Tunable Platform Tolerant Antenna Design for RFID and IoT Applications Using Characteristic Mode Analysis

Radio frequency identification (RFID) is a key technology to realize IoT (Internet of Things) dreams. RFID technology has been emerging in sensing, identification, tracking, and localization of goods. In order to tag a huge number of things, it is cost-effective to use one RFID antenna for tagging different things. Therefore, in this paper a platform tolerant RFID tag antenna with tunable capability is proposed. The proposed tag antenna is designed and optimized using characteristic mode analysis (CMA). Moreover, this tag antenna consists of a folded patch wrapped around FR 4 substrate and a feeding loop element printed on a paper substrate. The inductive feeding loop is stacked over folded patch and it provides impedance match with RFID chip. Because of separate radiating and feeding element, this tag antenna has a versatility of impedance matching with any RFID chip. Furthermore, this tag is able to cover American RFID band (902–928MHz) and can be tuned to European RFID band (865–868MHz) by adding tunable strips. In order to demonstrate platform tolerant operation, the read range of RFID tag is measured by mounting it on different materials.Themaximum read range of RFID tag is 4.5m in free space or on dielectrics and 6.5m above 200 × 200mm metal plate, respectively.


Introduction
The m odern wireless technologies are driving this world towards a new paradigm which includes Internet of Things (IoT) and Internet of Everything (IoE). In IoT world, every thing will have its own unique identity. The Internet of Things (IoT) will provide an emerging integrated wireless platform, where physical and virtual things can be uniquely identified on a global scale and are connected with a global network. IoT becomes prevalent in both research and industries due to its unique emerging applications like sm art cities [1,2], con nected vehicles [2], healthcare monitoring [3][4][5], and so on [2]. Radio frequency identification (RFID) is one of the vital technologies of IoT since it enables a device (object) not only to share its unique digital code across a network wirelessly, but also to capture its physical status for localization on a global scale [6]. RFID in combination with IoT has opened a new paradigm and new era of applications [7]. A descriptive modal to enable an IoT network integrated with RFID tech nology is shown in Figure 1.
The thing's basic information will be stored in electronic RFID tag in form of electronic product code (EPC) or uni versal identification (UID) and this information can be read by wireless RFID reader, which can be accessed from any where through Internet [8][9][10].
Passive ultrahigh (UHF) RFID tags are more attractive for most of applications because of their long-read range and high data rate capabilities. Moreover, the low cost, easily printable structure of UHF RFID tags enables them to inte grate or embed inside tagged objects, to give them a unique attribute. However, UHF RFID tags are m uch more sensitive towards host tagging objects such as metals, wood, and glass. This sensitivity, on one side, is good for exploiting UHF RFID tags as sensing device. O n the other hand, this sensiti vity towards different surfaces requires a separate UHF RFID User n tag design for each tagged entity [7,11]. Therefore, it is advan tageous and cost-effective to design one low cost, platform tolerant antenna solution to tag most of the things.
In literature, there are many solutions available to solve this problem of platform tolerant UHF RFID tag especially for counter metallic surface effects [12,13]. In [14], a dual layer PIFA antenna was introduced for platform tolerant appli cations. However, the performance of this design is highly depending on location of vias. Also, the dual layer structure with introduction of vias makes it a costly solution. A single layer coupled loop based antenna fabricated on 3 m m thick FR 4 substrate was proposed in [15], for tagging metal and low permittivity dielectric materials. Although this proposed antenna works on metal with read range of 6 m, its major drawback is substrate thickness. In [16], a stepped impedance coupled patch antenna was designed on a thin substrate. However, this coupled patch has small bandwidth especially on metallic surfaces. Another folded patch antenna with dig ital edge serration was designed in [17], for tagging metallic objects. This antenna was wrapped around 3 m m thick sub strate with RFID chip m ounted on vertical edge, which leads to a structure which is expensive and difficult to fabricate. Also, the vertical chip m ounting configuration is more vul nerable to damage. Another solution was proposed in [18] by employing a hybrid dual port tag antenna, which consists of a dipole embedded with a patch antenna. This design has large size with 3.2 m m substrate thickness.
There are many techniques to design an efficient platform tolerant antenna such as impedance analysis, employing m et amaterials (electromagnetic bandgap (EBG), artificial mag netic conductance (AMC)), and circuit analysis.
Characteristic mode analysis (CMA) in combination with theory of characteristic modes (TCM) is becoming a preva lent systematic tool to design efficient term inal antennas. The use of TCM in antenna design was not new topic. It was first proposed by Garbacz and later redefined by Harrington and Mautz [19]. However, in some recent years, TCM was revisited to get physical insight into antenna designing. TCM is more attractive and systematic approach because of its following features [19,20]. (1) It provides a more systematic design approach rather than a brute-force. (2) It also allows designer to get physical insight into antenna's operation. (3) It can help to find resonance frequency of specific mode. (4) It also allows finding the optimum feed arrangement to excite these specific modes. Moreover, there are very few designs available using CMA for RFID [21,22] and IoT applications [23].
In this paper, a tunable platform tolerant folded patch antenna is proposed for UHF RFID and IoT applications using characteristic mode analysis. This antenna consists of a folded patch (wrapped around FR 4 substrate) and a small inductive feeding loop. The feeding loop printed on a paper substrate is stacked over folded patch configuration. The proposed folded patch is optimized using CMA to resonate at 915 MHz. The small feeding loop is connected with RFID chip to provide a conjugate impedance match.
Because of separate radiating and feeding element, the proposed tag antenna has a versatility of impedance matching with any RFID chip. Furthermore, this tag is able to cover US RFID band and can be tuned to European RFID band by add ing tunable strips. In order to validate platform tolerant operation, the read range of RFID tag is measured by m ount ing it on different materials. The maximum read range of RFID tag is 4.5 m in free space or on dielectrics and 6.5 m above 200 x 200 mm metal plate, respectively. Moreover, this tag can also be equipped with IoT based sensors to sense envi ronm ental effects and processes, thanks to its platform toler ant capability. The low cost, tunable, and platform tolerant feature of this antenna makes it suitable for RFID and IoT applications.

