Low-Profile Interdigitated UHF RFID Tag Antenna for Metallic Objects

This paper presents a novel miniature interdigitated ultra-high frequency (UHF) radio frequency identification (RFID) tag antenna that can be placed on metallic objects. The tag structure comprises two horizontal strip lines, each loaded with seven identical open stubs, and an integrated circuit (IC) chip connected directly to the feed lines in the middle of the structure. The perfect match to the IC chip’s impedance is realized by adjusting the length of the loaded open stubs and the spaces between the stubs. Molding the antenna’s geometry can be applied to realize conjugated impedance with any sort of IC chip due to the flexibility of the tag structure. It is fabricated on a single Polytetrafluoroethylene (PTFE) substrate. Moreover, the proposed structure does not involve any metallic vias or shorting walls, which makes its construction simple and suitable for mass production. The tag of the size of <inline-formula> <tex-math notation="LaTeX">$55.2mm\times 44.2mm\times 1.5mm$ </tex-math></inline-formula> yields a total realized gain of −4.11 <italic>dB</italic> at the North American band (902 – 928 <italic>MHz</italic>) while being placed on a <inline-formula> <tex-math notation="LaTeX">$20cm\times 20cm$ </tex-math></inline-formula> metallic plate. The measured detection distance is <inline-formula> <tex-math notation="LaTeX">$8.14~m$ </tex-math></inline-formula> on metallic objects and <inline-formula> <tex-math notation="LaTeX">$5~m$ </tex-math></inline-formula> in the free space. A good match between the measured and simulated results is observed.


I. INTRODUCTION
Nowadays, ultra-high frequency (UHF) radio frequency identification (RFID) technology is widely implemented due to its diverse advantages. Exemplary applications include retail, inventory management, and patient monitoring. From the practical perspective it is preferable to realize tags that can be mounted onto different materials while ensuring appropriate functionality. However, the nature of the tagged surface affects the performance of the tag. This effect is pronounced in the case of tagging metallic surface, where The associate editor coordinating the review of this manuscript and approving it for publication was Yang Yang . the desired surface current of tag deteriorates and affects the gain, input impedance, antenna efficiency, etc. [1], [3]. Hence, designing a small low-profile tag that can be applied to metallic objects while maintaining its performance, is still a challenge.
In the literature, different approaches have been investigated to overcome the presence of metallic structure. The artificial magnetic conductor (AMC) substrate with an offset of metallic vias is utilized to improve the tag's performance (gain/read range) [4], [7]. The slab of AMC possesses a high dielectric constant and an air gap or foam separated between the meandered dipole antenna and AMC substrate. In addition, the employment of electromagnetic bandgap VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ (EBG) could prevent the impedance mismatch and the tag's gain G a is also improved [8]. The EBG structure has been employed as three-layer mushroom-like EBG material [9], EBG cells surrounding the antenna with metallic vias [10], or by etching a lattice of circular slots in the EBG ground surface [11]. These approaches significantly increase the complexity, thickness, and tag's cost. In [12], a reflector is integrated with a tag slab to improve performance; however, its construction is bulky. Furthermore, various methods have been developed to improve the tag's performance, e.g., a thick slab with loss tangent [13], foam or air foam inserted between the tag and the tagged surfaces [14], [15], air gap separating two substrates [16], parasitic slab [17], and planar inverted-F antenna (PIFA) [18]. Unfortunately, these approaches increase the difficulty of the fabrication process. Folded-patch antenna has been proposed to enable miniaturization, with the shorting stubs mainly applied to shift the resonance frequency down [19], [22]. Further size reduction is realized by increasing the tag thickness to 3 mm [23] or by wrapping three conductor layers around two substrates of foam [24], [25]. However, the stub position significantly affects the tag's performance. Also, the structure is not practical for certain applications due to dual-layer structure and considerable thickness. In addition, utilization of adhesives to preserve the wrapped structure is questionable. Here, we aim at designing an UHF RFID tag antenna on a single slab using a flexible design approach, which is simple in construction, inexpensive, and convenient for mass production.
The main task of the tag antenna is to efficiently scatter the received power back from the interrogator. In this regard, ideal matching between the tag antenna and integrated circuit (IC) chip is essential to maximize the power transmission [26]. Generally, the impedance of the IC chip that is integrated with the tag's antenna has a high negative reactance (50 to 200 ) and low resistance (3 to 150 ). The simple structure of the folded dipole antenna boosts its utilization, but the backing metallic surface could affect the antenna's input impedance. In addition, a design of a compact UHF RFID dipole antenna with the size not exceeding 2, 500 mm 2 is a challenge because the compact low profile tag normally exhibits a high resonant frequency. To alleviate this issue, different approaches have been suggested for dipole antennas to attain matching with the reactance of the mounted IC chip, but maintaining compact size, such as nested loop [27] and T-match [28]. In [29], the outer strip lines are connected to C-shaped resonators where the presence of the outer strip lines assures matching to the IC chip's impedance. In [30], a folded dipole antenna has been proposed with symmetrical circular loads; the structure comprises an inner circular part surrounded by outer trace. Optimizing these loads can control the antenna impedance to match the capacitive reactance of the IC chip. The three-arm folded dipole antenna is incorporated with an additional element which assists in precise tune of the antenna's impedance [31]. The two meandered arms and feed line of modified dipole fine tune tag's complex impedance [16]. The tag antenna in [32] consists of a matching loop connected directly to the folded dipole, which is combined with two patches. In [33], a loop antenna with two pairs of meandered lines can match the IC chip's impedance through a miniature shunt-stub. In general, a flexible low-profile UHF RFID tag antennas that can be readily matched to a variety of IC chip are highly desirable.
In this work, a novel miniature low-profile tag antenna is proposed for the UHF North American band (902 -928 MHz), which utilized an interdigitated structure. The open stubs are mainly lengthened to increase the inductance of the tag antenna, where they work as inductors. This resulted in downward shifting of the tag's resonance frequency. The latter can be easily adjusted owing to the exclusion of the shorting walls or metallic vias, instead the flexible antenna structure ensures that an optimum reflection coefficient response can be obtained. The tag structure consists of horizontal strip lines, each loaded with seven identical open stubs. An inexpensive polytetrafluoroethylene (PTFE) substrate is employed to implement the tag. Unlike for many designs reported in the literature, no complex fabrication process is required. The proposed structure works effectively while mounted on metallic surfaces. The simulation results exhibit good agreement with the measurements.

