# High-Density 3D Printable Chipless RFID Tag with Structure of Passive Slot Rings
## 3 Design and Fabrication of 12-bit Chipless Tag Printed on F4 BM Substrate
To correctly understand its influence, the simulation is conducted with different thicknesses of the substrate, including 0.5, 1 and 1.5 mm, respectively. The corresponding resonant frequencies are listed in Table 2. It can be seen that there is slight difference on the slot ring with same size but with different substrate thickness. But the coding qualities are all fine on the three substrates. A variety of tags, with codes (11111111111), (1010101010), (010101010101), (00000011111) and (0100000010), are 3D printed by adopting the above design rules. Figure 11 shows the photos of the 3D printed tags. To get a clear picture on the actual size, a coin is also included in Figure 11. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Thickness** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** \\ \hline h = 0.5 mm & 2.14 & 2.31 & 2.6 & 2.71 & 2.97 & 3.29 & 3.68 & 4.13 & 4.75 & 5.58 & 6.72 & 8.92 \\ h = 1 mm & 2.12 & 2.26 & 2.44 & 2.63 & 2.9 & 3.21 & 3.57 & 3.97 & 4.52 & 5.26 & 6.29 & 8.39 \\ h = 1.5 mm & 2.07 & 2.19 & 2.35 & 2.56 & 2.83 & 3.15 & 3.53 & 3.93 & 4.48 & 5.21 & 6.25 & 8.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Resonance frequency of different substrate thickness. Figure 10: The spectra corresponding to \(\varphi\) from 0\({}^{\circ}\) to 60\({}^{\circ}\) when \(\theta=20^{\circ}\). The substrate is F4 BM. For example: Figure 11a is a reference tag. In Figure 11b, some slot rings are removed. The RCS spectra characteristic of those removing resonators are disappeared. The RCS spectra characteristic of the existing resonators still exist and its logic state is \"1.\" This combination of presence and absence of rectangular slot ring resonators constitutes the encoding of the chipless tag. To verify the above analysis experimentally, the actual measurement is conducted in the anechoic chamber. Figure 12 shows the facilities to characterize the fabricated tags, including vector network analyzer (VNA) (model: Agilent E8362B) and two horn antennas (model: LB-8180-NF and LB-10180-NF) acting as transmitting and receiving antennas. The VNA can replace the chipless RFID reader in the laboratory. The distance between Tx and Rx horn antennas connecting the VNA is 10 cm. The chipless tag is fixed on the foam of the frame. The distance between the two horn antennas and the chipless tag is 50 cm. The Ref [35] indicated that an accurate RCS value of the tag can be obtained using Equation (3):  
\[\sigma^{tag}= \begin{bmatrix}\frac{s_{21}^{tag}-s_{21}^{iso}}{s_{21}^{ref}-s_{ 21}^{iso}}\end{bmatrix} \cdot\sigma^{ref} \tag{3}\]  
Figure 11: Photograph of the Rectangular slot ring tags with different IDs. (a) ‘11111111111’, (b) ‘10101010101010’, (c) ‘000000111111’, (d) ‘010101010101’, (e) ‘01000000010’. Figure 12: Block diagram of the measurement setup. Distance between two horn antennas is 10 cm and distance between horn antenna and tag is 50 cm. where \(\sigma^{ref}\) is the complex RCS value of the metallic reference plate obtained with analytical formula or simulation. Firstly, the background items that produce static noise is removed. \(S_{21}^{iso}\) is the isolation of two-horn antennas measured without any tag. \(S_{21}^{ref}\) is measured when the identification item is present. Finally, \(S_{21}^{tag}\) is measured when the tag is adhered on the identification item. Figure 13a-e show the characterization results corresponding to the tags with ID (1111111111), (101010101010), (000000111111), (010101010101) and (01000000010), including both simulation results and experimental results. For the tag coded 1111111111111, its result is shown in Figure 13a. The resonant frequency points include 2.12, 2.26, 2.44, 2.63, 2.9, 3.21, 3.57, 3.97, 4.52, 5.26, 6.29 and 8.39 GHz. Figure 13a shows the measurement and simulation results of the tag ID111111111111. When the 12 resonators are present, the coding states are all \"1.\" That is, there are 12 notches in the spectral structure.