# Electromagnetic Soil Characterization for Undergrounded RFID System Implementation
## 4 Practical Verification of the Link Budget Model Using a Buried Metal-Backed RFID Tag Antenna  
In order to test the validity of the link budget analysis model, a simple practical test for a buried tag localization and data reading is made. In this experiment we used the tag antenna proposed in [15]. The tag antenna was fabricated using photolithographic method. This tag was designed to be backed by a metallic surface, as it is placed on the pipe surface. A photo for the fabricated tag antenna (before RFID chip mounting) is illustrated in Figure 6.  
The tag antenna port is a differential port and it is intended to be connected to MONZA3 chip [16], which has an input impedance of (32-j216)ohm. The proposed tag should have impedance close to the complex conjugate of the chip impedance. To measure practically the input impedance of the antenna, an imaging theory procedure explained in [17] is adopted. In this theory, the dipole antenna is divided into two symmetric parts, where each one of them is equivalent to a monopole antenna. This half is placed over a finite ground plane made of copper, and connected to the VNA to measure its input impedance as indicated in Figure 7. The main objective of this method is to use the unbalanced monopole antenna to create a balanced dipole antenna by this ground plane. The input impedance of the dipole can be evaluated from:  
\[Z_{\text{Dipole}}=2\times Z_{\text{monopole}} \tag{3}\]  
The input impedance of the fabricated tag antenna is shown in Figure 8. The antenna reflection coefficient is calculated using the formula stated in [15]. The measured reflection coefficient of the  
Figure 5: The calculated link budget analysis of different soil types. (**a**) Yellow sand, (**b**) Red sand, (**c**) Black soil, (**d**) Gray soil, (**e**) Black gravels, (**f**) White gravels, (**g**) Red/Orange gravels. Figure 6: The fabricated metal backed Radio Frequency Identification (RFID) tag antenna. proposed antenna shows very good performance around the operating frequency of 915 MHz. Figure 9 presents the reflection coefficient S\({}_{11}\) of the proposed tag antenna. Figure 8: The input impedance of the proposed tag antenna (real and imaginary). Figure 7: Practical measurement of the input impedance of the proposed tag antenna. Figure 9: The reflection coefficient S\({}_{11}\) of the proposed tag antenna. To perform the practical experiment, the tag antenna with the RFID chip is buried under a 40 cm layer of yellow sand contained in the center of a plastic container as shown in Figure 10. The plastic container has the dimensions of 60 cm \(\times\) 40 cm \(\times\) 50 cm. For tag localization and data collecting, we built the system presented in Figure 7.0 This measurement system is based on the Impinj speedway R420 system for RFID reading [18]. This system, shown in Figure 11, is connected to a log periodic antenna with a gain of 7 dBi at 915 MHz to operate as a reading antenna. The RFID reading system is connected to a PC through a network router. Finding an exact location is easier than ever before. The reader transmits a signal to the buried tag and the tag returns a signal to the reader. The position is confirmed, even through the toughest soil conditions. The soil sample is assumed to be dry, and the experiment is done at room temperature. The Impinj tag reading system R420 can provide a transmitted power up to 30 dBm. The reading antenna is mounted above the sand container at a certain distance \(R\). The transmitted power travels from the reading antenna through the air then penetrates the soil to hit and activate the buried RFID tag. The tag starts to send the information back to reading system through both soil then air. The expected received power at the reading system can be calculated as:  
Figure 7.1  
where FSPL is the Free Space Power Loss (in dB) and it can be calculated from [19]:  
Figure 7.2  
Figure 7.3 The RFID reader system setup. Figure 7.4 The sand tank used. (**a**) before burying the tag. (**b**) after burying the tag.