Theory o f Characteristic Modes (TCM).
Characteristic modes can be defined as orthogonal surface current modes of an arbitrary shaped conducting body, which totally depend on its shape and size and are independent of any feed source.
Moreover, characteristic modes (CM) can be derived by solving eigenvalue equations, obtained from M ethod of Moments (MoM) based impedance matrix as follows: where R and X are real and imaginary components of MoM impedance matrix; Jn are eigencurrents and A" are eigenval ues: Furtherm ore, because of the orthogonality of characteristics modes, they can be used to derive total surface current / of an antenna as follows: where is model excitation coefficient and it can be obtained as expressed by The product Jn determines the coupling between nth mode and excitation. It also provides information about which mode of antenna is excited by feed or incident electric field (£ ').

Characteristic Mode Analysis fo r Rectangular
Plate. The theory of characteristic modes (TCM) has been employed to calculate characteristic modes of a rectangular plate (160 x 25 m m 2) m ounted on a FR 4 (2 m m thick, loss less) substrate. The multilayer solver ofCST Microwave studio has been used for modal analysis. Figure 2 shows the normalized current distribution (at 915 MHz) of first four associated characteristic modes of rectangular plate (as shown in subset of Figure 3). In order to understand behavior of these modes, eigenvalues will be used in combination with normalized current distributions. The eigenvalue associated with resonating modes is zero; it is concluded that the smaller the eigenvalue associated with a mode is, the more efficiently it will contribute to radiation. Moreover, the sign of eigenvalue determines w hether a mode is storing electrical energy (A" < 0) or magnetic energy (A" > 0). The eigenvalues associated with aforementioned modes of rectangular plate in free space are shown in Figure 3. The mode J1 resonates near 1.1GHz as eigenvalue is zero near 1.1 GHz. Because of symmetry, the mode J1 will propagate even if we use half of the length of rectagular plate. As can be seen from Figure 2, the mode J2 behaves like an inductive mode; it can also be confirmed from its associated eigenvalue (A" > 0). Furthermore, mode J3 represents capacitator like current distribution with eigenvalue (A" < 0). Mode J4 is a resonating mode at 1.6 GHz.
Another param eter known as modal significance is also very useful in determining how efficiently a mode resonates. Also, with the application of external excitation source, modal significance determines the contribution of particular mode to total radiation. The modal significance associated with characteristic modes of 160 x 25 m m 2 rectangular plate is shown in Figure 4.
Mode 1 and Mode 4 are resonating modes and will con tribute to radiation, when excited by a source, since their modal significance approaches 1 (as shown in Figure 4).
Modal significance can be defined as follows: Additionally, characteristic angle computed from eigenvalues as expressed by (6) is also very useful in determining exact resonance frequencies especially where more than one mode needs to be excited (e.g., circular polarized antenna design). The characteristic angle associated with first four modes of 160 x 25 m m 2 rectangular plate are presented in Figure 5. For resonating modes a n _ 180°, therefore, Mode 1 and Mode 4 are resonating modes at 1.1 GHz and 1.6 GHz, respectively.
3. RFID Tag Antenna Design Using CMA a capacitance) is created and it becomes dipole like patch as shown in subset of Figure 6. As can be seen from Figure 7, the two modes are resonating at 920 MHz and 1.1 GHz, re spectively. Furthermore, it can be inferred from normalized current distribution (shown in Figures 2 and 7) that Mode 1 of rectangular plate and Mode 2 of dipole patch are same. It means that the introduction of slot has no effect on this mode. Inductive Mode 2 of rectangular plate becomes a resonat ing mode with introduction of capacitance introduced by small slot.

