High-Q Slot Resonator Used in Chipless Tag Design

: A retransmission chipless tag with multiple U-shaped slot resonators is proposed to cut down the cost of traditional tags with chips. Multiple side-by-side U-shaped slot structures of different lengths are printed on the microstrip line, and the two terminals of the microstrip line are connected correspondingly with two orthogonal ultra-wideband (UWB) transceiver antennas to form the retransmission chipless tag. The U-shaped slot resonator has high Q values and narrow impedance bandwidth. The bandwidth that each resonator adds to the protection bandwidth is 300 MHz. Several 6-bit coding U-shaped slot resonator chipless tags are designed and fabricated for comparison and measurement. Results show that the simulation and the measurement are in agreement. The slot width of the U-shaped slot resonator and the distance between the resonators are reduced, resulting in deepened spectrum notch depth of the resonator. Decreasing the dielectric constant of the substrate or increasing the thickness of the substrate increases the spectrum notch depth of the resonator.


Introduction
Radio frequency identification (RFID), a wireless communication technology for noncontact automatic identification and tracking, identifies remote tags by using radio frequency (RF) wave and extracts the encoded data from the backscatter wave. The system is composed of a reader, RFID tags, and a data processing system. The working principle of the RFID system is shown in Figure 1. A continuous wave interrogation signal is transmitted to the tag through the reader antenna. The antenna of the chipped tag receives the interrogation signal and generates an induced current to obtain energy. In this way, the tag is awakened to work. The modulated coded information of the chipped tag is transmitted to the reader with backscatter wave, which decodes the modulated information of the tag. Finally, the encoded data of the tag is obtained by the system [1,2]. lectronics 2021, 10, x FOR PEER REVIEW determined by the cost of the conductive material constituting its resonant chipless tag, the cost has been greatly reduced due to the removal of the s the tag. Moreover, it also has the advantages of the traditional chip tag. H less tags have shortcomings in terms of data capacity and tag size [7]. In recent decades, many researchers have proposed various types of which are divided into two categories: time-domain (TD) [8][9][10][11][12][13], and frequ (FD) chipless tags [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] The Ref. [8] proposed a near-field chipless tag system with sequential r time domain. It is realized by multiple linear half-wavelength microstr RFID systems do not require manual intervention and have the advantages of nonline-of-sight (NLOS) reading and high data capacity. Therefore, chipless tags have the of coupling between the resonators. Retransmissible tags require orthogonal polarization antennas to retransmit information to the orthogonal polarization antennas of the reader. The polarization fields of the reader and the tag must be aligned, otherwise the system sensitivity will be reduced and the error code rate will rise.
It is unrealistic to fully align the orthogonal antennas in practical applications. This situation has promoted the search for chipless tags that are insensitive to polarization [17][18][19]. Current research is based on backscatter chipless tags.
The inherent low isolation between the transmitter and receiver in a chipless tag reader greatly reduces the dynamic range and sensitivity of the reader. The strong excitation signal leaking to the receiver will reduce the sensitivity of the reader to detect weak backscattered signals and reduce the reading range.
This paper has designed a retransmission chipless tag with multiple U-shaped slot bandstop resonators. The U-shaped slot bandstop resonator unit used in the tag has a high Q-value, low coupling effect, high spectrum efficiency, and high encoding capacity. The high isolation of the receiving and the transmitting signals of the tag are due to the orthogonal transceiver antenna. Furthermore, the tag uses transmitting and receiving antennas, resulting in prolonged reading distance. Figure 2 shows the working principle diagram of multiple U-shaped slot resonators with retransmitting chipless tags. First, the reader sends a UWB interrogation signal with a uniform spectrum. Afterward, the receiving antenna of the tag receives the interrogation signal, which is transferred through the resonance circuits to the tag's transmitting antenna. During the transmission process, the uniformly UWB spectrum structure is changed by the U-shaped slot resonators, and the encoding information on the tag is loaded into the spectrum of the UWB interrogation signal. The UWB signal loaded with encoded information is retransmitted back to the RFID reader by the tag. Then, the data information can be demodulated through the decoding algorithm. The spectral signature of the multiple Ushaped slot resonators has band-stop characteristics at the different resonance frequencies.

