Jasmin Grosinger
©SHUTTERSTOCK.COM/DIZAIN
The Internet of Things (IoT) refers to a structure that links everyday objects and the Internet. To enable the IoT, a massive deployment of wireless IoT nodes is required; predictions indicate that trillions of these nodes will be needed. This trend raises sustainability issues on environmental, economic, and societal levels. Concerning environmental and economic problems, a significant design challenge is to create wireless IoT nodes that operate at ultralow power to avoid the ecotoxicity of batteries and the prohibitive maintenance costs of battery replacement [1]. Another design challenge is to reach high levels of integration based on low-cost CMOS technologies [2] to limit the carbon footprint and costs associated with the nodes’ production and end of life. Concerning societal issues, a significant design challenge is to guarantee the security and privacy of data in IoT nodes.
The microwave community’s work aims to solve, at least to some extent, these sustainability issues by presenting design solutions for the IoT. Researchers have shown RF design solutions for ultralow-power wireless communication systems, focusing on specific needs and challenges in IoT applications. For example, RF design solutions have been presented, focusing on how novel IoT system designs can be rapidly tested in their respective application environments [3]. Other solutions show how batteryless or passive IoT nodes can be created that provide security and privacy of data [4] and passive sensing capabilities [5]. In addition, solutions demonstrate how to create passive miniaturized IoT nodes [6] and passive IoT nodes based on new sustainable materials [7].
Thus far, specific ultralow-power wireless communication systems have been investigated and classified for the particular wireless communication technology used. The examples are passive far-field [3], [5], [6], [7], [8], passive near-field [6], [8], and active far-field communication technologies [9], [10], which vary in terms of the communication distance, data range, and type of devices used. One specific ultralow-power wireless communication system of significant interest for IoT systems is high-frequency (HF) RFID systems based on passive near-field communication (NFC) technology [11].
HF RFID systems are grouped according to their communication standards, operating at a frequency of 13.56 MHz. These standards differ, for example, in terms of their modulation schemes and bit rates and are specifically tailored to achieve specific communication ranges. The systems are designed to meet vicinity standards ISO/IEC 15693 and ISO/IEC 18000-3, which cover communication ranges of up to 1 m, and proximity standard ISO/IEC 14443, which covers the communication of up to 10 cm. Furthermore, NFC systems are closely related to HF RFID proximity systems, which were released to meet ISO/IEC 18092.
Although various HF RFID systems comply with different standards, the operating principle governing all HF RFID systems is the same. The systems rely on the magnetic resonant coupling between a reader and a passive, batteryless, transponder (tag), often called a smartcard or label [12]. Figure 1 shows a photo of an example HF RFID system.
Figure 1. A photo of an example HF RFID system. The system relies on a communication link between an ACR122U RFID reader and an HF RFID tag, which consists of a coil and a chip, i.e., a Mifare Desfire EV1 chip.
The reader transfers RF power and data toward the tag, providing all the energy needed to operate the passive tag. Following Maxwell’s equations [13], [14], the reader generates an alternating magnetic field, penetrating the cross section of the reader coil area and the area around the coil [12]. If a resonant tag is placed within the field of the reader coil, the tag draws energy from the magnetic field following the law of induction [14]. The voltage induced in the tag coil generates the voltage supply for the tag chip and data transfer from the reader to the tag. The tag communicates its data to the reader using load modulation [12]; the tag chip modulates the tag coil current by varying its input impedance according to the tag data stream (e.g., the tag ID). The change in tag coil current can be sensed at the reader coil as an impedance variation, allowing the remote tag to modulate the voltage at the reader coil.
Conventional-sized HF RFID tags based on the proximity standard must be classified as low-power devices. The tag chip experiences an input power of several 100 ${\mu}{\text{W}}$, allowing the implementation of sophisticated security mechanisms to ensure data security and privacy [4]. In contrast, HF RFID tags based on the vicinity standards experience a chip input power of approximately 10 ${\mu}{\text{W}}$ or less. We thus classify them as ultralow-power wireless communication devices.