CMA o f Proposed Folded Patch.
The dipole patch is work ing at 920 MHz (US RFID bands 902 MHz to 928 MHz); however its length is more in order to use it as RFID tag. Due to symmetry, Mode 1 (resonating at 920 MHz) of dipole patch will still resonate, even ifhalf of the dipole patch is used (80 mm). In order to reduce the length further the patch is folded with same length on FR 4 substrate as shown in subset of Figure 8. The eigenvalue plot of resulting folded patch antenna is illustrated in Figure 8. To confirm its operation as platform tolerant design, the eigenvalue plot of one res onating mode is depicted in Figure 8, after placing this folded patch in free space and above a finite perfect electric conduc tor (PEC) boundary. It can be observed from eigenvalue plot that the resonance frequency of Mode 1 is almost same for both in free space and above PEC boundary conditions. However, the slope of eigenvalue plot of Mode 1 is differ ent for two different boundary conditions. Moreover, Mode 1 of folded patch has a steep slope, above metal plate as com pared with free space, which means that folded patch has more bandwidth in free space as compared with above metal.

CMA o f Proposed Folded Patch with Tunable Strips.
As can be seen from normalized current distribution of folded patch antenna expressed in Figure 9, since the current in Mode 1 is m aximum near folded end and decreases towards the slot, it behaves like cavity backed slot antenna. The effective length of current path can be increased by adding two tunable strips on both sides of folded patch near the slot as depicted in subset of Figure 10. Hence, the antenna can be tuned to European RFID band (866 M Hz-868 MHz). The tuning of folded patch to European RFID band can also be confirmed from eigenvalue and modal significance plot ( Figure 10) of this folded patch after adding tuning strips.

Simulation Results
In order to realize an RFID tag antenna, a feeding strategy should be proposed for the folded patch obtained after characteristic mode analysis. Since m ost of RFID tag chips have capacitive impedance, requiring the tag impedance to be inductive for conjugate match, therefore, inductive coupled loop is a good option to feed this antenna. Besides, the conjugate matching, a separate coupled inductive loop, also provides a versatility of impedance matching with any RFID chip, just by altering loop parameter, without any change in m ain radiator (folded patch).
The complete antenna geometry of proposed folded patch RFID tag is shown in Figure 11. Finally, the proposed design   consists of a folded patch as main radiator and a small inductive feeding loop in order to match with RFID chip. Furthermore, Figure 12 shows the surface current distri bution after placing the inductive feeding loop above folded patch; it is clear from Figure 12 that the feed loop excites the same mode as expressed in characteristics mode analysis of folded patch antenna (Figure 9). The proposed folded patch is wrapped around a low cost FR 4 substrate (er = 4.3, S = 0.025). Furthermore, the inductive feeding loop is printed on a 0.25 m m thick paper substrate.
The detailed dimensions of proposed design are illus trated in Figure 13. Antenna design parameters involved in simulation are optimized further using frequency domain solver of CST microwave studio and are listed in Table 1. Moreover, Alien Higgs H3 has been used as RFID chip with impedance (30-201 j) computed from equivalent circuit of parallel 1500 ohms resistance and 0.85 pF capacitance.
As shown in Figure 14, the simulated input impedance plot of this tag design confirms the working antenna in American RFID band (US) without tunable strips and can be tuned to European RFID (EU) band with tuning strips. The real and imaginary impedance of antenna is ranging from 30 to 40 Ohms and 180 to 205 Ohms, respectively, in required RFID bands, which is best m atched with Alien Higgs-3 RFID chip.
To verify antenna performance as platform tolerant de sign, for simulation purpose, the antenna is placed on two different materials: (1) a 200 x 200 mm2 plate with relative permittivity er = 4, representing a low permittivity dielectric; (2) a 200 x 200 mm2 metal plate. For both cases, the power reflection corresponding to conjugate match of this proposed design is shown in Figure 15. The 3 dB bandw idth of antenna is more than 40 M Hz for both cases in US band, with a little bit shift in center frequency for metal case. However, the bandw idth of antenna is enough to protect it from detuning. Moreover, the antenna also provides 3 dB bandwidth of 30 M Hz in EU band. Also, in this case, there is a small shift in center frequency as compared with metal and dielectric case.
Furthermore, the directivity of this tag antenna is 5.87 dBi, after m ounting it on a 200 x 200 mm2 metal plate as depicted in Figure 16.