Working Principle of a Retransmission RFID System Chipless Tag
The resonant frequency point and the data bit are in a 1:1 correspondence (n resonators = n bits), which appears as a notch in the spectrum. When the reader recognizes that the spectrum-characteristic notch exists, the logic state is "1", and the absence of the spectrum characteristic notch is represented by the logic state of "0". The polarization characteristics of the reader transmitting antenna and the tag receiving antenna are consistent, and the reader receiving antenna's polarization characteristic is consistent with the tag transmitting antenna. The transceiver antennas of the reader have orthogonally polarized to each other to maximize the isolation from the continuous interrogation and the encoded modulation signals and avoid mutual interference in the antennas [22]. The polarization characteristics of the tag transceiver antenna are also the same.
According to the Friis transmission formula [37], when the distance between the antennas meets the far-field conditions, and without environmental interference, the power density of the receiving antenna of the RFID system reader is where P T,reader is the power of the tag-transmitting antenna, G tag is the gain of the tagtransmitting antenna, and R is the distance between the transmitting and the receiving antennas.
Electronics 2021, 10, 1119 where B is defined as the −10 dB impedance bandwidth of the U-shaped s is the center frequency of U-shaped slot resonators.

U-Shaped Slot
The U-shaped slot is widely used in the patch antenna design [38,39]. parameters of a U-shaped slot of chipless tag are shown in Figure 3, where of the microstrip line, Wt is the length of the microstrip line, and Lu is th U-shaped slot. Wm is the slot width of the U-shaped slot, wu is the length side of the U-shaped slot, Ht is the distance from the U-shaped slot in the m and h is the thickness of the substrate.   The effective area of the receiving antenna of the RFID system is The power of the receiving antenna of the RFID system reader is R r = P T,reader G reader L P G tag λ 4πR 2 (3) where G reader is the gain of the reader-receiving antenna, L p is the power transmission coefficient, and λ is the wavelength.
The Q-value can be calculated by Equation (4) where B is defined as the −10 dB impedance bandwidth of the U-shaped slot resonator; f is the center frequency of U-shaped slot resonators.

U-Shaped Slot
The U-shaped slot is widely used in the patch antenna design [38,39]. The structural parameters of a U-shaped slot of chipless tag are shown in Figure 3, where L t is the width of the microstrip line, W t is the length of the microstrip line, and L u is the length of the U-shaped slot. W m is the slot width of the U-shaped slot, w u is the length of the bottom side of the U-shaped slot, H t is the distance from the U-shaped slot in the microstrip line, and h is the thickness of the substrate. The U-shaped slot is widely used in the patch antenna design [38,39]. The structural parameters of a U-shaped slot of chipless tag are shown in Figure 3, where Lt is the width of the microstrip line, Wt is the length of the microstrip line, and Lu is the length of the U-shaped slot. Wm is the slot width of the U-shaped slot, wu is the length of the bottom side of the U-shaped slot, Ht is the distance from the U-shaped slot in the microstrip line, and h is the thickness of the substrate.

Design and Simulation of the Chipless Tag for U-Shaped Slot Resonators
h Substrate GND U-shaped slot resonators

