Design of a Three-Dimensional Uniform UHF Near-Field RFID Reader Antenna

Tis paper proposes a three-dimensional uniform ultra-high frequency (UHF) near-feld radio frequency identifcation (RFID) reader antenna. Te antenna achieves a uniform electric feld in the x and y directions by placing a single branch microstrip line along the x - axis and y -axis directions, respectively. It reaches a uniform electric feld in the z -direction by a centrosymmetric four-branch microstrip line. Te proposed antenna achieves three-dimensional direction uniformity through a reconfgurable method. Te impedance matching bandwidth range of S 11 < − 10dB for simulation and measurement includes 0.66 to 0.98GHz, which can meet the near-feld RFID operation frequency band demand. Te isolation degrees between ports are less than − 24.6dB within the UHF RFID frequency band (0.86 to 0.96GHz). In addition, the antenna also has the characteristic of low gain in the far feld, and the maximum gain in the far feld is less than − 27dBi when operating at diferent ports. Te test results show that the proposed antenna three-dimensional uniform volume of dipole tags above the antenna is 99mm × 99mm × 20mm, and the reading volume of the near-feld tags is 40mm × 40mm × 5mm. When the tags are placed on a book, there will be a slight variation in the reading range of the tags.


Introduction
Recently, ultrahigh-frequency (UHF) radio frequency identifcation (RFID) technology has been widely concerned because of its rapid identifcation characteristics.Also, it has moved from obscurity into mainstream applications that help speed the handling of manufactured goods and materials.It is a well-known wireless application in traceability, logistics, and access control [1][2][3].
UHF RFID systems are divided into near-feld and farfeld systems according to the diferent recognition distances.Near-feld systems are generally used for recognition distances less than one meter.In recent years, near-feld application scenarios like bright bookshelves and intelligent vending machines have become more and more extensive [4][5][6].Te performance of the near-feld application system mainly depends on the near-feld reader antenna [4,[7][8][9].Hence, designing a reliable UHF RFID near-feld reader antenna is worthwhile.
According to the diferent coupling methods, the nearfeld RFID reader antenna is divided into a magnetic and an electrically coupled antenna.Magnetic coupling antenna communicates with magnetic tags through alternating magnetic felds in the near-feld region.Shi et al. proposed a zero-phase shift circular antenna to generate a strong and uniform magnetic feld [10,11].Yao et al. [12] realized uniform magnetic feld distribution in the near-feld region through two meandering open microstrip lines with reverse current.Te magnetic coupling antenna can read a large number of magnetic labels through a uniform magnetic feld distribution in the near-feld area, but its low reading distance limits its application scenarios.
Electrically coupled antenna exploits alternating electric felds for information interactive with tags.A meander microstrip line loaded with a 50-ohm resistance antenna is designed in [13] can generate a strong and uniform E-feld in the near-feld region.An antenna based on the EM coupling between an open-ended MS feed line and periodic planar metal strips was designed and applied to the smart bookshelf [14].However, these types of antenna polarization modes are linear polarization, which restricts the placement orientation of linear polarization tags during the actual reading process.To solve this problem, Yao et al. designed a variety of multipolarized near-feld antennas by introducing a 90 °phase shifter between the currents fowing along the opposite side of two branches [15][16][17].In addition, some circularly polarized antennas are proposed for RFID systems [18][19][20].Tese antennas achieved the detection of linearly polarized tags in an arbitrary orientation parallel to the antenna surface.However, they are unable to accurately detect linearly polarized labels perpendicular to the antenna surface, which limits the detection of linearly polarized labels in arbitrarily three-dimensional orientation.
In this paper, we propose a near-feld RFID reader antenna with low gain and uniform three-dimensional electric feld components in the near-feld region.Te proposed antenna is composed of two mutually perpendicular single-branch microstrip lines, one centrally symmetrical four-branch microstrip line, and four rectangular patches.Trough the time-sharing operation of three feeding ports, the uniformity of the three-dimensional components of the electric feld is realized.Te measurement results show that the −10 dB impedance bandwidth of the antenna is 0.66 to 0.98 GHz, which covers the standardized bandwidth of UHF RFID.When linearly polarized electrically coupled tags are placed arbitrarily in threedimensional space and detected by the proposed antenna, the read volume for 100% read rate of Alien A9662 tags is 99 mm × 99 mm × 20 mm, and the reading volume for 100% read rate of the near-feld tags Alien SIT is 40 mm × 40 mm × 5 mm.Te proposed antenna can detect tags perpendicular to its surface, making tag detection more accurate and application scenarios more diverse.