Measurement and Discussion
A prototype of proposed RFID tag was fabricated on a ground substrate with one shorted end as shown in Figure 17. Alien Higgs H3 RFID chip is connected with small feeding loop printed on a paper substrate. This paper substrate containing feed loop is then pasted on slot of folded patch.
To validate the performance of antenna on different materials, theinputim pedanceofthistagw asm easuredusing Agilent E8363B vector network analyzer by following the procedure expressed in [24].
The impedance measurement setup is depicted in the inset of Figure 18. The outer coats of two coaxial cables were soldered together, while the single end of each cable was attached with open ends of feeding loop. The return loss of antenna was computed by formula given in [24] using param eter measured from this setup.
As compared with simulated results (Figure 18), a better impedance match was observed for 865 to 868 MHz band, whereas the value of measured return loss is more for 902 to 928 MHz band. Overall, the measured results show a good agreement with simulation results with a little reduction in bandwidth. This maybe due to some fabrication error or cable loss. Moreover, the measurement procedure also introduces some discrepancies.
Furthermore, the glue used to paste the feeding loop also introduces some change in permittivity and thickness of sub strate.
The theoretical read range of RFID tag can be estimated using Friis equation as follows: where Pr, Gr are transm itted power and gain of RFID reader, Ga is gain of tag antenna, and Pth is m inim um threshold power of chip. However, another better and more practical approach to estimate maximum read range with maximum perm itted EIRP is to calculate read range first for small value of EIRP with a fixed distance as expressed by where £ max is maximum read range of the tag, £ ref is refer ence fixed distance measured in lab, EIRPmax is maximum perm itted EIRP = 4 Watts for most of the regions, and EIRPref is reference EIRP of range measuring equipment. A Tagformance Pro device (from Voyantic Company) based setup was employed to measure maximum read range in lab as illus trated in Figure 19. A frequency sweep is run by the Tagformance with a fixed distance (£ ref) provided by foam spacer. The theoretical read range is estimated by system software using formula expressed by (8). The measured read range of proposed tag above 200 x 200 mm2 metal plate using Tagformance setup is shown in Figure 20. As is clear from Figure 20, the measured read range of proposed tag is 6. To verify the robustness of this tag solution, the read range of RFID tag was measured after m ounting on different materials in outdoor and indoor lab environments with 4 W EIRP. The measurem ent setup includes a laptop, Impinj R420 reader, and a circular polarized reader antenna. The read range m easurements are carried out at 920 MHz.
Furthermore, the different m ounting materials used in range measurem ent were of plastic, wood, glass, and book (200 x 200 m m 2). The maximum measured read range on aforementioned materials is illustrated in Figure 21. More over, the m aximum read measured on metal plate is 6.5 m, while the measured read range on all other materials is more than 4.5 m in indoor environment. However, there is a small reduction in read range observed in outdoor environment.
Moreover, the measured read range pattern of this tag design is shown in Figure 22. A good unidirectional read range pattern with maximum read distance of 6.5 m is acquired in xz-plane. Also, it can be observed from Figure 22 that the maximum read angle is tilted since the surface current distribution is more near folded edge.
The comparison of proposed design with some other RFID tags in terms of size and performance is listed in Table 2.     In Internet of Things (IoT), every object can be connected to Internet through a unique digital code to identify it on global scale. RFID in combination with IoT has opened a new para digm and era of applications. In order to tag a huge number of things, it is cost-effective to use one RFID tag antenna for different things. Therefore, in this paper, a platform tolerant folded patch antenna with tunable capability is proposed for UHF RFID and IoT applications using characteristic mode analysis. The proposed design consists of a folded patch wrapped around FR 4 substrate and a small inductive feeding loop printed on a paper substrate. The proposed folded patch isoptim izedusingC M A toresonateat915 MHz. Sincethistag has a separate radiating and feeding element, therefore it can be reusable and has a versatility of impedance matching with any RFID chip. Furthermore, in order to prove platform toler ant capability of proposed RFID tag, its read range is mea sured after m ounting on different material. The maximum read range of RFID tag is 4.5 m in free space or on dielectrics and 6.5 m above 200 x 200 m m metal plate, respectively. The platform tolerant capability of this antenna makes it favorable for integration with IoT sensors, which can be used to sense environmental effects and processes. Moreover, the low cost and tunable features of this antenna make it suitable candidate for RFID and IoT applications.