# Passive Harmonic RFID System for Buried Assets Localization
## 5 Discussion  
An estimation of the cost of the system (for medium-large quantities) is given in Table 2. The price of the tablet, smartphone, or PC for data acquisition has not been included, because the software (i.e., SDRsharp) can run on different platforms and does not have specific requirements. The cost of the reader is around two hundred dollars and the tag is about four dollars. The cost of the reader is significantly lower than a UHF reader (between $1000 and $2000). The cost of a tag is comparable to a UHF tag for metallic applications, however, specific Integrated Circuit (IC) should be designed in order to improve the integration and reduce cost. Figure 17: Screenshot of measurements with SDR# software: (**a**) spectrogram without the tag and with the tag, (**b**) cut of the spectrum without the tag, (**c**) spectrum with the tag. Figure 18: Comparison of tag measured spectrum response at 2\(f_{0}\) = 1732 MHz as a function of the offset frequency (\(\Delta f\)) for three configurations: (i) tag in the air, (ii) tag buried under dry soil, and (iii) tag buried under the wet soil (VWC 15%). In order to compare the proposed solution with the state-of-art, Table 3 compares several techniques proposed in the literature for buried utility localization. The proposed system is easy to use and the cost is significantly lower than that of GPR systems used for pipe localization that, in addition, require specialized operators to interpret the measurements. In order to improve the resolution, GPR systems for pipe detection commonly use antennas between 900 MHz and 2000 MHz, therefore, the detection depth depends on soil losses and water content as in the proposed system. An enhancement system based on resonators has been proposed in the literature [5], obtaining a detection depth of 0.6 m at around 900 MHz. Enhancement techniques for improving the detection of GPR signals based on antennas loaded with delay lines are proposed in the works of [17; 18], obtaining detection depths of 1.1-1.2 m in dry soils. Recently, surface acoustic wave (SAW) RFID tags at 915 MHz have been proposed in the literature [51], obtaining a detection depth of 1.3 m. In the work of [24], UHF RFID metal tags are used for manhole detection. The highest depth underground that is detected is 10 cm. Also, depths of about 10 cm are reached for tags embedded in bricks and mortar in the work of [29]. Special antenna designs for buried tags using air spacers as proposed in the work of [23], or antenna designs used in this work can reduce the detuning effect due to the soil. However, the higher receiver sensitivity in the harmonic system allows the detection at higher depths compared with UHF systems. Few experimental results have been found in the literature employing buried harmonic tags. Experimental results with a harmonic tag buried at 3 cm under the sand are given in the work of [42], transmitting 7 dBm at a 2.4 GHz ISM band. In the work of [17], a harmonic tag based on silicon PN diode at 150 MHz is proposed for detection of buried object, but only experiments in free space are presented. Harmonic tags based on a varactor diode at 400 MHz have been proposed for the underground detection in the work of [43], but again experimental results in free space have only been presented up to 76 cm. Several low-frequency (LF) RFID systems (125-135 kHz) have been investigated for the underground detection in the work of [52; 53]. However, their detection depths are often far from the 1.5 m obtained with the commercial system of 3M [20] at LF. This system exploits the noticeably less attenuation at LF compared with that at UHF or microwave systems to achieve this depth. In order to obtain higher detection depths or detection under ice, an active transponder must be used [14]. The main drawback of an active transponder is the finite lifetime of the batteries and the cost of the tag compared with passive RFID tags. Another potential application of the proposed sensor is smart floor applications for indoor mapping, localization, and guidance [29].