) values close to the PP one (see Tables 5 and 6), the GAA is thinner than [16, 31, 38, 42, 49, 50, 51] and more compact than [16, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42]. It exhibits wider bandwidth than [32, 39] and equal to [34] for identical thickness being more compact. It almost doubles the gain of [31, 49], being thinner. Furthermore, it provides very high radiation efficiency (even using Al with thickness instead of PEC, as usual and in [34]) which is crucial. The MTR-GAA preserves the HPBW of the GAA with high gain, radiation efficiency (it overcomes [31, 40], and equals [50, 51] neither of which has the protection of a radome) and proper bandwidth.  
\begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Freq (GHz)**} & \multicolumn{3}{c}{**G (dBi)**} \\ \cline{2-4}  & **GAA** & **GAA+Radome** & **GAA+Metaradome** \\ \hline
**24.05** & 12.5 & 11.7 & 12.7 \\ \hline
**24.15** & 12.5 & 11.5 & 12.5 \\ \hline
**24.25** & 12.1 & 11.0 & 12.0 \\ \hline \end{tabular}
\end{table}
Table 4: Radiation properties results in measurements.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Ref.**} & **Size (mm3)** & \multirow{2}{*}{\(\varepsilon_{\rm r}\)} & **BW (GHz)** & **BW (\%)** & **S11 (dB)** \\ \hline
[16] & 60 \(\times\) 60 \(\times\) 0.787 & 2.2 & 1.99 & 8.2 & \(-\)34 \\
[31] & 20 \(\times\) 20 \(\times\) 0.68 & 2.9 & 2.0 & 8.3 & \(-\)35 \\
[32] & 98.7 \(\times\) 14.4 \(\times\) 0.52 & 2.2 & 0.28 & 1.2 & \(-\)25 \\
[34] & 60 \(\times\) 60 \(\times\) 0.52 & 2.2 & 0.96 & 4.0 & \(-\)26 \\
[37] & 74.9 \(\times\) 74.9 \(\times\) 3.3 & - & 0.2 & 1.0 & \(-\)8 \\
[39] & 160 \(\times\) 43.2 \(\times\) 0.5 & 2.3 & 0.32 & 1.3 & \(-\)20 \\
[40] & 230 \(\times\) 31 \(\times\) 0.254 & 2.17 & 0.39 & 1.6 & \(-\)20 \\
[41] & 73.5 \(\times\) 55 \(\times\) 0.254 & 2.2 & 0.83 & 3.4 & \(-\)42 \\
[42] & 143 \(\times\) 18.5 \(\times\) 3.625 & 2.54 & 2.25 & 9.7 & \(-\)17 \\
[49] & 32.4 \(\times\) 15 \(\times\) 1.016 & 2.2 & 2 & 8.0 & \(-\)28 \\
[50] & 24 \(\times\) 21 \(\times\) 43.3 & 2.2 & 2.75 & 11.7 & \(-\)38 \\
[51] & 40 \(\times\) 6 \(\times\) 0.8 & 2.2 & 4.44 & 18 & \(-\)20 \\
[52] & 86 \(\times\) 86 \(\times\) 0.254 & 2.2 & 0.77 & 3.2 & \(-\)25 \\ This work & 40.8 \(\times\) 40.8 \(\times\) 0.52 & 2.2 & 0.94 & 3.9 & \(-\)24 \\ This + MTR & 40.8 \(\times\) 40.8 \(\times\) 1.74 & 2.2 & 0.70 & 2.9 & \(-\)35 \\ \hline \end{tabular}
\end{table}
Table 5: Size, relative dielectric permittivity, bandwidth and impedance matching.