A. ANTENNA STRUCTURE
This research proposes an interdigitated tag structure for miniaturization purposes. Fig. 1 depicts the geometry. The antenna is implemented on an inexpensive 1.5 mm thick PTFE slab (relative permittivity 2.55, loss tangent 0.0015 at 10 GHz). Two horizontal strip lines loaded with seven identical vertical open stubs and the feed lines form the tag antenna. The Higgs 4 strap chip is connected directly to the terminal of the feed lines in the middle of the antenna structure. The selected IC chip features a reading sensitivity P IC of −20.5 dBm and an input impedance of 20.97 − j193. 16 . The dimensions of the feed lines are precisely chosen to be suitable to mount a Higgs 4 strap chip. The width W 1 of feed lines is 4 mm, whereas the gap G between the feed lines is set to 3 mm. The loaded open stubs work as inductors resulting in increasing the inductive reactance of the antenna, which is useful for tuning the resonant frequency [34]. The open stubs can be characterized as lump elements L os and C os which are placed in parallel [35]. The inductance L os of the each open stub is computed as [36], where L i is the stub's length (3.21cm), W is the stub's width (0.3cm), and t is the thickness of the copper layer (0.0035cm). Thereby, the value of L os is 23nH . The formed capacitance C os between ground and each stub is computed as [35], where A T is the stub's size, h is the substrate's thickness, and ε = ε o ε r with ε r being the dielectric constant of the substrate. The capacitance C os of each stub is 1.52pF. After the lumped elements values are determined, the open stub's impedance (Z onestub ) can be calculated as, The length L of the feed lines determines the length of the loaded open stubs which enables optimization. The tuning of the operating frequency is performed by adjusting L, which controls the length of the feed lines and the loaded open stubs, as well as the width W of the stubs. The parametric analysis of the space S 1 between the stubs, and the space A between the horizontal strip lines and the end of load open stubs is carried out to minimize the reflection coefficient response. Tuning of these parameters allows the antenna to realize an optimal impedance matching with the selected IC chip, where it works as an input impedance network. This structure has the flexibility to match any sort of IC because of the inductive coupling approach [37], [39]. The presented structure is designed to operate at the North American band (902 -928 MHz).