Design and Simulation of the Chipless Tag for U-Shaped Slot Resonators
Multiple side-by-side U-shaped slot resonators placed parallel to each other are etched on the microstrip line to form a multi-bit chipless tag, visible in Figure 4, where the L 1 is the width of the microstrip line, W 1 is the length of the microstrip line, L is the length of the shortest U-shaped slot resonator, and m is the side length difference between adjacent U-shaped slots. w is the slot width of the U-shaped slot resonator, and all U-shaped slot resonators have the same slot width. g is the length of the bottom side of the U-shaped slot resonator and is fixed and consistent with the length of the microstrip line. The distance between adjacent U-shaped slot resonators is b. Different resonance frequencies can be obtained by changing the length of a U-shaped slot resonator. A trapezoidal microstrip is used as a transition belt for impedance matching between the microstrip transmission line and the 50-ohm feeder, which plays the role of a gradual transition to impedance, and the reflection will be suppressed. tronics 2021, 10, x FOR PEER REVIEW Multiple side-by-side U-shaped slot resonators placed paralle etched on the microstrip line to form a multi-bit chipless tag, visible the L1 is the width of the microstrip line, W1 is the length of the micr length of the shortest U-shaped slot resonator, and m is the side le tween adjacent U-shaped slots. w is the slot width of the U-shaped slo U-shaped slot resonators have the same slot width. g is the length of the U-shaped slot resonator and is fixed and consistent with the leng line. The distance between adjacent U-shaped slot resonators is b. D frequencies can be obtained by changing the length of a U-shaped trapezoidal microstrip is used as a transition belt for impedance ma microstrip transmission line and the 50-ohm feeder, which plays th transition to impedance, and the reflection will be suppressed.  Figure 5 shows a schematic distribution of a single U-shaped slot resonator. The electric field is U-shaped slot, indicating that the U-shaped slot has resonated.
The transmission characteristic curve of the single U-shaped sl shown in Figure 6. When the side length of the single U-shaped slot the fundamental frequency of the resonator is 8.28 GHz, and the U-shaped slot resonators are designed on the F4BM substrate. The relative dielectric constant, loss tangent, and substrate thickness are 2.2, 0.0007, and 1.00 mm, respectively. The High-Frequency Structure Simulator (HFSS) is used for the modeling and the simulation of a single U-shaped slot resonator. Figure 5 shows a schematic of the electric field distribution of a single U-shaped slot resonator. The electric field is strongest near the U-shaped slot, indicating that the U-shaped slot has resonated.   The transmission characteristic curve of the single U-shaped slot resonator S21 is shown in Figure 6. When the side length of the single U-shaped slot resonator is 7 mm, the fundamental frequency of the resonator is 8.28 GHz, and the notch depth of the spectrum characteristic achieves −22 dB. Moreover, the impedance bandwidth B is 0.04 GHz. Q-value is about 207.   Considering that this structure generates second harmonics at 17.67 GHz, the fundamental frequency of the shortest U-shaped slot cannot exceed 17.67 GHz. Otherwise, the second harmonic of the longest slot interferes with the encoded data onto the shortest slot and eventually causes bit errors. Therefore, the available coding frequency band is 8.28 GHz to 17.67 GHz. The bandwidth occupied by a single resonator is 40 MHz. If the guard band of the spectrum-characteristic notches of adjacent resonators is 260 MHz, the 31-bit frequency position code can be formed in the available frequency band, and the largest code states can reach 2 31 . According to the literature [40], for very thin conductors (that is, thickness→0), the expression of the characteristic impedance Z 0 is as follows: where ε re is the effective dielectric constant, η = 120π Ω is the wave impedance in free space, u = L 2 / h, and The accuracy of this expression applies to ε r ≤ 128 and 0.01 ≤ u ≤ 10. From Equation (11), the width L 2 of the Z 0 = 50 Ω trapezoidal microstrip can be obtained. Due to the limitation of processing accuracy, the size adjustment step length of numerical simulation cannot be lower than the process requirements of chipless tag manufacturing. According to the relevant theoretical formula of the U-shaped slot [38,39], after the HFSS software simulation and design, the structural parameters of the 6-bits U-shaped slot resonator are finally determined in Table 1.  8 show the resonance curves of the 4-bits coding tags, namely ID111111, ID111000, ID101010, and ID010101. The ID111111 tag is considered the reference tag. The six resonance spectral notches of the reference tag ID111111 are 6.09, 6.37, 6.76, 7.21, 7.79, and 8.30 GHz, respectively. The notch depth of the spectrum characteristic is between −19 dB and −32 dB. The corresponding spectrum width of each resonator is narrow. The spectrum notch is deep, and the spectrum utilization rate is high. Freq/GHz ID111000 (a) (b) (c) Figure 7. Simulation of the resonance curves of the 6 bits U-shaped slot resonator chipless tags with a short-circuit logic state of "0": (a) ID111111 and ID101010; (b) ID111111 and ID010101;(c) ID111111 and ID111000.