Reader Antenna Design
2.1.Antenna Confguration.Te proposed multiport reconfgurable antenna confguration is shown in Figure 1.It has three layers structure.Te frst layer is composed of two single-segment microstrip lines and a central symmetrical four-segment microstrip line, which is fabricated on an FR4 substrate with a thickness of 2 mm.Te end of each microstrip branch relates to a 50 Ω load.Terefore, the surface of the microstrip line will generate a traveling wave current, which can realize the broadband of the antenna.Te second layer is four parasitic patches with a thickness of 0.035 mm of metal copper, which are fabricated on an FR4 substrate with a thickness of 2.6 mm.Te last layer is the grounding plate with copper material.Te total thickness of the antenna structure is 4.6 mm.Te antenna has three feeding ports to provide electromagnetic energy to the radiation structure.Port 1 is located in the center of a centrally symmetrical fourbranch microstrip line.Port 2 is located to the right of the single branch along the x-axis direction.Port 3 is below the single-stub microstrip line along the y-axis direction.Tey are fed coaxially through the 50 Ω SMA connector.Te optimized dimensions of the antenna are shown in Table 1.

Antenna Design Discussion
. Before the antenna design in this paper, the principle of uniformity of the electric feld in the near feld was frst studied.Assuming that there is an equivalent model of a fnite-length current source with a terminal connected load placed along the x-axis in space, the amplitude of the current source is distributed in a sinusoidal manner as shown in the following equation: Divide the current source into countless small current elements, and the radiation electric feld at a certain point in space is the superposition of the small current source's radiation electric feld at that point as shown in the following equation: Finally, utilizing the conversion formula between the rectangular coordinate system and the spherical coordinate system, the electric feld value excited by the current source at an arbitrarily point in the rectangular coordinate system can be obtained.When there is in-phase current on the current element, the electric feld value is maximum at the center above the current element, and the farther it deviates from the center position, the smaller the electric feld value.Tis paper uses two single-branch microstrip lines with mutually perpendicular ends loaded to generate a uniform two-dimensional electric feld in the near-feld region.To ensure in-phase current on a single branch, the length of a single branch node satisfes the following inequality.Actual length can be obtained through simulation optimization.
Te principle of a uniform electric feld E z perpendicular to the antenna surface is not the same as E x and E y .For the convenience of application, most of the reader/writer antennas are low-plane antennas, so it is not feasible to directly generate a vertical electric feld using the vertical current.Assuming there is a planar magnetic ring above the antenna, the electric feld perpendicular to the antenna surface can be obtained according to Maxwell's equation in (5).According to the Ampere theorem of (6), it can be analyzed that four centrally symmetric current sources with adjacent phase diferences of 90 °can generate a magnetic ring above them.Terefore, it is possible to utilize a centrally symmetric fourbranch microstrip line to generate a uniform E z .To make the current on the surface of each branch in phase, its length is designed to be about a quarter wavelength as shown in (7).Due to the introduction of parasitic patches, reverse currents appear on the surface of branches.To counteract this reverse current, each branch is designed as an arrow type.

