# Passive Harmonic RFID System for Buried Assets Localization
## 4 Low-Cost Harmonic Reader  
Unfortunately, harmonic readers are not yet commercially available. A proof-of-concept reader based on a generator with a power amplifier and a spectrum analyzer is implemented. A low-cost implementation based on an SDR device (RTL-SDR based on chipset R820T2 RTL2832U) and free open source SDR software (SDRsharp) is presented in Figure 15. A photograph of the system is shown in Figure 15b. The transmitter is based on a PLL synthesizer where a low-cost 868 MHz ISM band FM transmitter (FM-RTFQ1-868) from Telecontrolli is employed. A chain of amplifiers composed by Minicircuit Gali-84+ and Broadcom ALM-31122 are used to reach maximum output power (26 dBm). An attenuator is included to avoid the saturation of the amplifiers and to control the output power. Custom combline passband filters were designed using microstrip technology. A bandpass filter centered at 868 MHz is added to filter spurious emissions at the second harmonic. The receiver is composed of a bandpass filter at the second harmonic and two amplifiers. The first amplifier (Minicircuits GALI-84+) presents a high compression point to avoid the saturation of the chain. The second block amplifier is the Minicircuits ERA-3 SM. The microstrip combline filters are designed to be connected both before and after the transmitter/receiver antenna. A dual-polarized RFID antenna (model FEIG gain 8.5 dBi) is used in transmission, as well as a custom circular-polarized patch antenna (a circular path with a hybrid) with gain 6 dBi in reception. A photograph of the system mounted on a trolley is shown in Figure 16. In order to control the soil's water content, the tag is submerged in a plastic container within a hole in the ground. The water content of the soil of the container is known. The tag is placed again at a distance of 70 cm from the TX/RX antennas and buried 60 cm under the ground. Figure 17 shows a screenshot of the SDRsharp corresponding to the  
Figure 14: (**a**) Measured tag response under different conditions (tag in the air, buried in the soil, and without tag) as a function of the transmitted power (equivalent isotropic radiated power—EIRP) and a function of the input frequency (**b**) for the EIRP of 33 dBm.  
measurement of the tag's response when the prototype of the reader passes over it. When the tag is out of range, a central peak caused by the parasitic coupling appears in the center of the spectrum (Figure 17b), whereas when the antennas are on top of the tag, a couple of peaks at the modulation frequency arise around the central frequency (Figure 17c). A version of SDRsharp is available also for Android devices; therefore, the software can run on a small tablet or smartphone in order to reduce the weight of the system in a future potential commercial product.  
Figure 16: Photography of the system and measurement setup.  
Figure 15: Block diagram (**a**) and photography (**b**) of the low-cost reader implemented.  
Finally, Figure 18 shows a comparison between the tag responses using the proposed low-cost reader for three configurations. The comparisons between previous measurements in Figure 18 demonstrate the limitation of this reader because the PLL presents a higher noise level. This affects the measurements performed under wet soil because the tag cannot be detected when the water content is higher than 15%.