An equivalent circuit generally models an HF RFID system to facilitate system design. Figure 2 shows an example equivalent circuit model [6], [11]. The HF RFID reader consists of a reader chip, matching network, and reader coil modeled by the inductance ${L}_{\text{R}}$, resistance ${R}_{\text{R}}$, and capacitance ${C}_{\text{R}}$. The tag is composed of a tag coil modeled by ${L}_{\text{T}}$, ${R}_{\text{T}}$, and ${C}_{\text{T}}$, and the tag chip is modeled by the equivalent parallel circuit of ${R}_{\text{Chip}}$ and ${C}_{\text{Chip}}$. As mentioned previously, the tag coil is penetrated by the alternating magnetic field generated by the reader coil. The fraction of the magnetic field aligned perpendicular to the tag coil induces the voltage in the coil, resulting in the chip input voltage ${V}_{\text{Chip}}$.
Figure 2. An example equivalent circuit of an HF RFID system [6], [11]. The equivalent circuit consists of a reader and a tag. The reader is composed of a reader chip, matching network, and reader coil modeled by the inductance ${L}_{\text{R}}$, resistance ${R}_{\text{R}}$, and capacitance ${C}_{\text{R}}$. In contrast, the tag is composed of a tag coil modeled by ${L}_{\text{T}}$, ${R}_{\text{T}}$, and ${C}_{\text{T}}$, and the tag chip is modeled by the equivalent parallel circuit of ${R}_{\text{Chip}}$ and ${C}_{\text{Chip}}$ with the chip input voltage ${V}_{\text{Chip}}$. Inductive coupling between the reader and tag coils is characterized by the coupling coefficient k [12].
The coil models in Figure 2 use the simplest form of equivalent circuits to model the reader and tag coils [15]. The equivalent circuit consists of an inductance L R|T that models the coil inductance, a resistance R R|T that models conductive and dielectric losses in the coil metal and tag substrate, and a capacitance C R|T that models parasitic capacitive coupling between the coil windings. Based on this circuit model, the respective resonance frequencies of the reader and tag coils can be determined by [16] \[{f}_{{\text{R}}{|}{\text{T}}} = \frac{1}{{2}{\pi}}\sqrt{\frac{1}{{L}_{{\text{R}}{|}{\text{T}}}{C}_{{\text{R}}{|}{\text{T}}}}{-}\frac{{R}_{{\text{R}}{|}{\text{T}}}^{2}}{{L}_{{\text{R}}{|}{\text{T}}}^{2}}}{.} \tag{1} \]
Additional components in the coil equivalent circuits must be considered in the case of more sophisticated coil designs. These can include RFID tags exploiting system-on-chip and system-in-package concepts [6] or broadband coil models investigating the coexistence and interoperability of HF RFID systems with wireless power transfer systems [17].
The power transfer efficiency h generally characterizes the wireless power transfer in HF RFID systems. The efficiency depends mainly on the coil’s characteristics, positions, and orientations in space. In free space, the power transfer efficiency between two circular coils aligned coaxially is [13], [18], [19] \[{\eta} = \frac{1}{{16}{R}_{\text{R}}{R}_{\text{Chip}}}\frac{{\left({{\mu}_{0}{2}{\pi}^{2}{f}_{\text{R}}{N}_{\text{R}}{N}_{\text{T}}{r}_{\text{R}}^{2}{r}_{\text{T}}^{2}}\right)}^{2}}{{\left({{r}_{\text{R}}^{2} + {z}^{2}}\right)}^{3}} \tag{2} \] where ${R}_{\text{chip}}$ is the chip resistance, and ${\mu}_{0}$ = 4${\pi}$E-7 H/m is free-space permeability. ${\text{N}}_{{\text{R}}{|}{\text{T}}}$ is the respective number of coil windings. ${\text{R}}_{{\text{R}}{|}{\text{T}}}$ is the respective coil radius. z is the distance of the tag along the reader coil center axis (i.e., z = 0 corresponds to the center of the reader coil). The efficiency will decrease with lateral and angular coil misalignments [19]. It is also associated with the read range of the RFID system. The read range of an RFID system is defined as the maximum communication range between the reader and the tag.
