# Flexible UHF RFID Tag for Blood Tubes Monitoring
## 3 Design of the Tag Antenna
### Optimization
For this inductive behavior, currents can flow from one pad to the other, and the resonant frequency shifts to lower values, as shown by the black line in Figure 3. Due to the high value of the relative permittivity of the blood, the resonant frequency of the antenna becomes too low, and the input impedance exhibits values which are too large for both the real and the imaginary parts at the frequency of interest. This shift in resonant frequency could be compensated by reducing the length \(L_{1}\) of the meandered substrate arms. However, we have verified that the size reduction has a strong negative impact on the efficiency of the resulting tag antenna. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** \\ \hline \(L\) & 49.5 & \(s\) & 5 \\ \(W\) & 15 & \(g\) & 2.25 \\ \(L1\) & 18.75 & \(n1\) & 5 \\ \(W1\) & 8.5 & \(n2\) & 3.5 \\ \(L2\) & 18.25 & \(d\) & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optimized parameters of the proposed tag antenna (Figure 2b) with the NXP UCODE G2 XL IC chip. Figure 3: Simulated input impedance \(Za\) of the designed tag antenna (Figure 2b) for different gap values \(g\). (**a**) real part of \(Za\); (**b**) imaginary part of \(Za\). The other dimensions are defined in Table 1.  
An important conclusion can be draw from this study. While in many applications the use of inductive tags is preferred, since it leads to smaller sizes, this is no longer the case for this application, where the tag must operate in the presence of blood. Due to the high real relative permittivity value of the blood, the inductive tag tends to be very small, therefore leading to low efficiencies [8]. However, the capacitive configurations, as the one proposed in Figure 2b with \(g>0\), lead to tags with higher radiation efficiencies, still exhibiting small sizes. This is due to the natural size reduction effect that occurs when the tag operates near to the blood. A second benefit of this capacitive design is its high sensitivity to the real relative permittivity value of the blood. This high tag sensitivity could be used to reduce non-relevant information collected from empty tubes. In a similar way, the tag could be used as a sensor to monitor the amount of blood contained in a specific tube, as it will be demonstrated in Section 5. In the previous study, it has been observed that the gap \(g\) between pads can be used to control both parts (real and imaginary) of the input impedance \(Za\) of the tag antenna. However, for a practical design, it is needed to be able to adjust the real and imaginary parts independently. To achieve this, it is necessary to add a second design parameter. For the structure proposed in Figure 2b, the fine tuning of the input impedance \(Za\) of the tag antenna can be performed by means of the parameter \(W1\). Figure 4 presents the variation of both real and imaginary parts of the input impedance \(Za\) of the tag antenna as a function of \(W1\) and the frequency (0.5 GHz-2 GHz), when all other parameters are kept constant (Table 1). It can be observed in Figure 4 that the parameter \(W1\) can also control the resonant frequency of the antenna. Smaller values of \(W1\) lead to higher resonant frequencies, since the effective length of the pad is reduced. Details on the variation of the real and imaginary parts of the input impedance \(Za\) of the antenna with the parameter \(W1\) in the frequency band of interest (0.86 GHz-0.96 GHz) are shown in the insets inside Figure 4a,b. As it can be seen in the insets, the impact of this parameter (\(W1\)) in the real part of the input impedance is much lower, as compared to the influence introduced in the imaginary part. This behavior allows to optimize the values of the real and imaginary parts of the input impedance \(Za\) of the tag antenna, with respect to those of the impedance \(Zc\) of the IC chip, by following an iterative procedure involving the gap \(g\) and the parameter \(W1\).