# Passive UHF RFID Tag for Multispectral Assessment
## 3 Results and Discussion  
### Antenna Characterization  
The antenna in Figure 3 is the typical dipole antenna based on the development kit recommended for the SL900A chip by the manufacturer (AG 2014), with final dimensions of the antenna arms of 5.5 mm in width and 79 mm in length. The dipole arms have been bent to optimize the occupied area. The input impedance of the dipole at 868 MHz is \((31.1+\mathrm{j}9.4)\)\(\Omega\). This dipole is designed to achieve the same real part of the impedance as the RFID chip \((31.1-\mathrm{j}286)\)\(\Omega\)[53]. The imaginary part is compensated with an SMD matching inductor series 3650 of 51 nH and a quality factor of 60 at 900 MHz (TE Connectivity, Ltd., Schaffhausen, Switzerland) placed on one of its arms. Antenna gain is 0.661 dBi, directivity is 2.401 dBi, and efficiency is 66.97% at the working frequency. These parameters have been obtained by EM simulation with Advanced Design System 2013 (Keysight Technologies Inc., Santa Rosa, CA, USA). Figure 5 compares the dipole antenna measured response with the simulated one. The obtained resonance value at 868 MHz is \(-12.5\) dB, whereas the simulated one is \(-13.6\) dB. Additionally, we observed a wider range of response. The maximum read range can be calculated using a Friis free-space equation [54]:  
\[\mathrm{range}_{\mathrm{max}}=\frac{\lambda}{4\pi}\sqrt{\frac{\mathrm{G_{tag }\;G_{reader}\;P_{reader}\;\tau\;PLF}}{\mathrm{S_{tag}}}} \tag{2}\]  
where \(\lambda\) is the wavelength, \(G_{\mathrm{tag}}\) is the tag antenna gain, \(G_{\mathrm{reader}}\) is the reader antenna gain, \(P_{\mathrm{reader}}\) is the effective power transmitted by the reader, \(S_{\mathrm{tag}}\) is the RFID chip sensitivity, \(\tau\) is the power transmission coefficient, and PLF is the polarization loss factor. In our case, the transmission power is 26 dBm and the reader antenna gain is 7 dBi at 868 MHz, according to the manufacturer. The PLF includes the polarization mismatch between the reader antenna (circular polarization) and the tag antenna (linear polarization), with a value of \(0.5\) (\(-3\) dB). The minimum threshold power necessary to activate the tag and answer the identification inquiries of the EPC protocol is \(S_{\mathrm{tag}}=-15\) dBm. However, extra power is required to drive the SFE [35] and power up the microcontroller unit. This power is collected from the reader's radiated EM field. Therefore, the chip sensitivity to read out a value of the photodiode current  
Figure 4: The 45\({}^{\circ}\)/0\({}^{\circ}\) geometry setup used to carry out diffuse reflectance measurements with the plain-coloured cardboards and the banana.  
is increased to \(-3.98\) dBm. Assuming ideal conditions (\(\tau=1\) and G\({}_{\text{tag}}=0.661\) dBi as obtained by the EM simulation) and considering the chip sensitivity to be \(-3.98\) dBm, the read range should be \(1.5\) m, according to Equation (2). The measured read range of \(1.1\) m for the RFID tag is slightly smaller than the simulated one, perhaps due to the non-ideal behavior of the printed conductive layer, reducing the tag antenna performance. To measure this range, the RFID reader antenna was attached to a tripod and placed in front of the tag at the same height. The measurements were taken in an anechoic chamber.