Principle of Tree-Dimensional Uniformity of Electric
Field.When unlike ports operate, the current distribution on the surface microstrip line is shown in Figure 2. From Figure 2(a), it can be seen that when Port 1 is working, the current on the four branches with central symmetry is reversed.According to Ampere's law, this current distribution will generate a magnetic feld above the antenna as shown in Figure 3(a).It can be observed that the magnetic feld in the horizontal plane above the antenna presents a circular shape, and there will be a uniform vertical electric feld around the circular magnetic feld as shown in Figure 4(a).Tis alternating electric feld activates the chip inside the linearly polarized electrically coupled tag placed along the zdirection by generating an induced electromotive force, thereby completing the information exchange between the reader and the tag.From Figure 2(b), it can be seen that when Port 2 is operating, electromagnetic energy is mainly focused on the single branch microstrip line along the x-axis direction and the left branch in the four branch microstrip line, and the total current fows along the x-axis direction.At this point, the magnetic feld vector and electric feld vector distribution above the antenna are shown in Figure 3(b) and Figure 4(b), respectively.A uniform electric feld along the xaxis can detect linearly polarized labels placed along the xdirection within a certain area.As above, Figure 2(c) shows that when Port 3 is operating, electromagnetic energy is mainly concentrated on the single branch microstrip line along the y-axis direction and the lower branch of the four branch microstrip line, and the total current fows along the y-axis direction.At this point, there is a magnetic feld in the x-direction above the antenna as shown in Figure 3(c).A uniform electric feld in the y-direction will be generated above the antenna as shown in Figure 4(c).Ensure that labels placed in the y-direction can be accurately detected.From the electric feld vector map above the antenna, it can be seen that the distribution of the electric feld vector is disorderly when very close to the antenna.It is possible to read the linear polarization labels placed in arbitrary orientations.As the distance increases, the direction of the electric feld vector gradually becomes regular.At this time, only tags whose polarization direction is consistent with the direction of the electric feld can be detected.From the above analysis, it can be seen that when the three ports operate in a timesharing manner, a uniform three-dimensional electric feld can be generated on the surface of the proposed antenna.
Te four rectangular patches in the second layer of the antenna structure are used to enhance the electric feld strength above the antenna and expand the uniform distribution range of the electric feld.In addition, it can also generate resonance with the top-layer radiator, expanding the impedance bandwidth of the antenna.Figure 5 shows the current distribution on the surface of four parasitic patches when three ports are operating separately.When port 1 is working, there is a reverse current on a pair of centrally symmetrical patches, and the phase diference of International Journal of Antennas and Propagation the surface currents of adjacent patches approximately 90 °, which is similar to the surface current of the four-branch microstrip line, and it has the same component as the surface current of the four-branch microstrip line.When Port 2 or Port 3 is operating, the current on the patches is mainly concentrated on the bottom left or top left patches.Te induced current will also have an impact on the feld distribution above the antenna.Te electromagnetic energy in space mainly comes from the surface current of the radiation structure.9.With the SH2 changes, the port impedance bandwidth hardly changes, but there is a slight fuctuation in the value of the refection coefcient.Te impact of SH2 on isolation S 12 and S 13 in the 1-1.6 GHz frequency band is greater than that of low frequency.Te isolation S 23 decreases with the increase of SH2.Comparing Figures 8 and 9, it can be observed that the thickness of the frst substrate SH1 has a greater impact on antenna performance than the thickness of the second substrate SH2.Te impact of the dimensions of the substrate on antenna performance is shown in Figures 10 and 11. Figure 10 is the study of the width W of the substrate.Te change of W value has almost no efect on the impedance bandwidth.With the change of W, the refection coefcient will slightly move up and down.Te change in W value also has little impact on the isolation degrees S 12 and S 13 but has a certain impact on S 23 .When W � 117 mm, isolation degrees S 23 at low frequencies are signifcantly better than when W is other values.Te infuence of the change in length L of the substrate on the refection coefcient and isolation of the proposed antenna is consistent with the infuence of W as shown in Figure 11.Terefore, the size of the substrate has a sure impact on the isolation S 23 at low frequencies.

Far-Field
Tree-Dimensional Direction Map.Te farfeld three-dimensional direction map of diferent ports operating the proposed antenna is presented in Figure 12.As can be seen that the maximum far-feld gain is less than −27 dBi, indicating that the antenna has the characteristics of low gain in the far feld, and it will avoid misreading tags in the far feld area in actual application scenarios.Terefore, the antenna meets the performance requirements of the near-feld reader-writer antenna.

Electric Pattern.
When only Port 1 is active, a plane parallel to the antenna surface and away from the proposed antenna above 2, 10, 50, and 100 mm, and the E z distribution diagrams as shown in Figure 13(a).Te E z feld is evenly distributed in a controlled area, and the reading range decreases as the distance increases.Figure 13(b) presents the E x scalar distribution at the same height in the same plane when only Port 2 operates.Te E x feld strength above the antenna is uniform and controllable.Figure 13(c) shows the E y distribution when only Port 3 operates.Similarly, the controllable area above the antenna has a uniform E y feld strength.
Figure 14 illustrates the 3D distribution of the electric feld at 2, 10, 50, and 100 mm for the antenna without parasitic patches.By comparing the uniform distribution range of the electric feld in Figures 13 and 14, can be seen that the uniform distribution range of the feld decreases after removing the parasitic patches.International Journal of Antennas and Propagation

