# Research on Penetration Loss of D-Band Millimeter Wave for Typical Materials
## 2 Materials and Methods  
The cause of wireless path loss is the radiation diffusion of electromagnetic waves and the channel characteristics in the transmission path, so that the received power is smaller than the transmitted power. The free-space path loss model describes the channel propagation characteristics in an ideal propagation environment. Its expression is given as  
\[FSPL(d,f)=20 log_{10}(4\pi df/c) \tag{1}\]  
where  
\(d\) is the wireless transmission distance;  
\(f\) is the transmission frequency;  
\(c\) is the speed of light. It can be observed from the above formula that the free space path loss is only related to the transmission distance \(d\) and the transmission frequency \(f\). When the transmission distance or transmission frequency doubles, the loss is increased by 6 dB. The free-space propagation model is suitable for the wireless environment with an isotropic propagation medium (such as a vacuum), which does not exist in reality and is an ideal model, but the air medium is similar to an isotropic medium. Moreover, atmospheric attenuation is closely related to altitude, air pressure, temperature, and water vapor density above the Earth. Figure 1 shows atmospheric absorption for free-space paths at sea level (z = 0 km) and at 10 km above sea level under dry conditions (water vapor density w = 0 g/m\({}^{3}\)) and standard conditions (w = 7.5 g/m\({}^{3}\)). As shown in Figure 1, it is a graph of signal attenuation per kilometer as a function of frequency. It can be observed that the attenuation of electromagnetic wave signals of different frequencies in water vapor and oxygen is different, and there are absorption peaks at several frequency points in the relationship between the resonant frequencies of water vapor and oxygen, and the D-band is just between the two absorption peaks, ranging from 0.01 dB/km to around 2 dB/km. So, the D-band is suitable for long distances up to 100 m millimeter wave communication. In the real environment, the path loss is related to the presence or absence of occluders, the type of occluders, the thickness of the occluders, as well as the angle of the sender and receiver. The FSPL model does not reflect the actual propagation characteristics. For obstacles of different materials in the transmission path, two modeling schemes of  
Figure 1: Free-space path atmospheric absorption for dry and standard environments at z = 0 km and z = 10 km. three rays and four rays are used in Ref. [19]. With the proposed scheme, the transmission model of the D-band millimeter-wave signal with obstacles at a distance of 100 m can be simplified, as shown in Figure 2.  
The attenuation model given in Figure 2 is adaptive for the different materials with different thickness. For example, the relative permittivity and permeability of iron are very large, and the D-band millimeter-wave signal will have a very large transmission loss under the shielding of the steel plate. It can be considered that the D-band millimeter-wave signal cannot penetrate the steel plate, so the three-ray diffraction model is applied to the steel plate shielding. For relatively thin insulator materials, such as 5 mm-thick glass and 3 mm-thick wood, their penetration loss is not very large, i.e., between 2 dB and 5 dB, so the four-ray method is used for D-band modeling [20]. Four rays include three edge diffraction paths and one transmission path. Unlike thicker insulating materials, such as 5 cm-thick slate and 1.75 cm-thick wood, D-band mmWave signals also have very large transmission losses. Therefore, we also believe that the D-band mmWave signal is impenetrable in this case, while the three-ray diffraction method is suitable for simulating thicker insulator materials. The penetration performance of millimeter waves is very poor with increasing frequency. Therefore, in our experiments, the penetration loss of millimeter waves in the 140-160 GHz frequency band is large, and it is also influenced by the dielectric constant, thickness and other parameters of the blocking material.