## 4 Measurement Result of Radio Wave Absorption  
### 28 GHz Band Radio Wave Absorption  
The sweep generator was set so that the center frequency of the signal output from the transmitting antenna was 28 GHz, and the sweep frequency was from 26.5 GHz to 29.5 GHz.  
The marker for the level measurement of the Scalar network analyzer was set to 28 GHz, and the level resolution was set to 10 dB/1 Division. Figures 11-14 show a copy of the Scalar network analyzer screen during measurement. Figure 11a 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 at 0 dB. Calibration was performed based on the Japanese Industrial Standard R1679 using a polished aluminum plate. Figure 11b is a screen copy of the calibration and the measurement of only the expanded polypropylene used for the substrate. It can be confirmed that there is no change from the value at the time of calibration even if the foamed polypropylene is installed. This indicates that the expanded polypropylene does not affect the radio wave absorption effect. The decibel value in parentheses indicates the amount of radio wave absorption.  
Figure 11: Changes in the amount of radio wave absorption depending on the board: (**a**) Calibration (\(-0.02\) dB); (**b**) Foamed polypropylene (\(-0.06\) dB).  
Figure 10: Sample stand installed on an infrared control type rotary table.  
Figures 12-14 are screen copies of the CNT spray paint, carbon black paint, and graphite paint when the amount of radio wave absorption is measured. The decibel value in parentheses is the amount of radio wave absorption. The value is attenuated by absorbing radio waves from the reference value obtained by the calibration with the sample material. Although the measured value shows a minus, the minus is omitted for the radio wave absorption amount.  
It can be seen that the amount of radio wave absorption of the CNT spray paint is the largest, as shown in Figure 12. With CNT spray paint, the amount of radio wave absorption was the largest at 5.91 dB with a coating thickness of 35 \(\upmu\)m. Figure 13 shows the amount of radio wave absorption with the carbon black paint. When the coating thickness was 16 \(\upmu\)m, a maximum radio wave absorption amount of 3.29 dB was obtained, which is smaller than that of the CNT spray paint. Figure 14 shows the amount of radio wave absorption  
Figure 14: Radio wave absorption of graphite points: (**a**) Coating thickness 15 \(\upmu\)m (1.27 dB); (**b**) Coating thickness 36 \(\upmu\)m (1.44 dB); (**c**) Coating thickness 74 \(\upmu\)m (0.57 dB).  
Figure 12: Radio wave absorption of CNT spray paint: (**a**) Coating thickness 16 \(\upmu\)m (0.52 dB); (**b**) Coating thickness 35 \(\upmu\)m (5.91 dB); (**c**) Coating thickness 76 \(\upmu\)m (3.29 dB).  
Figure 13: Radio wave absorption of carbon black paint: (**a**) Coating thickness 16 \(\upmu\)m (3.29 dB); (**b**) Coating thickness 35 \(\upmu\)m (2.32 dB); (**c**) Coating thickness 76 \(\upmu\)m (1.62 dB).  
with the graphite paint. When the coating thickness was 36 \(\upmu\)m, a maximum radio wave absorption amount of 1.44 dB was obtained, which is the smallest value among the three carbon materials.  
From these facts, it can be seen that the amount of radio wave absorption varies greatly depending on the type of sample material and also the coating thickness.