Measurement Results
To verify the correctness of the simulation results, the proposed antenna is processed, and its actual performance indicators are measured.Figure 15 shows the physical processing picture of the proposed antenna prototype.A 50 Ω SMA adapter is soldered at the feed port of the antenna, and a 50 Ω chip resistor of the 0805 packages is soldered at the end of the microstrip line.
In this paper, the impedance-matching bandwidth of the antenna is measured using the Agilent 8753ES vector network analyzer.Connect the adapter to the coaxial line of the vector network analyzer to measure each port refectance coefcient of the proposed antenna.Te comparison of the         International Journal of Antennas and Propagation simulation and measurement results of the refection coefcients of each port of the antenna as shown in Figure 16.
Te measurement results display that the bandwidth with a refection coefcient of less than −10 dB ranges from 0.66 to 0.98 GHz, including the UHF RFID (860−960 MHz) frequency range.Figure 17 represents the simulation and test results of the isolation degree between diferent ports.Te test results show that the isolation degrees S 12 , S 13 , and S 23 between ports are less than −24.6 dB in the 0.86 to 0.96 GHz frequency band.Compared to the simulation results, the slight deviation between the refection coefcient and the isolation degree, which is caused by fabrication error.Te detection range of tags is one of the important performance indicators of the reader antenna.Tis paper uses two types of labels to detect the actual reading performance of the proposed antenna, respectively, Alien A9662 and Alien SIT.
Figure 18 shows the testing scenario of the label Alien A9662.Te proposed antenna is connected to the Impinj Speedway R420 reader [22] with an output power of 30 dBm and a frequency of 920−925 MHz through a coaxial line.Te reader is connected to the computer through a network cable to detect the reading range of labels placed in diferent directions.A foam board with a dimension of 226.5 mm × 189 mm is placed parallel to the antenna surface, its surface is equally divided into 15 grids, and a 17 mm × 70 mm Alien A9662 dipole tag is pasted to the center of each grid along the x-axis as shown in Figure 18(a).Move the foam plate along the z-axis to get the reading area of labels at diverse heights, repeat the test 10 times, record the label reading range at each height, and calculate the average read rate.Te reading range of the tags placed along the y-axis can be tested by rotating the foam board in Figure 18(a) by 90 °.Paste the label vertically on a foam board with a dimension of 165 mm × 165 mm and divided it into 25 grids.Similarly, move the foam board along the z-axis direction to measure the reading volume of the label pile along the z-axis direction as shown in Figure 18(c).Figure 19 depicts the detection results of tags placed along the x, y, and z directions.From the fgure, it can be seen that when tags are placed along the x-axis, the reading volume with a 100% reading rate is 226.5 mm × 113.4 mm × 40 mm.When tags are placed along the y-axis, the reading volume is 226. 5    placed along the z-axis, the reading volume is 99 mm × 99 mm × 70 mm.Terefore, the reading volume of the label with a 100% reading rate in the three-dimensional direction is 99 mm × 99 mm × 20 mm.Te testing scenario of the tag Alien A9662 placed on books is shown in Figure 20. Figure 21 shows the detection results of the reading range of the tag placed on the book.At this time, the reading volume with a 100% reading rate in the three-dimensional direction of the tag is 99 mm × 99 mm × 20 mm.Compared with Figure 19, it is found that when the Alien A9662 tags are placed on a book-like medium, there is a slight deviation in the tag detection results, but it has little impact on the reading range.
Figure 22 gives the detection scenario of the Alien SIT tag.Te tags are pasted on the foam plate parallel to the antenna surface along the x, y, and z directions, respectively.Te dimensions of the foam plate above the antenna are 80 mm × 80 mm, and its surface is divided into 16 grids.When testing the reading range of labels in diferent directions, a 12 mm × 9 mm Alien SIT near-feld label in the corresponding direction is pasted at the center of each grid.Figure 23  Te reading volume of the proposed antenna with a 100% reading rate in the three-dimensional direction of the Alien SIT near-feld antenna is 40 mm × 40 mm × 5 mm.Te testing scenario of the tag Alien SIT placed on books as shown in Figure 24. Figure 25 shows the measurement results of the foam board with a near-feld label placed on the book, and the size of the foam board is consistent with Figure 22.It can be found that books have little impact on the reading range of tags in the x-and y-axis directions, but they have a certain impact on the reading of tags in the z-axis direction.Table 2 shows a comparison between the manufactured antennas and other previous works.It can be seen that the antenna proposed in this paper has the lowest far-feld gain, which can greatly reduce the misreading rate in practical applications.Te proposed antenna is smaller in size compared to other antennas.Although the reading volume for the same tag is smaller, it can read tags in an arbitrarily direction.Te size of the antennas in paper [15,17] is not much diferent, and both can normally detect tags parallel to the antenna surface.However, for diferent types of tags, the reading volume varies greatly.

