# Flexible UHF RFID Tag for Blood Tubes Monitoring
## 4 Fabrication and Experimental Characterization of the Proposed Tag Antenna
### Reading Range  
The critical parameter that determines the performance of an RFID tag antenna is the reading range [7], which corresponds to the maximum distance at which the RFID reader can detect the modulated backscattered signal and successfully identify the tag. The reading range \(r\) can be obtained by means of the Friis free-space formula [2]:  
\[r=\frac{\lambda_{0}}{4\pi}\sqrt{\frac{P_{r}\ G_{r}\ G_{tag}\ \tau}{P_{c}}}\, \tag{3}\]  
where \(\lambda 0\) is the free space wavelength, \(Pr=1\) W is the power transmitted by the reader, \(Gr=4.5\) dBi is the gain of the reader antenna, \(Gtag\) is the gain of the tag antenna, and \(\tau\) is the power transmission coefficient defined in the previous subsection. Besides, \(Pc=-17\) dBm (NXP UCODE G2 XL IC chip) is the minimum power threshold needed to provide enough power to the chip (sensitivity). The simulated and measured reading ranges obtained for the proposed tag attached to a tube in the frequency range from 0.8 GHz and 0.9 GHz are shown in Figure 10. The measurements include the results obtained for the smallest and largest tube standards manufactured by the Becton Dickinson (BD) company. It can be observed that the measurements are in good agreement with predictions. The measured reading range reaches a peak value of 2.2 m at the frequency of 867 MHz when the tag is attached to the smallest size tube full of blood, while the largest size tube has a peak value of 2.3 m at 860 MHz. At 867 MHz, the largest size tube presents a reading range of 2.25 m, which is similar to the measured value (2.2 m) for the smallest size tube. For both tube sizes, the reading ranges drastically decrease when the tubes are empty. This confirms that the proposed design can be applied to different tube standards, and that it is possible to detect the tag at reasonable distances when the tube is full of blood. In addition, in both cases the reading range drastically reduces for empty tubes. This behavior can be used to reduce the amount of information collected in the tracking system, coming from empty tubes.  
Figure 9: Simulated and measured realized gain \(G_{r,tag}\) for the proposed tag antenna attached to full and empty blood tubes.  
To check the performance of the proposed tag attached to a tube full of blood in a natural environment, measurements out of anechoic chambers have been realized in two different scenarios. The first scenario consists in the measurement of a single individual tag, while the second one is the measurement of the tag located behind a group of four tubes full of blood. Figure 11 depicts the reading ranges for these two scenarios. As it can be seen (Figure 11), there is no substantial change in the performance with respect to previous results (Figure 10) obtained for an individual tag by means of the Voyantic Tagformance measurement system. The reading range reaches the same peak value of 2.2 m, but shifted to a higher frequency of 875 MHz. On the other hand, the results of the second scenario confirm that a group of filled tubes located in the vicinity of the tag does not greatly affect its performance. In this test, the maximum reading range and the resonant frequency slightly decrease (with respect to the tag placed alone) to 2.1 m and 870 MHz, respectively.  
Figure 11: Measured reading ranges for the proposed tag attached to a tube full of blood in two different scenarios: Single tag placed alone, and tag located behind four filled tubes.  
Figure 10: Simulated and measured reading ranges obtained for the proposed tag attached to two standards (\(\emptyset\) = 13 mm and \(\emptyset\) = 16 mm) of full and empty blood tubes.