# Fully Printed Flexible Single-Chip RFID Tag with Light Detection Capabilities
## 3 Results and Discussion  
### Antenna Performance  
Figure 4 presents the printed RFID tag including the external components, labelled for better understanding. Final dimensions of the inkjet-printed dipole antenna arms are 5 mm width and 87 mm length. The dipole arms have been bent twice to optimize the occupied area. Figure 5 shows the S-parameter response of the dipole antenna, both simulated and measured responses. The measured resonant frequency fulfills the simulations results, although we found a difference on antenna impedances. The measured input impedance is (55 + j23) \(\Omega\), whilst he antenna impedance obtained by simulation is (31.1 + j23) \(\Omega\). This difference could be associated with the parasitic resistance of the SMD inductor and with the printed silver electrical behavior. In the Electromagnetic (EM) simulation, a silver metallic layer is modeled as bulk material, whilst the printed patterns differ from this ideal assumption, causing increases in the ohmic loss [21]. Therefore, an extra loss would be introduced in the RF link between reader and tag due to this mismatch. This drawback could be overcome optimizing the antenna design through new EM simulations or using a different matching network topology with more versatility such as a T-Network. Finally, the chosen SMD inductor has an inductance of 47 nH, size of 0603, and a quality factor of 60 at 900 MHz (SRF = 1.9 GHz). The maximum read range, according to the theory of communication in RFID systems [20; 27; 28], is calculated using the following Equation (2):  
\[range_{\text{max}}=\frac{\lambda}{4\pi}\sqrt{\frac{PLF\cdot G_{tag}\cdot G_{ reader}\cdot P_{reader}\cdot\tau}{s_{tag}}}, \tag{2}\]  
Figure 4: (**a**) image of the tag with all its elements labelled; (**b**) image of the tag. where Stag is the RFID chip sensitivity; the minimum received power level to activate the tag is \(-15\) dBm for the chosen RFID chip. However, in this particular case, the harvested energy from the EM field radiated by the RFID reader is necessary to retrieve the sensor information. The necessary power to drive the sensors comes from the internal RF rectifier of the RFID chip. According to the manufacturer, operating at 1.5 V in passive mode requires 200 \(\upmu\)A to power up the chip and the internal ADC converter in charge of processing the acquired signals. Therefore, we have defined a new tag sensitivity of \(-2.21\) dBm, which is calculated defining the RF rectifier efficiency of \(-40\%\). Gtag refers to the tag antenna gain, P reader is the effective radiated power by the reader and G reader is the reader antenna gain. Tag antenna parameters obtained by EM simulation are the following: gain is \(-0.98\) dBi, directivity 1.67 dBi and efficiency 54.47% at the working frequency. The reader transmission power is 26 dBm and the reader antenna gain is 7 dBi at 868 MHz, according to the manufacturer. The polarization mismatch between the reader antenna (circular polarization) and the designed tag antenna (lineal polarization) is also included. This is represented as the polarization loss factor (PLF) in (2) and its value is 0.5 (\(-3\) dB). The factor \(\tau\) takes into account the losses related to the mismatch between the chip and the antenna. The maximum read range has been only 80 cm whilst the estimated one is 1.4 m. The estimated maximum read range assumes a perfect matching between chip and antenna (\(\tau=1\) in Equation (2)), but, as previously stated, the different real parts of antenna and RFID chip impedances causes a mismatching (\(\tau=0.72\) in Equation (2)) that reduces the read range to 1.2 m. The reasons of the reduced read range, apart from the impedance mismatch, are related to the poor antenna gain. Both limitations are caused by the low thickness obtained with printing one single layer, according to the model presented in [21], below 0.4 \(\upmu\)m. Several strategies can be followed to overcome this limitation like printing several layers or increasing the printing resolution [29].