The reader’s matching network typically consists of three parts [20]. Starting from the reader coil, the matching network consists of damping resistances applied to reduce the quality (Q) factor of the reader coil and broaden the reader’s bandwidth. The Q factor is optimized to maximize the magnetic field strength while maintaining an acceptable bandwidth for the tag data transfer. Typically, a Q factor of 10–30 is chosen for reader coils, which offers a good tradeoff [12]. In addition, the matching network consists of a matching network that allows the impedance of the reader coil to be matched to the 50 Ω input impedance of the reader chip. Furthermore, the matching network includes an electromagnetic compatibility filter to reduce harmonics.
The tag has to be resonant to draw energy from the reader’s magnetic field efficiently, i.e., with a tag resonance frequency of ${f}_{\text{Tag}}$ = 13.56 MHz (in contrast to ${f}_{\text{T}}$, which is the tag coil resonance in (1). The tag resonance frequency ${f}_{\text{Tag}}$ can be calculated using the equivalent circuit shown in Figure 2 [11]: \[{f}_{\text{Tag}} = \frac{1}{{2}{\pi}}\sqrt{\frac{{1}{-}\left({{C}_{\text{Chip}} + {C}_{\text{T}}}\right){R}_{\text{T}}^{2}}{{L}_{\text{T}}\left({{C}_{\text{Chip}} + {C}_{\text{T}}}\right)}}{.} \tag{3} \]
The tag chip is modeled by an equivalent parallel circuit of ${R}_{\text{chip}}$ and ${C}_{\text{chip}}$. The tag coil inductance LT and tag capacitances ${C}_{\text{chip}}$ and CT are adjusted to create a tag resonance at 13.56 MHz. This adjustment is made by building a dedicated resonance capacitance into the chip in state-of-the-art tags, attaining the desired tag resonance, as presented in Figure 3 [11]. HF RFID tag chips generally offer multiple input capacitance values to support different coil sizes according to the ISO/IEC 7810 standard. The typical values of tag capacitances ${C}_{\text{chip}}$ and CT are, for example, 20 pF for standard ID-1 coils, as shown in Figure 1, with a credit card size of 85.60 mm × 53.98 mm.
Figure 3. The tag characteristics for six different chip examples from various vendors and an ID-1 tag coil [11]. (a) The calculated chips’ tag resonance frequencies ${f}_{\text{Tag}}$ versus input voltage ${V}_{\text{Chip}}$. (b) The chip resistances ${R}_{\text{Chip}}$ and capacitances ${C}_{\text{Chip}}$ of the three proximity tag chip products (DESFire, Ultralight, Classic) at 13.56 MHz. In contrast, (c) shows the resistance and capacitance of the three vicinity tag chip products (Tag-it, K50, FM248). The chip resistances and capacitances were measured with an RF current–voltage impedance measurement method and achieved, compared to previous work, higher measurement accuracy of better than 1.5%. The measurement data are available online at IEEE DataPort doi: 10.21227/4yvmc819.
The recommended tag resonance frequency range for standard ID-1 coils of proximity tags is 13.56 MHz ${f}_{\text{Tag}}$ <16 MHz [21]. The recommended resonance frequency is higher than the reader operating frequency, i.e., 13.56 MHz, to counteract card-loading effects, keeping the interaction of nearby tags low [12]. These resonance frequency values can also be seen in Figure 3, showing tag resonance frequencies between 15 and 16 MHz for small chip input voltages. Figure 3 depicts the characterization of different chip examples from various vendors [11], showing the measurements results of six chip examples, three proximity standard-based chips (DESFire, Ultralight, Classic), and three vicinity standard based (Tag-it, K50, FM248). The tag resonance frequency is shown versus the chip input voltage ${V}_{\text{Chip}}$ for the different chips in combination with an ID-1 coil, following (1). Figure 3 also shows the measured parallel resistance ${R}_{\text{Chip}}$ and capacitance ${C}_{\text{Chip}}$ of the proximity and vicinity standard-based chips versus input voltage. Resistance and capacitance show a strongly nonlinear behavior versus voltage ${V}_{\text{Chip}}$. When approaching the maximum operating voltage of the chip, the influence of an internal voltage limiter becomes dominant to protect the chip integrated circuit from overvoltage [4]. The limiter effect is visible for the proximity chips for the measured input voltage range. As a side effect, the limiter detunes the tag resonance frequency at the respective clamping voltage to avoid overvoltage [11]. The three proximity chip examples show a maximum limiter clamping voltage of roughly 3 V, while vicinity chips have a clamping voltage of approximately 10 V.
