1. Introduction

Radio Identification(RFID) was proposed for higher efficiency in keeping and distributing consumer and industrial products en mass and avoiding time-consuming and tedious bar-code reading in warehouses and discount malls^[1]. It is demanded by numerous specific realms, for instance, distribution of medicine, clothes, expensive alcohols, whole-sale products, and so forth. Identifying a personnel at the entrance area for security reasons is the recipient of the RFID technologies. These days, Internet of Things(IoT) is being mentioned much and this makes people turn their eyes to the expectations of RFID.

The spectrum of RFID technologies is wide, and dependent on the specific requirements, the frequency band, the use of a battery in a tag and the read-range will be decided. For tracking military containers, the 400 MHz area is chosen with batteries. To check CDs and books out, 13.56 MHz is chosen for the tags on them. Neither as high as 2.4 GHz nor as low as 13.56 MHz, an upper UHF-band including 920 MHz is popular in RFID applications. Not like 13.56 MHz, this UHF-band is contact-free reading. To end up with meters of a distance in reading, we need antennas for tags. Dipoles and their modifications with the T-matching branch are adopted many times as in [2]^[2]. The quality of antennas excited by the chips decides that of the entire RFID system and the design methods of tag antennas seem mature^[3,4].

Antennas work really well, when they are separated from other antennas. But, in many cases, tags are stacked and placed right next to other tags, because they are stored in a large volume. As one tag is located in the near-field of other tag, the structures tend to electromagnetically interfere with each other^[5]. So, in this paper, we suggest a method to lower the nasty interference of two closely tag-dipoles and make the performances of the dipole antennas acceptable. By trying different techniques such as periodic EBGs and notch-generating stubs, we will reach an optimal geometry. A vertically laid H-shaped metal structure as an isolator is used to cut the electromagnetic coupling between the two dipoles in tandem with just a 2 mm-distance. The isolator is so thin to be inserted between the two dipoles. In the paper, we show you the enhancement of isolation by over 15 dB for 920 MHz. Also, each of the dipoles keeps the resonance for radiation at 920 MHz due to a good impedance match. These characteristics will help the RFID system work properly for cases where RFID tags are put on top of each other.

2. Case Study and Recommended Solution

A dipole is made to resonate at 920 MHz as follows.

Fig. 1(a) shows the dipole antenna which has a metal pattern on one side of substrate FR4(1 mm thick). A 7.4-cm long dipole causes the dip at 920 MHz as the resonance as shown in S₁₁ of Fig. 1(b). The omni-directional far-field pattern from the antenna has 1.97 dBi of peak-antenna gain as in Fig. 1(c). Because this basic antenna is made based on an almost ideal condition for resonance and electromagnetic radiation, the levels of S₁₁ and antenna gain are good.

그림. 1. 기본구조인 다이폴 안테나 (a) 구조 (b) 반사계수 (c) 전자파 방사패턴

Fig. 1. Dipole antenna as the basic structure (a) Geometry (b) S₁₁ (c) Far-field pattern

The dipole is no more alone. Two dipoles are located 3.5 mm apart. This gap is extremely small compared to the wavelength.

In Fig. 2(a), two dipoles of the same shape are positioned by a very small gap. Negative effects occur with a very high unwanted coupling(|S₂₁|=-3.75 dB) and impedance matching degraded by 11.25 dB as in Fig. 2(b). Fig. 2(c) shows the beam-pattern from either of two antennas is not very bad. Though the interference as the unwanted coupling is very high, the gain is not ignorable due to the folded-dipole effect, which is unintentional. Most of all, the interference between the two dipoles is large, and numerous techniques should be tried as follows.

그림. 2. 근접한 두 개 다이폴 안테나 (a) 구조 (b) 반사계수 및 결합계수 (c) 전자파 방사패턴

Fig. 2. Dipole antennas with a small spacing (a) Geometry (b) S₁₁ and S₂₁ (c) Far-field pattern

2.1 Line with stubs and U floating lines as an isolator

One-end open stubs are used to generate a notch as a narrow stop-band.

In Fig. 1(a), stubs are one eighth of the transmission line and this line has the same length as the dipole. This isolation device is sandwiched by the two dipoles. The line is parallel with the two dipoles. The isolation does not become good as S₂₁ is nearly –2 dB as in Fig. 3(b). Two lines are bent as letter U and put under the (-) arm and (+) arm of the dipole.