B. DESIGN STAGES
The designed tag antenna is simulated and analyzes using CST Microwave studio. The Higgs 4 chip with two copper straps was precisely modeled through simulation, showed in [40]. When performing the simulation, the antenna was mounted on a 20cm × 20 cm perfect electrical conductor (PEC) surface with a total thickness of 1 mm. Notably, the tag performance in terms of reflection coefficient, gain, read patterns, realized gain, directivity, or read range, is not affected by changing the thickness of the backing metallic surface [26].
The simulation starts with two horizontal strip lines loaded with three open stubs, and the Higgs 4 strap chip connected directly to the terminal of feed lines. At this stage, the resonant frequency of the structure was 2.116 GHz as shown in Fig. 2(a), which is considered too high for RFID implementations. Next, two extra open stubs were added to each horizontal strip line to increase the inductance of the tag antenna and shift the resonance downward. In this stage, each horizontal strip line was loaded with five identical open stubs. As shown in Fig. 2(a), the resonant frequency is shifted further downward to 1.436 GHz. However, the operating frequency is still higher than the RFID band. Adding further open stubs results in shifting the resonant frequency to the RFID operation band (860 -960 MHz) because of the increment in the antenna inductive reactance. In this stage, seven identical open stubs were connected to the horizontal strip line as seen in Fig. 2  (902 -928 MHz) as seen in Fig. 2(b). The optimal match with the IC chip was achieved at 915 MHz. Additional inductive reactance is found at each stage when adding extra open stubs. This way, a minimum reflection coefficient value is realized, and a tag with an acceptable gain is obtained.

C. CURRENT ANALYSIS
This section presents the simulated surface current distribution. The analysis is performed while the tag is mounted on the upper surface of the PEC layer. Fig. 3(a) shows the results for the first version of the tag (horizontal strip lines loaded with three open stubs). The surface current is equal to 848 A/m. In the second stage as seen in Fig. 3

D. PARAMETRIC STUDY
The CST software was used to perform parametric analysis of the proposed structure. The antenna response was optimized by tuning feed lines length L, which is directly related to the open stubs' length with a flexible tuning mechanism, open stubs width W , and the spaces A and S 1 . The main factor in shifting the resonant frequency downward is the increment of L as shown in Fig. 4(a)  seen in Fig. 4(b). Further, the effect of A was studied where the length of the identical open stubs loaded on the horizontal strip lines is determined. A was adjusted to realize the best reflection response as shown in Fig. 5(a), with the achieved value of −39.2 dB. The parameter S 1 controls the strength of the mutual coupling between the open stubs, and facilitates obtaining a perfect impedance match by maximizing the power transmission coefficient (τ = 1) as depicted in Fig. 5(b). Interdigitated structure's performance is effectively improved by varying the value of S 1 and W as exhibited in Fig. 6. The G a improved from −9.2 to −4.07 dB while changing S 1 value from 0.2 to 0.4 mm as seen in Fig. 6(a). Besides that, the tag's performance in terms of realized gain is improved from −8.45 to −4.11 dB for varying W from 2.7 to 3 mm as seen in Fig. 6(b).