The main criterion applied while designing HF RFID tag coils is to optimize the delivery of a sufficient chip input voltage. The power transfer efficiency h can usually be increased by increasing the magnetic field provided by the reader (i.e., increasing the reader output power), decreasing the distance between the reader and tag coils, expanding the radius of the tag coil, or increasing the number of windings [18]. Increasing the number of coil windings is often the most effective way to increase h as local authorities limit the maximum magnetic field emission, and specific applications often limit communication distances and tag sizes. The most efficient planar coil design can be created by optimizing the number of windings and the width of the conductor if fixed dimensions are provided [18]. A high Q factor of the tag is not favorable as a narrow-band HF RFID tag is highly prone to detuning effects when placed close to the dielectric, conducting, or ferromagnetic materials. More robust tags can be achieved by optimizing the inductance and resistance of the coil to achieve a typical Q Tag of roughly 20 or less [18], [22].
In general, metal environments severely impair the performance of HF RFID systems [23]. Eddy currents are induced on the metal surface in contact with or close to a coil to satisfy the boundary condition, which states that the magnetic field normal to the metal surface must be zero [14]. The induced eddy current opposes the magnetic field generated by the coil, which significantly damps the magnetic field in the vicinity of the metal surface, leading to a reduction in the coil inductance, which is a function of coil geometry and the number of windings. According to (1) and (3), this reduction in inductance leads to an increase in the coil resonant frequency [24] and, subsequently, to a decrease in power transfer efficiency [25], shown in Figure 4.
Figure 4. An example of the impact of a metal environment on the wireless power transfer efficiency of a vicinity-standard-based HF RFID system [25]. (a) Arrangement of the reader and tag coils distanced at 50 cm. The distance ${\Delta}$ of the tag coil and the metal obstacle is 10 mm. (a) Power transfer efficiency at 13.56 MHz versus lateral misalignment ${\Delta}_{y}$ and angular tag misalignment j to the reader, i.e., compare numerical calculation with and without the metal obstacle. Next to the numerically calculated results, the simulation results from CST are presented, aligning well with the analytical model shown in [25]. sim.: simulation; num.: numerical; calc.: calculation.
Figure 4 shows an example of the impact of a metal environment on the wireless power transfer efficiency of a vicinity-standard-based HF RFID system [25]. The rectangular single-turn reader and tag coils have a distance of 50 cm. Their wireless power transfer efficiency is investigated at 13.56 MHz for a lateral misalignment of the tag coil to the reader coil in the y-direction characterized by the distance ${\Delta}_{y}$ and an angular misalignment of tag and reader coils characterized by the angle j of the tag coil plane and the z-direction [19]. In addition, a metal obstacle is located behind the tag; the metal obstacle’s position is always constant to the tag coil. There is a considerable decrease in power transfer efficiency of 50 × 10−3 in the case of the metal next to the tag compared to no metal [25].
Different design strategies have been presented to realize HF RFID systems that operate robustly in metal environments. Table 1 shows these design strategies listed according to the years when the prototypes were first published. The table gives the metal location at the reader, tag, or between the reader and the tag. It also includes information on the reader and tag coils, their sizes, the number of coil windings, and reader’s transmit power. In addition, the list presents the exploited design strategy to realize a robust operation in the specific metal environment, detailing the use of ferrite material. Finally, the presented custom-built designs are compared in their read range with off-the-shelf devices in the metal environment, or the prototypes are compared in their read ranges in the metal environment versus free space. A read range analysis of Table 1 shows that a considerable read range improvement was achieved from 2011 onward using ferrite material, adaptive matching networks, and booster antenna designs.
Table 1. A summary of HF RFID systems that operate robustly in metal environments (listed chronologically by publication date). The read range of the custom-built designs is compared to the read range in free space (metal environment/free space*) or with an off-the-shelf design in the specific metal environment (custom-built/off-the-shelf design).