Conclusion
Tis paper proposes a reconfgurable microstrip antenna to achieve a uniform electric feld in the three-dimensional direction and presents its confguration, principle, characteristics, simulation, and measurement results of the antenna.Te simulation results show that the uniformity of the electric feld in the three-dimensional direction of the antenna is quite good.Te measurement results illustrate that the reading area of the tag above the antenna is concentrated, and the tags in the far feld cannot be detected, which verifes the theory's correctness.

Figure 2 :
Figure 2: Current on the surface of the microstrip line when diferent ports operate.(a) Only Port 1 operates.(b) Only Port 2 operates.(c) Only Port 3 operates.

Figure 3 :
Figure 3: Distribution of magnetic feld vectors in the horizontal plane z � 100 mm above the antenna when diferent ports work.(a) Only Port 1 operates.(b) Only Port 2 operates.(c) Only Port 3 operates.

Figure 4 :
Figure 4: Distribution of electric feld vectors in the horizontal plane z � 2 mm and z � 100 mm above the antenna when diferent ports work.(a) Only Port 1 operates.(b) Only Port 2 operates.(c) Only Port 3 operates.

Figure 7
depicts the isolation degree between various ports S 12 , S 13 , and S 23 .Te port isolation of the antenna within the 860−960 MHz bandwidth is less than −31.6 dB, indicating very low interference between ports.

Figure 7 :
Figure 7: Isolation between adjacent ports of the proposed antenna.

Figure 12 :
Figure 12: Far-feld three-dimensional direction map of diferent ports operating the proposed antenna.(a) Only Port 1 operates.(b) Only Port 2 operates.(c) Only Port 3 operates.

Figure 13 :
Figure 13: Te plane parallel to the antenna surface away from the proposed antenna above 2, 10, 50, and 100 mm electric distribution map.(a) E z distribution map.(b) E x distribution map.(c) E y distribution map.

Figure 14 :
Figure 14: Te plane parallel to the antenna surface away from the antenna without parasitic patches above 2, 10, 50, and 100 mm electric distribution map.(a) E z distribution map.(b) E x distribution map.(c) E y distribution map.

Figure 15 :Figure 16 :
Figure 15: Processing physical picture of the proposed antenna.
plots the detection results of placing the tags directly above the antenna.It can be seen that when the tags are placed along the x-direction, the reading volume with a 100% reading rate of the label is 40 mm × 40 mm × 20 mm.When the tags are placed along the y-direction, the reading volume with a 100% reading rate of the label is 40 mm × 40 mm × 15 mm.When the tags are placed along the z-direction, the reading volume with a 100% reading rate of the label is 40 mm × 40 mm × 5 mm.

Figure 17 :Figure 18 :Figure 19 :
Figure 17: Simulated and measured results of the isolation between distinct ports of the proposed antenna.

Figure 20 :Figure 21 Figure 21 :
Figure 20: Te testing scenario of the tag Alien A9662 placed on books.(a) Tags are placed along the x-axis.(b) Tags are placed along the y-axis.(c) Tags are placed along the z-axis.

Figure 22 :Figure 23 :Figure 24 :
Figure 22: Te testing scenario of the tag Alien SIT.(a) Tags are placed along the x-axis.(b) Tags are placed along the y-axis.(c) Tags are placed along the z-axis.

Figure 25 :
Figure 25: Te detection results of the Alien SIT tags placed on books.(a) Tags are placed along the x-axis.(b) Tags are placed along the yaxis.(c) Tags are placed along the z-axis.

Table 1 :
Dimensions of optimized antenna design.
Figures 8 and 9plot the refection coefcient and isolation of the proposed antenna at diferent substrate heights.Figure8is a graph of diferent substrate heights SH1.As SH1 increases, the impedance bandwidth of Port 1 widens, and the refection coefcient S 11 shifts downwards.When SH1 � 2 mm, the refection coefcients S 22 and S 33 are signifcantly better than when SH1 is at other values, and the impedance bandwidth of Ports 2 and 3 is also the widest.When SH1 takes diferent values, the isolation will undergo irregular changes, but it is less than −25 db in the 860−960 MHz frequency band.Te efect of the second layer substrate height SH2 on antenna performance is shown in Figure 3.2.Parametric Study.

Table 2 :
Comparison between the proposed and previous works.
International Journal of Antennas and Propagation