## 5 Discussion
### 75 GHz Band Measurement  
Table 2 summarizes the amount of radio wave absorption in the 75 GHz band measured for each coating thickness.  
Figure 21: Radio wave absorption of sample material created by our CNT spray method (8.51 dB).  
Since variations in the measured values were confirmed depending on the measurement points, the measurement points were set at two points, the upper part and the lower part, with the center as the boundary, and the average value was calculated.  
The maximum value of radio wave absorption in the 75 GHz band was confirmed when 16 \(\upmu\)m of CNT spray paint was applied. It can be seen that the effect of the coating thickness is that the radio wave absorption amount decreases when the coating thickness is 35 \(\upmu\)m and increases again when the coating thickness is 76 \(\upmu\)m (See Table 2). It is presumed that this is because the coating conditions did not match the CNT spray paint, causing irregularities on the coated surface and diffusing the radio waves, resulting in an increase in the apparent amount of radio waves absorbed. Figure 22 shows the surface photographs of the sample coated with 16 \(\upmu\)m of CNT spray paint and the sample coated with 76 \(\upmu\)m.  
In the sample material with a coating thickness of 15 \(\upmu\)m, a slightly darker-colored part (the dented part) of about 0.25 mm can be confirmed, but in the sample material with a coating thickness of 76 \(\upmu\)m, parts of about 1 mm are scattered. In addition, the light-colored part (convex part) can be confirmed more clearly. The JIS R1679 recommends that the surface roughness of the sample material be 1/10 or less of the free space wavelength. Since the free space wavelength of 75 GHz is 4 mm, the surface roughness of the sample must be 0.4 mm or less. In the sample material with a coating thickness of 76 \(\upmu\)m, a part with a surface roughness of about 1 mm was confirmed. It is considered that radio waves were diffused due to this unevenness. These cases will also be reconfirmed using the spray nozzle developed by [PERSON] and [PERSON]. The radio wave absorption of other test materials also showed the same tendency as 28 GHz, and the result was that the radio wave absorption effect could not be obtained with graphite paint. It was confirmed that the difference in the amount of radio wave absorption depending on the coating thickness is different from that in the 28 GHz band at the peak point. However, it also changes depending on the coating thickness.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & & & \multicolumn{8}{c}{**Coating Thickness (\(\upmu\)m)**} \\ \cline{3-10}  & & \multicolumn{3}{c}{**16**} & \multicolumn{3}{c}{**35**} & \multicolumn{3}{c}{**76**} \\ \cline{3-10}  & & **Upper Part** & **Lower Part** & **Average Value** & **Upper Part** & **Lower Value** & **Upper Part** & **Lower Part** & **Average Value** \\ \hline \multirow{3}{*}{Radio wave absorption (dB)} & CNT spray & \multirow{3}{*}{6.78} & \multirow{3}{*}{7.14} & \multirow{3}{*}{6.96} & \multirow{3}{*}{1.45} & \multirow{3}{*}{1.18} & \multirow{3}{*}{1.32} & \multirow{3}{*}{2.26} & \multirow{3}{*}{3.76} & \multirow{3}{*}{3.01} \\  & paint & & & & & & & & & & \\ \cline{1-1}  & Carbon black & \multirow{3}{*}{1.25} & \multirow{3}{*}{1.32} & \multirow{3}{*}{1.29} & \multirow{3}{*}{3.03} & \multirow{3}{*}{2.96} & \multirow{3}{*}{3.00} & \multirow{3}{*}{1.47} & \multirow{3}{*}{1.40} & \multirow{3}{*}{1.44} \\ \cline{1-1}  & & & & & & & & & & & \\ \cline{1-1}  & Graphite paint & \multirow{3}{*}{0.77} & \multirow{3}{*}{0.61} & \multirow{3}{*}{0.69} & \multirow{3}{*}{0.89} & \multirow{3}{*}{0.93} & \multirow{3}{*}{0.91} & \multirow{3}{*}{0.71} & \multirow{3}{*}{0.67} & \multirow{3}{*}{0.69} \\ \hline \hline \end{tabular} Note: Graphite coating thickness is 15 \(\upmu\)m, 36 \(\upmu\)m, 74 \(\upmu\)m.  
\end{table}
Table 2: Radio wave absorption in the 75 GHz band.  
Figure 22: Surface photograph of the sample material coated with CNT spray paint (magnification: 100 times).