The use of ferrite material leads to considerable read range improvement. The ferrite material provides magnetic isolation of the coil from the metal, preventing eddy currents from originating. This improvement was shown for HF RFID systems for the first time by NXP Semiconductors researchers in 2011 [30]. They proposed inserting a flexible ferrite sheet between the printed NFC coil and the metallic battery pack of a mobile phone, showing an improved read range of 3 cm, in contrast to no established communication in the case of a conventional printed coil in the mobile phone. Compared to using an additional ferrite sheet, in 2014, researchers from the Graz University of Technology and Infineon Technologies presented an NFC tag coil directly silver-inkjet printed on a ferrite substrate [33]. This approach improved the NFC tag’s mechanical and economic properties, leading to a minimum tag thickness and lower assembly effort. Also, the silver-inkjet printing process is cheaper than other fabrication processes, such as etching [38].
Figure 5 depicts the presented silver inkjet-printed NFC tag prototype in [33] and its performance comparison with a silver inkjet-printed photo paper-based tag in a metal environment. Both tags contain printed coils connected to Infineon NFC chips with temperature sensors. A mobile phone was used as the NFC reader to read the chip’s temperature values. A 100-${\mu}$m-thick copper sheet is arranged below the tag coils. Figure 5 shows the temperature readout with both NFC tags. The researchers could read out the NFC tag with the ferrite coil in case the copper sheet was directly attached to the back of the coil. In contrast, they could only read out the NFC tag with the photo paper coil for a spacing of 1.4 cm between the coil and the copper sheet. The figure also shows that the reading distance of the ferrite coil in a metal environment is more significant than that of the photo paper coil. The copper sheet at the back of the photo paper tag antenna deteriorates the photo paper tag’s performance. Additional simulations and measurements in a nonmetal environment showed that the ferrite coil performed equally well as a custom-built NFC coil printed on a photo paper substrate, despite further losses in the ferrite substrate. The ferrite coil outperformed the photo paper coil in a metal environment, showing a read range of 10 cm in contrast to 0 cm in the case of the copper directly attached to the tags (see Table 1).
Figure 5. (b) A silver inkjet-printed NFC tag prototype [33]. The figure shows a performance comparison with (a) a silver inkjet-printed photo paper-based tag in a metal environment. The sizes of the four winding coils were 45 mm × 75 mm connected to NFC Infineon chips equipped with temperature sensors. A mobile phone acted as an NFC reader, reading the temperature values of both tags.
Using ferrite material in combination with booster antenna technology further improves HF RFID systems in metal environments. Typically, we use booster antennas to extend the read range of miniaturized RFID tags by increasing the effective tag antenna area [6]. A booster antenna consists of two parts, i.e., a large pickup antenna and small coupling structure [39]. The pickup antenna focuses the available energy by the reader, while the coupling structure acts as an alternative transmit-receive part of the particular miniaturized tag.
Researchers from the Graz University of Technology and NXP Semiconductors first exploited this favorable combination of ferrite material and booster antenna technology [37]. In 2020, they presented a stack of two booster antennas and ferrite material, a so-called HF RFID repeater, to enable communication of an HF RFID reader with a tag enclosed in a metal housing (i.e., the metal between the reader and the label). The HF RFID repeater consists of two booster antennas, as shown in Figure 6. One booster antenna is located inside the metal housing with a small opening. In contrast, the other booster antenna is located outside the metal housing. Their coupling coils are centrally aligned to the opening, while their pickup coils are aligned with the reader and tag coils. In addition to the booster antennas, a ferrite core and ferrite sheets make up integral parts of the repeater. The ferrite sheets isolate the various coils from the metal environment, while the ferrite core guides the magnetic field through the small opening in the metal housing. This combination enabled the repeater to provide a reader-repeater-tag system with an extended read range of 5 cm compared to the read range of 4.2 cm of the same reader-tag system in free space (i.e., without a repeater and metal).