Experimental Results
Figure 9 is the test system of a retransmission chipless tag based on multi-state resonators. The experimental setup architecture was composed of a UWB reader, Tx/Rx antennas and a chipless tag. A Ceyear vector network analyzer 3672D was used as an alternative to the UWB reader. The two ports of the network analyzer were, respectively, connected to the L-shaped slot-loaded stepped-impedance UWB antennas [41], which were orthogonal to each other to improve the transceiver isolation of the reader. The tag was connected to two orthogonal L-shaped slot-loaded stepped-impedance UWB antennas through two microwave adapters. In order to prevent the received signal and the transmitted signal of the tag from interfering with each other, the two-sided UWB antennas of the tag were also orthogonal to each other. Both the reader antennas and the tag antennas were fixed on the foam, 10 cm apart. The gain of the L-shaped slot-loaded stepped-impedance UWB antenna was 4 dBi to 8.54 dBi in the range of 2.39 GHz to 13.78 GHz.  Figure 9 is the test system of a retransmission chipless tag based on multi-state resonators. The experimental setup architecture was composed of a UWB reader, Tx/Rx antennas and a chipless tag. A Ceyear vector network analyzer 3672D was used as an alternative to the UWB reader. The two ports of the network analyzer were, respectively, connected to the L-shaped slot-loaded stepped-impedance UWB antennas [41], which were orthogonal to each other to improve the transceiver isolation of the reader. The tag was connected to two orthogonal L-shaped slot-loaded stepped-impedance UWB antennas through two microwave adapters. In order to prevent the received signal and the transmitted signal of the tag from interfering with each other, the two-sided UWB antennas of the tag were also orthogonal to each other. Both the reader antennas and the tag antennas were fixed on the foam, 10 cm apart. The gain of the L-shaped slotloaded stepped-impedance UWB antenna was 4 dBi to 8.54 dBi in the range of 2.39 GHz to 13.78 GHz.

Experimental Results
ics 2021, 10, x FOR PEER REVIEW Figure 9. Test system. The experimental setup architecture was composed of a tag. The two ports of the network analyzer were, respectiv The experimental setup architecture was composed of a UWB reader and a chipless tag. The two ports of the network analyzer were, respectively, connected to the UWB antennas, which were orthogonal to each other to improve the transceiver isolation of the reader. The chipless tag needed two orthogonal transceiver antennas; similarly, the chipless reader also needed two orthogonal transceiver antennas which would not affect each other. The polarization characteristics of the transmitting antenna of the reader and the receiving antenna of the tag were the same. The polarization characteristics of the receiving antenna of the reader and the transmitting antenna of the tag were the same. The receiving antenna of the reader received the signal that was transmitting from the tag; however, the transmitting antenna of the reader would not receive any signal from the tag.
In accordance with the simulated U-shaped resonator chipless tag structure, several typical coded chipless tags with a coding capacity of 6 bits were produced, namely ID111111, ID010101, ID101010, ID111000, and the photo is shown in Figure 10. A simulated current vector of the U-shaped slot resonator is shown in Figure 11. The experimental setup architecture was composed of a UWB reader and a chipless tag. The two ports of the network analyzer were, respectively, connected to the UWB antennas, which were orthogonal to each other to improve the transceiver isolation of the reader. The chipless tag needed two orthogonal transceiver antennas; similarly, the chipless reader also needed two orthogonal transceiver antennas which would not affect each other. The polarization characteristics of the transmitting antenna of the reader and the receiving antenna of the tag were the same. The polarization characteristics of the receiving antenna of the reader and the transmitting antenna of the tag were the same. The receiving antenna of the reader received the signal that was transmitting from the tag; however, the transmitting antenna of the reader would not receive any signal from the tag.
In accordance with the simulated U-shaped resonator chipless tag structure, several typical coded chipless tags with a coding capacity of 6 bits were produced, namely ID111111, ID010101, ID101010, ID111000, and the photo is shown in Figure 10. A simulated current vector of the U-shaped slot resonator is shown in Figure 11.   The simulation and the measurement results of the 6-bit U-shaped slot resonators are shown in Figure 12. Figure 12a shows that the six U-shaped slot resonators correspond to six resonance points. The simulation and the measurement results are consistent. A 100 MHz frequency deviation appeared at the 5th resonance point. The value is within the allowable resonator bandwidth, so it does not affect the encoding state of the tag. Figure 12b-d show that, after removing the U-shaped slot, the corresponding simulated and measured resonance points also disappeared, and that the simulation and measurement results are the same. Figure 12 illustrates that the bandwidth of each resonator in the measurement result is broadened compared to that of the simulation result, and the simulation result of the lost arm is larger than the measurement result. The UWB antenna is connected to the resonant circuits by the connector. Although the connector can produce the insertion loss during the measurement, the spectral characteristics of the measured tag can correctly reflect the coding data of the tag, which does not affect the use of the tag. lated and measured resonance points also disappeared, and that the simulation and measurement results are the same. Figure 12 illustrates that the bandwidth of each resonator in the measurement result is broadened compared to that of the simulation result, and the simulation result of the lost arm is larger than the measurement result. The UWB antenna is connected to the resonant circuits by the connector. Although the connector can produce the insertion loss during the measurement, the spectral characteristics of the measured tag can correctly reflect the coding data of the tag, which does not affect the use of the tag.