그림. 3. 개방형 스터브가 부착된 선로형 간섭저감 구조 (a) 구조 (b) 반사계수(흑색) 및 결합계수(적색)

Fig. 3. Isolation device having two open-ended stubs (a) Geometry (b) S₁₁(Black) and S₂₁(Red)

The length of the U-shaped lines is the same as that of the dipole. In Fig. 4(a), they are rotated by 90o from the orientation of the dipole so as to take the coupling energy between the two dipoles and resonate it on the U-shaped lines and dismiss its radiation to the air not the other dipole. Fig. 4(b) shows resonance frequency shifted much as with black & green curves and isolation is bad like Fig. 3. Other structures need trying.

그림. 4. U자형 선로 쌍 간섭저감 구조 (a) 구조 (b) 반사계수(흑색) 및 결합계수(적색)

Fig. 4. Isolation device having two open-ended stubs (a) Geometry (b) S₁₁(Black) and S₂₁(Red)

2.2 Capacitive EBGs and Inductive EBGs

Periodic patches can make a capacitive EBG.

Fig. 5(a) has the capacitive EBG with larger cells. The cell size is the 26th of the wavelength. A very poor impedance match as –2 dB is obtained, and isolation is 5 dB as in Fig. 5(b). Impedance match is broken as smaller cells(the 54th of the wavelength.) are used in the EBG as in Fig. 5(c), but better isolation as in Fig. 5(d). This is not good news to antenna designers, though a 10 dB isolation is given, because a serious impedance mismatch makes the resonance of the dipole antennas go away. Opposite the capacitive surface, we introduce the inductive surface^[6,7].

그림. 5. 용량성 차단면의 큰 쎌 및 소형 쎌 구조 (a) 큰 쎌 구조 (b) 큰 쎌 구조의 경우 다이폴 반사계수(흑색) 및 결합 계수(적색) (c) 소형 쎌 구조 (d) 소형 쎌 구조의 경우 다이폴 반사계수(흑색) 및 결합계수(적색)

Fig. 5. Capacitive EBGs of larger and smaller cells (a) Geometry of larger cells (b) S₁₁(Black) and S₂₁(Red) of the larger cell case (c) Geometry of smaller cells (d) S₁₁(Black) and S₂₁(Red) of the smaller cell case

Fig. 6(a) has the inductive EBG with larger cells of a 3-turn coil. The cell size is the 26th of the wavelength. The resonance frequency of impedance match is shifted to 700MHz~800MHz as in Fig. 6(b). Isolation becomes 5 dB. As impedance match is broken at 920 MHz, the dipole antennas cannot be operated as antennas. The size of the cell is reduced to half of Fig. 6(a) and there are more cells in the surface. The entire structure of the coil cells looks similar to that of the patch cells, because there are a lot of cells filling the surface as in Fig. 6(c). Because of the aforementioned geometrical similarity between Fig. 6(a) and Fig. 6(c), their s-parameters look similar. Like Fig. 6(b), Fig. 6(d) has poor impedance match and increased isolation. Different from the techniques up to this point, to solve impedance match for the resonance and isolation at once, another method is needed.

그림. 6. 유도성 차단면의 큰 쎌 및 소형 쎌 구조 (a) 큰 쎌 구조 (b) 큰 쎌 구조의 경우 다이폴 반사계수(흑색) 및 결합계수(적색) (c) 소형 쎌 구조 (d) 소형 쎌 구조의 경우 다이폴 반사계수(흑색) 및 결합계수(적색)

Fig. 6. Inductive EBGs of larger and smaller cells (a) Geometry of larger cells (b) S₁₁(Black) and S₂₁(Red) of the larger cell case (c) Geometry of smaller cells (d) S₁₁(Black) and S₂₁(Red) of the smaller cell case

2.3 Proposing a proper isolating device

We suggest a new type of decoupling device to be inserted between the two dipoles.

