# High-Density 3D Printable Chipless RFID Tag with Structure of Passive Slot Rings
## 4 Discussion
\begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \multicolumn{1}{c}{**Structure**} & \multicolumn{1}{c}{**Excitation**} & \multicolumn{1}{c}{**Harmonic**} & \multicolumn{1}{c}{**Ground**} & \multicolumn{1}{c}{**Coding Density**} & \multicolumn{1}{c}{**Capacity (bits/GHz)**} & \multicolumn{1}{c}{**Range (cm)**} \\  & & & & **(bi/cm\({}^{2}\))** & & **(bi/cm\({}^{2}\))** & \\ \hline C-shaped slot [26; 40] & limit & 3 th & no & \textgreater{}7 & 1.45 & 50 \\ C-shaped microstrip [41] & limit & 3 th & no & 3.3 & 0.46 & 65 \\ Circular slot [25] & no & no & no & 4.18 & 2 & 20 \\ Circular microstrip ring [42] & no & no & no & 2.1 & 2 & 40 \\ Rectangular microstrip ring [38; 39] & no & 5 th & yes & 0.49 & 1.25 & 50-55 \\
3D-printed rectangular slot ring (this work) & no & 5 th & no & 0.98 & 1.9 & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparisons of different structures’ characteristic. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Kind**} & \multicolumn{1}{c}{**Structure**} & \multicolumn{1}{c}{**Ground**} & \multicolumn{1}{c}{**Notch depth**} & \multicolumn{1}{c}{**Manufacture**} & \multicolumn{1}{c}{**RCS**} \\  & & & & & **Amplitude Consistency** \\ \hline Costa’s & Rectangular & yes & \(-18\) dB & etch & poor \\ This work & Rectangular & no & \(-20\) dB & 3D printing & good \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The Comparison between [PERSON]’s and this work. significant spectral characteristics on different media under different dielectric thicknesses. In summary, the chipless tag of the rectangular slot ring resonator proposed in this paper has the following advantages. This chipless tag is printed by the advanced 3D printer with the high processing precision. The encoding is not limited by the second harmonic, the third harmonic and the forth harmonic. So, the coding frequency band is wide. The electric field can be incident from any direction, without limitation on the orientation. The ground plate is not required on the back side. So, the full-printing can be realized on any substrates. It has large coding capacity and the long reading distance. We also demonstrate the feasibility of our design on other flexible substrates, such as paper and polyethylene terephthalate (PET). The key parameters of substrates are shown in Table 5. The simulation results are show in Figure 16. The parameters of paper include the thickness 1 mm, the relative dielectric constant 2.25 and the loss tangent 0.045. The parameters of PET used in simulation include the following: The thickness 1 mm, the relative dielectric constant 3.4 and the loss tangent 0.07. The spectra signature of the paper substrates can show the coding capability of the tag clearly. The corresponding resonant point of the largest rectangular slot ring in Figure 16b is unclearly and the other resonant points are still show correctly coding state. It can produce the coding error, therefore, the structure size of chipless tag must be adjusted. Figure 17 shows the simulation results of the designed 12-bit reference tag with coding (111111111111) on substrates with different relative dielectrics. Figure 17a,c,d show that the encode capacities are 1.84 bits/GHz, 2.14 bits/GHz and 2.35 bits/GHz when the relative dielectrics are 3, 5.5 and 6, separately. This means that the narrower operation band corresponds to the substrate with larger permittivity, which is accompanied by larger encode capacity. Figure 17b shows the result on FR4 substrates. The resonance point of the highest frequency is 7.54 GHz while the lowest frequency is 1.54 GHz. The encoded capacity is 2 bits/GHz. Figure 16: The simulation spectra of the twelve rectangular slot rings when the substrates are paper and PET, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Substrate** & **Relative Dielectric Constant** & **Loss Tangent** & **Thickness (mm)** \\ \hline F4 BM & 2.2 & 0.0007 & 1 \\ Paper & 2.25 & 0.045 & 1 \\ PET & 3.4 & 0.07 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameters of dielectric materials.