# High-Density 3D Printable Chipless RFID Tag with Structure of Passive Slot Rings
## 4 Discussion  
One of the merits of our design is the coverage of the 3D printing conductive material on the surface. To get a clear comparison with other similar work, the schematic pictures of our design and the strip structure [38; 39] are shown in Figure 15. Our work is illustrated in Figure 15a. [PERSON] et al. [38; 39] used rectangular structure as resonator to design chipless tag, which structure is shown in Figure 15b. In our design, the coverage of the deposited conductive structure on the substrate is above 95%. On the contrary, the coverage of the strip structure [38; 39] is below 5%. It can be seen that [PERSON]'s structure used rectangular microstrip as resonant part. Most of conductor on surface was etched away. Key parameters comparison of [PERSON]'s and this design are shown in Table 3.  
Figure 14: Statistic results on the distribution of the errors of the twelve resonant points. Figure 15: (**a**) Schematic of the designed tag, with top view and cross section, showing slot ring structure; (**b**) The structure proposed by [PERSON] [38; 39], using microstrip as resonator, with ground conductor on backside. For our design, as shown in Figure 15a, the tag is printed on the surface area. The resonant property can be observed clearly although no back-metal included. This is very unique when it is compared with other groups' results. For other results, back-metal is necessary to keep resonant property. On this sense, we believe the designed tag in this paper shows good resonant property including high sensitivity, narrow bandwidth, no interference, RCS amplitude consistency and fixed resonance frequency point. In our design, the depth of the spectrum notch is responsible for the high sensitivity. The resonant bandwidth determines its spectrum utilization. The steepness and smoothness of notch determines the small interference. The fixed resonant frequency point determines the correctness of the coding and decoding. The good resonant property can be observed on the printed tag without the back metal. The tag in this paper can be 3D printed directly on the surface of the items without any sacrifice on its properties. For [PERSON]'s structure [38; 39], it cannot be 3D printed because the back metal is necessary. By comparing the two structures in Figure 15, our design is easier to be fabricated, while keeping bigger scatter cross area. Table 4 shows the comparisons among different chipless tags, including the properties of excitation, harmonic, ground, coding density, capacity bits and range. It can be seen that the 3D-printed work shows the most promising properties among all the listed results. The C-shaped slot and the C-shaped microstrip structures are limited by the direction of the excitation. On this sense, the direction of the excitation must be aligned with the opening orientation of the C-shaped slot resonator. On the other hand, the excitation must be orthogonal with the opening direction of the C-shaped microstrip resonator. Although the nested C-shaped slot and the nested C-shaped microstrip chipless tags have a coding density of 7 bit/cm\({}^{2}\) and 3.3 bits/cm\({}^{2}\), they are limited by the third harmonic. So, the actual number of code bits is limited by the third harmonic frequency. The other obvious shortcoming is the error of the circular slot and the circular microstrip ring. To overcome the error, the precision of the designed geometries is highly required. Moreover, a ground plate is mandatory required on the back side for the rectangular microstrip ring structure of the chipless tag substrate. Although the circular microstrip ring and the circular slot ring have not the harmonic interference, this structure is not easy to resonate and the regular spectral characteristics appear for a specific structure. It is not suitable for general application. The rectangular slot ring that we proposed shows