Discussion
The U-shaped slot resonator is used in retransmission chipless tag RFID systems. It brings a lot of advantages to this design. To compare it to other similar work, Figure 13 shows the schematic diagrams of our design and the other U-shaped slot chipless tag [34]. Our work is shown in Figure 13a. Md Aminul Islam et al. [34] used embedded U-shaped slots based on self-resonance. The electric field must be excited along the direction of the U-shaped slot opening. In our work, the Rx UWB antenna received the interrogation signal and the cascade U-shaped slot resonators changed the structure of the UWB spectrum. Then, the Tx UWB antenna transmitted them to the reader. For our design, the strength of retransmission signal is much greater than the comparison tag in Figure 13b. The coupling effect between the resonators is weak. The chipless tag in our design has high sensitivity, is long in distance, is omnidirectional, etc.

Discussion
The U-shaped slot resonator is used in retransmission chipless tag RFID systems. It brings a lot of advantages to this design. To compare it to other similar work, Figure 13 shows the schematic diagrams of our design and the other U-shaped slot chipless tag [34]. Our work is shown in Figure 13a. Md Aminul Islam et al. [34] used embedded U-shaped slots based on self-resonance. The electric field must be excited along the direction of the U-shaped slot opening. In our work, the Rx UWB antenna received the interrogation signal and the cascade U-shaped slot resonators changed the structure of the UWB spectrum. Then, the Tx UWB antenna transmitted them to the reader. For our design, the strength of retransmission signal is much greater than the comparison tag in Figure 13b. The coupling effect between the resonators is weak. The chipless tag in our design has high sensitivity, is long in distance, is omnidirectional, etc.
rection of the U-shaped slot opening. In our work, the Rx UWB antenna received the in terrogation signal and the cascade U-shaped slot resonators changed the structure of th UWB spectrum. Then, the Tx UWB antenna transmitted them to the reader. For our de sign, the strength of retransmission signal is much greater than the comparison tag in Figure 13b. The coupling effect between the resonators is weak. The chipless tag in ou design has high sensitivity, is long in distance, is omnidirectional, etc. The resonance frequency of a U-shaped slot resonator is determined by the length o the slot and the influence of other parameters, such as the slot width (w) of the U-shaped slot and the distance (b) between the resonators, which are shown in Figures 14 and 15 When the length of the U-shaped slot resonator is 7 mm, the resonance curves corre The resonance frequency of a U-shaped slot resonator is determined by the length of the slot and the influence of other parameters, such as the slot width (w) of the U-shaped slot and the distance (b) between the resonators, which are shown in Figures 14 and 15. When the length of the U-shaped slot resonator is 7 mm, the resonance curves corresponding to the change in w from 0.1 mm to 0.7 mm in steps of 0.1 mm is shown in Figure 14. When the slot w changes, the width of the resonance point is unchanged. As the w of the slot narrows, the resonance frequency decreases, and the notch corresponding to the resonance point deepens. The resonance characteristics of b are adjusted to study the coupling effect of U-shaped slot resonators ( Figure 15). The b decreases and the notch depth of the resonators deepens, but the resonance frequency is almost unchanged, indicating that U-shaped slot resonators are minimally affected by the coupling effect. s 2021, 10, x FOR PEER REVIEW sponding to the change in w from 0.1 mm to 0.7 mm in ste 14. When the slot w changes, the width of the resonance p the slot narrows, the resonance frequency decreases, and resonance point deepens. The resonance characteristics coupling effect of U-shaped slot resonators ( Figure 15). depth of the resonators deepens, but the resonance frequ dicating that U-shaped slot resonators are minimally affec  The resonance characteristics of tags with different d and dielectric thicknesses ( Figure 17) are studied to direct U-shaped slot resonator structure on materials of differe Figure 16 shows that, when the dielectric constant of the 3.0, the resonance frequency decreases with increasing d printed materials are different, the frequency position ne frequency position code of the reference tag needs to be re the tag resonance curve obtained by changing the substra mm in steps of 0.5 mm. The resonance frequency corres resonator changes slightly when the thickness is changed increases as the thickness increases.  