Figure 6. (a) An HF RFID repeater prototype and (b) a read range measurement setup [37]. The repeater consists of two booster antennas, ferrite material, tuning capacitances, and a damping resistor. The capacitances tuned the resonant system to 13.56 MHz, while the damping resistor decreased the quality factor of the coupling coils. The measurement setup consisted of an NFC reader and an NFC tag. The repeater was mounted on two 1-mm-thick aluminum plates with a 4.5-mm opening.
Next to using ferrite material and booster antenna technology, adaptive matching networks show considerable improvement in HF RFID systems in metal environments. The coil inductance change due to the metal environment can be readjusted by adapting the matching network in the specific HF RFID device. Researchers from the Ştefan cel Mare University of Suceava presented this method in 2012 [31], [32]. They showed a considerable increase in the read range of an HF RFID system by exploiting an adaptive matching network at the reader in a metal environment.
Currently, researchers are working on wireless power transfer systems in general, and HF RFID systems in particular, using real-time adaptive matching networks controlled by deep neural networks [40], [41], [42]. Their focus is on combating the reader and tag coils misalignment in real time and the resulting decrease in power transfer efficiency. Researchers from the Georgia Institute of Technology presented a system for wireless power transfer operating at 13.56 MHz using helical wire coils [42]. Eventually, these real-time adaptive matching systems will help operate HF RFID systems reliably in metal environments.
Figure 7 shows an example real-time adaptive HF RFID system using an adaptive matching network at the reader that consists of shunt-variable capacitors and series inductors [40], [41]. A bidirectional coupler tracks the reflection coefficient at the matching network input. At the same time, an RF phase and gain detector receives the reflected power from the coupler and outputs a voltage signal to a microcontroller. The microcontroller acts on the adaptive matching network based on a trained deep neural network, using measurement data for testing. The measurement setup is shown in Figure 7. A semiautomated test setup measures the coil-to-coil transmission coefficient with a vector network analyzer for different combinations of intercoil distance d, azimuthal tilt angle i, and x-y axis misalignment ${\Delta}_{x}$ and ${\Delta}_{y}$.
Figure 7. (a) A real-time adaptive matching system and (b) a semiautomated measurement-driven setup for testing the deep neural network [40], [41]. The matching system is part of the HF RFID reader. Reader and tag coil transmission coefficient were investigated versus coil distance d, azimuthal tilt angle i, and x-y axis misalignment ${\Delta}_{x}$ and ${\Delta}_{y}$. The microcontroller acts on the adaptive matching network based on a trained deep neural network and improves the wireless power transfer of the system for different coil positions.
A reliable and robust operation of HF RFID systems is mandatory to use these systems’ benefits for solving IoT sustainability issues that come along with the massive deployment of IoT nodes. In particular, the recent development of HF RFID sensor tags [5], [43] in combination with booster antennas [6], [37] makes the technologies very interesting for IoT applications, allowing for a batteryless operation, high integration, and security and privacy of data. Thus, these systems must operate robustly in harsh applications such as metal environments. The successful design strategies for a robust operation use ferrite materials, booster antennas, and adaptive matching networks, considerably improving the HF RFID systems’ performance in metal environments.
Future work must focus on making HF RFID systems more sustainable. Using biodegradable tag materials would help reduce the tags’ and booster antennas’ carbon footprint. Although there have been presentations of biodegradable substrates, the realization of highly conductive, biodegradable substrates is still at the beginning. In particular, for passive, batteryless, devices, highly conductive materials are a must to realize operation. Other trends to increase the sustainability of HF RFID devices cover technologies like plastic electronics complementing semiconductors [44]. Also, the reader and tag would benefit from fully integrated real-time adaptive matching systems, leveraging robust system performance in harsh environments. In particular, tiny artificial intelligence algorithms consuming microwatts of power would be needed at the tag side. Ultimately, future work needs an interdisciplinary collaboration between RF, computers, and material scientists to realize a sustainable IoT.
[1] D. Bol, G. de Streel, and D. Flandre, “Can we connect trillions of IoT sensors in a sustainable way? A technology/circuit perspective (Invited),” in Proc. IEEE SOI-3D-Subthreshold Microelectron. Technol. Unified Conf. (S3S), 2015, pp. 1–3, doi: 10.1109/S3S.2015.7333500.
