# Passive Harmonic RFID System for Buried Assets Localization
## 3 Read Range
### Tag Read Range  
Figure 13 shows read range simulations as a function of the received power for dry and wet soil (case 2: 60% sand, 20% clay). The volumetric water content (VWC) in the case of wet soil is considered the worst case, close to the saturation (30%). Considering tags with a threshold power of \(-20\) dBm, the link budget is limited by the downlink (received power at the reader). Table 1 summarizes the main parameters used in calculations.  
These results show that the read range underground depends on the water content. In the case of wet soil, the read range is about 0.7 m for a receiver sensitivity of \(-110\) dBm (considering a 10 dB of margin over the noise floor). Indeed, it is different than traditional UHF RFID applications in the air, where the read range is limited by the threshold tag power.  
Although the attenuation of the soil at the second harmonic is higher than at the fundamental frequency, better read range performance is obtained for the harmonic tags because the receiver sensitivity is better than that of conventional UHF readers (\(\approx\)\(-73\) dBm).  
Measurements have been performed with the instruments shown in Figure 6 and the tag buried underground. To this end, the tag was placed in a cylindrical plastic container with a diameter of 0.6 m filled with soil with known water content. This container was placed inside a hole in the ground. The volumetric water content (VWC) is determined from the ratio between the soil and water introduced in the container. The antennas of the reader were placed 10 cm away from the ground. Figure 14 shows measurements of the received power (at frequency \(f_{0}+f_{m}\)) with the tag buried 60 cm underground for two volumetric water contents (5% and 25%). The first water content is close to the residual value of dry soil and the second is close to the saturation value for the soil used. This figure also compares a measurement of the tag in the air at the same distance from the antennas (70 cm) and the measurement of the noise floor (measurement without tag). The equivalent isotropic radiated power (EIRP) is swept from 19 dBm to 33 dBm, as shown in Figure 7a, and the frequency is swept in the European RFID band (865-869 MHz) for the case of EIRP of 33 dBm. The measured power level with the spectrum analyzer is referred to the antenna input after subtracting the gain of the chain of amplifiers and filters connected to the input of the spectrum analyzer to reduce the noise factor of the instrument. The effect of the attenuation of the soil can be observed, especially for high volumetric water content. An attenuation of about 19 dB (or 32 dB/m) and 25 dB (or 42 dB/m) between the case of the tag and the air and the tag buried for 5% and 25% VWC, respectively, is found. These values of attenuation are within the order of magnitude obtained in Figure 11. A difference of about 10 dB over the noise floor for the case of 25% of VWC is observed, which leaves a certain margin to detect the tag if there are variations in attenuation caused by changes in the composition of the soil. A nearly flat response is obtained in all frequency bands (Figure 14b).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline Transmitter EIRP & 2 & W \\ Frequency\(f_{0}\) & 865 & MHz \\ Tag antenna gain, \(G_{\text{tag}}\)\(\text{at}f_{0}\) & 6 & dB \\ Tag antenna gain, \(G_{\text{tag},\,2}\) at \(2f_{0}\) & 6 & dB \\ Reader antenna gain, \(G_{R}\) & 6 & dB \\ Volumetric water content VWC for the wet soil & 30 & \% \\ Distance to ground, \(d_{g}\) & 10 & cm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters used in the read range simulations. EIRP—equivalent isotropic radiated power; VWC—volumetric water content.  
Figure 13: Simulated received power at tag and reader for dry (solid lines) and wet soil (dashed lines).