The resonance characteristics of tags with different dielectric constants ( Figure 16) and dielectric thicknesses ( Figure 17) are studied to directly print the chipless tag of the U-shaped slot resonator structure on materials of different thicknesses, like a barcode. Figure 16 shows that, when the dielectric constant of the medium is 2.2, 2.55, 2.65, and 3.0, the resonance frequency decreases with increasing dielectric constant ε r . When the printed materials are different, the frequency position needs to be re-encoded, and the frequency position code of the reference tag needs to be re-encoded. Figure 17 illustrates the tag resonance curve obtained by changing the substrate thickness from 0.5 mm to 2 mm in steps of 0.5 mm. The resonance frequency corresponding to the U-shaped slot resonator changes slightly when the thickness is changed, but the resonance frequency increases as the thickness increases. The resonance characteristics of tags with different and dielectric thicknesses ( Figure 17) are studied to direc U-shaped slot resonator structure on materials of differe Figure 16 shows that, when the dielectric constant of the 3.0, the resonance frequency decreases with increasing d printed materials are different, the frequency position ne frequency position code of the reference tag needs to be r the tag resonance curve obtained by changing the substr mm in steps of 0.5 mm. The resonance frequency corre resonator changes slightly when the thickness is changed increases as the thickness increases.   Figures 18 and 19 show the resonance curves o length and the resonator with the shortest side leng stituting the chipless tag is increased to 31 bits. Th was found that the side length of the U-shaped s length and the longest side length is 2.9 mm and 1 difference between adjacent resonators is 0.27 mm U-shaped slot resonators is the same, at 2.9 mm. T tag is 113.94 mm × 21.2 mm. The cascades of the U by side. The resonant frequency of the shortest re 13.88 GHz. Meanwhile, the longest resonator corres GHz.   Figures 18 and 19 show the resonance curves of the resonator with the longest side length and the resonator with the shortest side length, as the number of resonators constituting the chipless tag is increased to 31 bits. Through HFSS software simulation, it was found that the side length of the U-shaped slot resonator with the shortest side length and the longest side length is 2.9 mm and 11 mm, respectively. The side length difference between adjacent resonators is 0.27 mm. At the same time, the width of all U-shaped slot resonators is the same, at 2.9 mm. The dimension of the 31-bits chipless tag is 113.94 mm × 21.2 mm. The cascades of the U-shaped resonators are arranged side by side. The resonant frequency of the shortest resonator in the 31-bits chipless tag is 13.88 GHz. Meanwhile, the longest resonator corresponds to a resonant frequency of 4.7 GHz.
Capacity bits/cm 2 = number of bits size(cm 2 ) Spectral capacity (bits/GHz) = number of bits frequency bandwith(GHz) (11) Figures 18 and 19 show the resonance curves of the length and the resonator with the shortest side length, as stituting the chipless tag is increased to 31 bits. Through was found that the side length of the U-shaped slot re length and the longest side length is 2.9 mm and 11 mm difference between adjacent resonators is 0.27 mm. At t U-shaped slot resonators is the same, at 2.9 mm. The dim tag is 113.94 mm × 21.2 mm. The cascades of the U-shape by side. The resonant frequency of the shortest resonato 13.88 GHz. Meanwhile, the longest resonator corresponds GHz.   Table 2 lists the performance comparisons of various types of retransmission chipless tags. The U-shaped slot resonator chipless tags proposed in this paper are simple in design, with high Q-value and small size. The capacity is 1.03 bits/cm 2 . The spectral capacity is 2.71 bits/GHz. Compared to other retransmission chipless tags, such as ref. [21], the designed 6-bit U-shaped slot resonator has insufficient spectral capacity. However, due to the high Q value of the U-shaped slot resonator, the spectrum utilization rate is high. The substrate size of the 6-bit tag is designed to be 30 × 19.5 mm 2 . Adding a resonator can increase the encoding capacity of the tag. However, the tag size of the U-shaped slot resonator will not be too large. The tag performance designed in this paper can be expanded.

Conclusions
In this paper, a compact U-shaped slot resonator chipless tag based on a retransmission structure is proposed. The resonance unit uses a U-shaped slot resonator with a high Qvalue. This type of resonator has a weak mutual coupling effect and is suitable for multi-bit chipless tag design. Given the influence of the second harmonic of the U-shaped slot resonator, this tag can only work in the 8.28-17.67 GHz frequency bands. Finally, 6 bits of U-shaped slot resonators are produced for testing, and results have fully verified the feasibility of this type of chipless tag. Considering that the chipless tag does not need to use any silicon-based circuit or semiconductor device groups, the cost of the tag is low. Moreover, the tag has the same printable characteristics as the barcode, thereby presenting