## 2 Background
The method of coupling radio waves between the reader and the tag is classified into near-field and far-field [13]. The subject of analysis in this study belongs to the far-field area. The advantage of far-field coupling is the absence of distance limitation and the gradual attenuation of the intensity of electromagnetic waves. UHF RIFD uses various types of antennas, such as I-tag, Squiggle-tag, M-tag, and X-tag [14]. According to [PERSON] et al. [15], the communication distance increases as the wavelength and transmission power, the reader's antenna gain, and the tag antenna gain increase, whereas the communication distance decreases as the power for the tag operation increases. The power transmission efficiency is the best when the impedance of a chip and an antenna are the same, and the performance chart was established by studying the correlation between antenna impedance, chip impedance, and communication distance [16]. According to the Friis free-space formula, communication distance R can be expressed as Equation (1). It can be simplified and expressed as \(RSS\) as Equation (2) [17]. \[\mathrm{R}=\frac{\lambda}{4\pi}\sqrt{\frac{P_{t}G_{t}G_{r}\tau}{P_{th}}} \tag{1}\]  
where \(\lambda:\) wavelength, \(P_{t}\): the power delivered to the terminals of an isotropic transmit antenna [dB], \(G_{t}\): the gain of the transmitting antenna in the direction of the receiving antenna, \(G_{r}:\) the gain of the receiving antenna in the direction of the transmitting antenna, \(P_{th}:\) the minimum power required for RFID tag operation, and \(\tau:\) power transmission coefficient  
\[RSS=-20\ \log_{10}\ R-20\alpha\log_{10}e+C[\mathrm{dBm}] \tag{2}\]  
where \(\alpha:\) attenuation that consists of conductivity, permeability, and permittivity. \(RSS\) is a unit of dBm that stimulates signal strength [18]. The higher number means a stronger signal strength. RSSI (Received Signal Strength Indicator) is an indicator of \(RSS\) and is a relative value of RSS defined by each chip supplier. In general, a reader that complies with the regulations of the Federal Communications Commission (FCC) can emit a signal with a strength of 30 dBm, while a typical tag's RSSI value can range from \(-30\) to \(-85\) dBm. According to one study on the UHF RFID reaching distance using a standard reader antenna, the reaching distance in the air may be significantly increased as the power increases. Moreover, in the UHF band, the reach performance was the best in the range of 960 to 970 MHz [13]. Various research results analyzed the factors affecting the reaching distance through simulation. Based on the analysis results of the influence of objects installed near the tag, the reading distance of the tag was shortened when the metal was present within 20 mm and the water was present within 30 mm [14]. According to [PERSON] et al. [15], the absolute distance difference affects the signal intensity rather than the change in the medium between the transmitting and receiving units. Five types of UHF RFID tags were installed on the packaging surface and 5 cm below the packagingto compare the signal attenuation effects of asphalt media, and 63% and 50% attenuation were confirmed in the two types of tags [19]. According to the results of the previous study, there were only a few studies on RFID signal attenuation based on medium. In particular, there is no such study in the fields of the ground or road pavement. In the field of GPR, there are many related research cases because it is directly related to media. The factors affecting the attenuation of electromagnetic waves are permittivity, conductivity, and permeability [20]. [PERSON] [21] attempted to measure the permittivity and conductivity of the soil. The fundamental electromagnetic characteristics of soil, i.e., permittivity and conductivity, were analyzed for frequency, water content, and soil density. Previous studies were conducted on conductivity, permittivity, and signal attenuation by material [22].