## 4 Measurement Result of Radio Wave Absorption
### 75 GHz Band Radio Wave Absorption  
In the 75 GHz band, the Sweep Generator was set so that the signal output from the transmitting antenna had a center frequency of 75 GHz and a sweep frequency of 74.1 GHz to 75.9 GHz. The marker for the level measurement of the Scalar network analyzer was set to 75 GHz, and the level resolution was set to 10 dB/1 Division. Figures 15-18 show screen copies of the Scalar network analyzer during measurement.  
Figure 16: Radio wave absorption of CNT spray paint: (a) Coating thickness 16 \(\upmu\)m lower part (7.14 dB); (b) Coating thickness 35 \(\upmu\)m upper part (1.45 dB); (c) Coating thickness 75 \(\upmu\)m lower part (3.76 dB).  
Figure 17: Radio wave absorption of carbon black paint: (a) Coating thickness 16 \(\upmu\)m lower part (1.32 dB); (b) Coating thickness 35 \(\upmu\)m upper part (3.03 dB); (c) Coating thickness 75 \(\upmu\)m upper part (1.47 dB).  
Figure 15: Changes in the amount of radio wave absorption depending on the board: (a) Calibration (+0.07 dB); (b) Foamed polypropylene (\(-\)0.01 dB).  
Figure 15a is a copy of the measurement screen at the time of calibration. It can be confirmed that the value in the measurement frequency band is linear when nearly 0 dB. The aluminum plate used for calibration is mirror-finished so that the surface unevenness is 1/10 or less (0.4 mm or less) of the wavelength in order to prevent diffused reflection from surface unevenness.  
Figure 15b is a screen copy of the calibration and the measurement of only the expanded polypropylene used for the substrate.  
In the 75 GHz band, due to the effect of installing the expanded polypropylene, it repeatedly increases/decreases slightly in the 350 MHz cycle. However, since the range of change is as small as about 0.3 dB, it was judged that it would not have a significant effect on the measurement results.  
Figure 16 shows the radio wave absorption of CNT spray paint, Figure 17 shows the carbon black paint, and Figure 18 shows the amount of radio wave absorption of graphite paint. Since variations in the measured values were confirmed at the measurement points, measurements were performed at the upper and lower parts of the sample, but the screen copies of Figures 16-18 show only the best values.  
With CNT spray paint, the amount of radio wave absorption was the largest at 7.14 dB with a coating thickness of 16 \(\upmu\)m. This is a little larger than the amount of radio wave absorption in the 28 GHz band (see Figure 16). With carbon black paint, a maximum amount of radio wave absorption of 3.03 dB was obtained when the coating thickness was 35 \(\upmu\)m (see Figure 17). In addition, graphite paint showed a maximum amount of radio wave absorption of 0.93 dB when the coating thickness was 36 \(\upmu\)m, but there was only a maximum difference of 0.22 dB from the other coating thicknesses, so radio waves can be obtained regardless of the coating thickness. Almost no absorption effect was obtained (see Figure 18).  
As mentioned above, even in the 75 GHz band, the amount of radio wave absorption shows the same tendency as in the 28 GHz band, and it can be seen that the amount of radio wave absorption of the CNT spray paint is the largest. In the 28 GHz band, the measurement frequency bandwidth was 3 GHz, but in the 75 GHz band, it was 800 MHz, which is relatively narrow, so the frequency characteristics could not be confirmed (see Figure 16). From these measurement results, it was also found that the amount of radio wave absorption varies greatly depending on the type of test material even if the frequency changes, and also changes depending on the coating thickness. Regarding the coating thickness, it was also found that the optimum thickness changes depending on the frequency.