III. RESULTS AND DISCUSSION
This section presents the simulated and measured results of the proposed tag antenna. The Voyantic Tagformance Pro measurement system is utilized to obtain the realized gain G r , tag sensitivity P tag , and read range r of the presented structure. The measurement is conducted while the tag antenna is mounted on a 20cm × 20cm metal plate. Fig. 7(a) depicts the realized gain, which is −4.11dB at 915 MHz according to simulation, and −6.58dB at 920 MHz, according to the measurement. The simulated G r in the free space is −7.83dB at 911 MHz. The difference between the measured and simulated results can be attributed to the IC chip impedance variations and etching/fabrication defects. Fig. 7(a) depicts the measured P tag which is equal to −13.91dBm at 920 MHz while being mounted on metallic surface.
The detection distance is computed using the Friis formula where λ is the wavelength, P EIRP refers to the RFID reader permitted output power (4 Watt), and P th is the threshold power of chosen Higgs 4 strap chip. Fig. 7(b) shows the measured detection distance of the proposed tag antenna, which is 8.14 m on a metallic surface and 5 m in the free space. The computed detection distance is 10.88 m on the metal sheet.
The backing metallic surface acts as a reflector that reduces back radiation of the tag and reflects the electromagnetic waves in-phase, thereby improving the tag's performance in contrast to the free space condition. Fig. 8 shows the read pattern of the proposed tag at 920 MHz. The tag is rotated in the x, y, and z axes during measurements to plot the measured read patterns in yz, xz, and xy planes while the reader is mounted at a fixed distance. The maximum detection distance appeared in the boresight direction (θ = 0 • ) and r is truncated beyond θ = ±90 •  because of the existence of the metallic surface which operates as a reflector. Fig. 9 illustrates the effect of varying dimensions L y and W x of the backing metallic surface on the tag's performance in terms of the reading distance. First, the length L y of the metallic plate varies from 10 to 20 cm, whereas its width W x is kept constant at 20cm, as shown in Fig. 9(a). The maximum detection distance is 8.14 m at the operating frequency. Notably, the tag's performance exhibited a similar response for various values of L y . Subsequently, W x varies from 10 to 20 cm for fixed L y = 20cm. The reading distance dropped to 4.56m when W x changed from 20 to 10 cm due to the effect on the electrical flux strength at θ = 0 • , which is only affected while varying W x as shown in Fig. 9(b) [38]. These results indicate that the resonant frequency is still stable when the dimensions of metallic surface are changed, in other words, the RFID tag response to reader still appears over same frequency range while either L y or W x of metallic surface are varied. This feature is preferable for RFID metalmountable implementations. Table 1 shows a comparison of state-of-the-art metalmountable UHF RFID tag antennas. Reference [41] reported a dual-layer tag antenna constructed from four PIFAs and the impedance matching obtained by shorting the stub lines to the ground. In [8], a novel folded dipole antenna with a symmetric dual-layer electromagnetic band gap (DLS-EBG), whereas its structure included an offset of vias. In these studies [8], [41], the general structure of the multi/double-layer tag antenna, is over 2 mm thick and contains metallic vias. Such construction is not desired for metal-mountable UHF tag antennas, and adjusting the input impedance/operation frequency is tedious. Likewise, their complex implementation affects the fabrication cost to a large extent. To overcome the metallic effect, the researchers in [14] used a foam spacer with a total thickness of 1 cm. The reported works in [18], [31], and [42] fabricated on a single-layer substrate and their structure included several metallic vias, which increase the fabrication complexity level and cost. Moreover, a folding polyethylene terephthalate around the foam with shorting stubs was reported in [23], [43], and [46]. However, the highest realized gain is 1 dB for the design proposed in [23], but its thickness is over 2 mm, which is not preferable for metal mountable implementations. This is the reason why the structure proposed in this research does not include shorting walls or metallic vias. Our tag antenna is fabricated on a single-layer of PTFE slab, which allows for its straightforward deployment on metallic objects. In addition, the structure does not require any further complex fabrication processes, therefore, it is suitable for mass production.

IV. CONCLUSION
In this research, a novel low-profile metal-mountable UHF RFID tag antenna is proposed. The antenna is formed as an interdigitated structure comprising two horizontal strip lines loaded with seven identical open stubs. The measured detection distance is 8.14 m on a metal structure and 5 m in the free space. The proposed structure has a flexible tuning mechanism, low fabrication cost, and it is simple in construction. The aforementioned features make is suitable for mass production, and a variety of applications.  His research interests include computational modeling, optimization, uncertainty quantification of engineered systems with an emphasis on methods for multifidelity modeling and machine learning, aerodynamic shape optimization, aerodynamic flutter, model-based nondestructive evaluation, microwave devices, and foodenergy-water nexus. He is currently a Professor with the Department of Engineering, Reykjavik University, Iceland. His research interests include CAD and modeling of microwave and antenna structures, simulation-driven design, surrogate-based optimization, space mapping, circuit theory, analog signal processing, evolutionary computation, and numerical analysis. He is currently serving as the Associate Editor for the IEEE JOURNAL OF RADIO FREQUENCY IDENTIFICATION. Since April 2021, he has been a Distinguished Lecturer of the IEEE Council on RFID. He is a fellow of the Academy of Sciences Malaysia and the ASEAN Academy of Engineering and Technology. His current research interests include RFID antennas, smart and re-configurable antennas, and multifunctional antennas.
EFFARIZA HANAFI (Member, IEEE) received the B.Eng. degree (Hons.) in telecommunications from The University of Adelaide, Adelaide, SA, Australia, in 2010, and the Ph.D. degree in electrical and electronic engineering from the University of Canterbury, Christchurch, New Zealand, in 2014. She joined the University of Malaya, Kuala Lumpur, Malaysia, where she is currently a Senior Lecturer in electrical engineering. She has authored or coauthored journal articles in ISI-indexed publications and refereed conference papers. Her research interests include multiple antennas systems, RFID, cognitive radio networks, cooperative communications in wireless communications, the Internet of Things, and 5G networks. VOLUME 10, 2022