Fig. 7(a) has a new isolating device as the middle layer. There are vertical H-shaped metals parallel to the dipoles, while the others 90° rotated from them. H-shaped metals are made on both sides of a 0.5 mm thick FR4. The longer H-shaped metal has L_isol as the 7/12 times half wavelength and W_isol is the same as that of the dipole. The shorter H-shaped metal has L_isol as the 2/12 times half wavelength and W_isol is the same as that of the dipole. These geometrical values are given by the parametric study and these make the s-parameters which show isolation and impedance match are satisfied at once as presented in Fig. 7(b). At the target frequency, S₁₁(Blue) and S₂₁(Green) are below – 10 dB together. This positive effects are confirmed by the measurement of S₁₁ and S₂₁ on the manufactured devices and dipoles using the vector network analyser. Fig. 7(c) shows S₁₁ and S₂₁ at the target frequency are below –10 dB. We can tackle the impedance match and isolation problems of the two close antennas. The far-field pattern is plotted as Fig. 7(d) which keeps the omni- directional pattern as expected with gain of –1.23 dBi. The negative value in gain is explained as follows. The electromagnetic fields from the current on the TX dipole antenna induce the coupling in the isolation device(an EBG). The energy to be used for radiation of the TX antenna is reduced by this unwanted induction. This results in the antenna gain of –1.23dB. Following the suit, this is acceptable to RFID wireless links.

그림. 7. 수직 H형 차단구조 (a) 두 다이폴 사이의 주기적으로 놓인 수직 H형 차단층 (b) 다이폴의 반사계수(청색) 및 결합계수(녹색) (c) 회로망 분석기로 측정한 반사계수(흑색) 및 결합계수(적색) (d) 방사파 패턴

Fig. 7. Vertical H-shaped isolating device (a) Geometry of larger cells (b) S₁₁(Blue) and S₂₁(Green) of the dipoles (c) S₁₁(Black) and S₂₁(Red) of the dipoles measured by the vector network analyzer (d) Far-field pattern of the dipoles

3. Conclusion

In this paper, we presented the development of an effective decoupling device for the two RFID dipoles in the proximity of each other to have good isolation as well as impedance match. While a stub-attached line, U-shaped floating lines, EBGs do not succeed in solving the problems, periodical vertical H-shaped metals can satisfy the impedance match at the UHF RFID frequency and inter-antenna isolation. This method was verified by the electromagnetic full-wave simulations and measurement. This method will be advantages in isolating the stacked RFID tags as a thin structure.

Appendix

In this section, we would like to prove the reason that the proposed decoupling device can be classified to the EBG (electromagnetic bandgap) from typical standpoints of view.

The boundary conditions we gave to the simulation of the unit cell of the EBG are the PEC(Perfect Electric Conductor) for the top and bottom of the computation space and the PMC(Perfect Magnetic Conductor) for the side-walls as others do in the electromagnetics community. When the electric field is vertically polarized, the PEC is right to be assumed for a PBC problem. The magnetic field is horizontally polarized and the PMC is right to be assumed for the PBC problem. This is equivalent to the plane-wave test setup. It is conducted as below.

그림. A-1. EBG 구조를 평가하는 실험공간

Fig. A-1. Test Configuration of the EBG structure

Then, the EM energy entering port 1 becomes equal to the plane-wave incidence. The plane-wave is incident to the structure of interest. Then, if it works as the EBG. The reflection coefficient obtained at port 1 is plotted as below.

그림. A-2. EBG 구조의 평면파 입사에 대한 반사계수

Fig. A-2. Reflection coefficient of the EBG structure with respect to the plane-wave incidence

As the figure presents, the reflection coefficient(called $\Gamma$) is nearly +1 in magnitude and 0o in phase, which means it is approximately +1. The small deviation from +1 is mainly caused by the dielectric loss and conductor loss which is a common phenomenon. Next is the dispersion diagram.

그림. A-3. EBG 구조의 분산도

Fig. A-3. Dispersion daiagram of the EBG structure

Based on the s-parameters of the plane-wave test, we can extract the dispersion diagram as the $\omega $-$\beta $ relationship. $\omega $ and $\beta $ mean the angular frequency and the propagation constant, respectively. It is displayed above. We can clearly see the EBG, that is to say, a stopband where $\beta $ is not working as observed in the dispersion curve. This implies that propagation is not possible and it coincides with the frequency region of 920 MHz that the coupling between the two antennas is the lowest. There, isolation becomes remarkably good.