# Passive Harmonic RFID System for Buried Assets Localization
## 6 Conclusions  
A passive harmonic tag has been presented for buried assets localization. In order to identify different utility services underground and facilitate the detection, the tag is modulated using a low-power oscillator that is fed from RF energy harvesting. The theoretical read range is estimated from the attenuation model found in the literature. The experimental results show that the harmonic tag reaches read ranges up to 60 cm under the soil with a 25% water content. A low-cost harmonic reader prototype based on an inexpensive SDR receiver was also developed.  
[PERSON] and [PERSON] performed the experiments, analyzed the data, and wrote the article. [PERSON] was the lead developer for the hardware used and the investigation. [PERSON] contributed to the system fabrication. [PERSON], [PERSON], and [PERSON] contributed to the writing of the review.  
This work was funded by the H2020 Grant Agreement 645771-EMERGENT and the Spanish Government Project TEC2015-67883-R.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Reference** & **Technique** & **Deep Underground** & **Comments** \\ \hline
[5] & GPR (time domain) & 0.6 m & Detection of pipes using tag resonators with GPR (GSSI model) at 900 MHz with tag resonators. \\ \hline
[17] & GPR (time domain) & 1.1 & Detection of a dipole antenna with a delay line buried using GPR (Sensors model) operating at a center frequency of 900 MHz in a dry sand. \\ \hline
[18] & GPR (time domain) & 1.2 & Detection of FAT dipoles with delay lines at 900 MHz with VNA in dry soil. \\ \hline
[19] & GPR (time domain) & 0.45 m & Detection of resonant tags with GPR (GEOTCH OKO-3 model) at 700 MHz in dry sand \\ \hline
[51] & SAW RFID 915 MHz & 1.3 m & SAW RFID at 443 MHz and 915 MHz in dry soil transmitting 30 dBm. \\ \hline
[20] & LF RFID & 1.5 m & Electronic marking system (EMS) by 3M. \\ \hline
[52] & LF RFID (134.2 kHz) & 0.24 m & Passive integrated transponder (PIT) for tracking soil movements. \\ \hline
[53] & LF RFID (135 kHz) & NA & Underground pipeline location. \\ \hline
[28] & HF RFID & 8 mm & Smart floor application with standard 13.56 MHz tags under the tiles. \\ \hline
[24] & UHF RFID & 10 cm & UHF RFID metal tags are used for manhole detection. \\ \hline
[29] & UHF RFID & 10 cm & Metal UHF tags embedded in bricks and mortar. \\ \hline
[27] & IR UWB radar & 1 cm & Detection of metallic reflectors under the floor for smart floor application with UWB radar. \\ \hline
[42] & Harmonic tag at 2.4 GHz & 3 cm & Harmonic tag with a reader transmitting 7 dBm. \\ \hline
[43] & Harmonic tag at 400 MHz & NA & Experimental results in free space up to 0.76 m at 400 MHz using a varactor based harmonic tag. \\ \hline
[17] & Harmonic tag at 150 MHz & NA & Harmonic tag at 150 MHz (preliminary experiments in free space). \\ \hline This work & Harmonic UHF & 30–60 cm & Harmonic tag with harvester and modulator. \\ \hline
[14] & Active tag at 315 MHz & NA & 500 mW transmitter under the ice. \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of different techniques for detecting underground objects. GPR—ground penetrating radar.