[2] M. T. Bohr and I. A. Young, “CMOS scaling trends and beyond,” IEEE Micro, vol. 37, no. 6, pp. 20–29, Nov./Dec. 2017, doi: 10.1109/MM.2017.4241347.
[3] G. Saxl, L. Goertschacher, T. Ussmueller, and J. Grosinger, “Software-defined RFID readers: Wireless reader testbeds exploiting software-defined radios for enhancements in UHF RFID systems,” IEEE Microw. Mag., vol. 22, no. 3, pp. 46–56, Mar. 2021, doi: 10.1109/MMM.2020.3042408.
[4] L. Zoescher et al., “HF/UHF dual band RFID transponders for an information-driven public transportation system,” Elektrotechnik Informationstechnik, vol. 133, no. 3, pp. 163–175, 2016, doi: 10.1007/s00502-016-0405-y.
[5] J. Grosinger and A. Michalowska-Forsyth, “Space tags: Ultra-low-power operation and radiation hardness for passive wireless sensor tags,” IEEE Microw. Mag., vol. 23, no. 3, pp. 55–71, Mar. 2022, doi: 10.1109/MMM.2021.3130686.
[6] J. Grosinger, W. Pachler, and W. Boesch, “Tag size matters: Miniaturized RFID tags to connect smart objects to the internet,” IEEE Microw. Mag., vol. 19, no. 6, pp. 101–111, Sep./Oct. 2018, doi: 10.1109/MMM.2018.2844029.
[7] F. Alimenti et al., “Smart hardware for smart objects: Microwave electronic circuits to make objects smart,” IEEE Microw. Mag., vol. 19, no. 6, pp. 48–68, Sep./Oct. 2018, doi: 10.1109/MMM.2018.2843978.
[8] A. Costanzo and D. Masotti, “Energizing 5G: Near- and far-field wireless energy and data transfer as an enabling technology for the 5G IoT,” IEEE Microw. Mag., vol. 18, no. 3, pp. 125–136, May 2017, doi: 10.1109/MMM.2017.2664001.
[9] Z. Popovic, “Cut the cord: Low-power far-field wireless powering,” IEEE Microw. Mag., vol. 14, no. 2, pp. 55–62, Mar./Apr. 2013, doi: 10.1109/MMM.2012.2234638.
[10] P. Greiner et al., “A system-on-chip crystal-less wireless sub-GHz transmitter,” IEEE Trans. Microw. Theory Techn., vol. 66, no. 3, pp. 1431–1439, Mar. 2018, doi: 10.1109/TMTT.2017.2748130.
[11] J. Grosinger, B. J. B. Deutschmann, L. Zoescher, M. Gadringer, and F. Amtmann, “HF RFID tag chip impedance measurements,” IEEE Trans. Instrum. Meas., vol. 71, 2022, Art. no. 2000911, doi: 10.1109/TIM.2021.3130664.
[12] K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification. Chichester, U.K.: Wiley, 2003.
[13] D. C. Yates, A. S. Holmes, and A. J. Burdett, “Optimal transmission frequency for ultralow-power short-range radio links,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 51, no. 7, pp. 1405–1413, Jul. 2004, doi: 10.1109/TCSI.2004.830696.
[14] S. Ramo, J. Whinnery, and T. Van Duzer, Fields and Waves in Communication Electronics. New York, NY, USA: Wiley, 1965.
[15] M. Bensetti et al., “A hybrid finite-element method for the modeling of microcoils,” IEEE Trans. Magn., vol. 41, no. 5, pp. 1868–1871, May 2005, doi: 10.1109/TMAG.2005.846284.
[16] T. Bauernfeind, W. Renhart, S. Schemthanner, M. Gebhart, and K. Preis, “Equivalent circuit parameter extraction for controlled detuned NFC antenna systems utilizing thin ferrite foils,” in Proc. 12th Int. Conf. Telecommun., 2013, pp. 251–256.
[17] R. Fischbacher, D. Pommerenke, R. Prestros, J. R. Lopera, W. Boesch, and J. Grosinger, “Broadband EC models of coil antennas for inductively coupled systems,” in Proc. Wireless Power Week (WPW), 2022, pp. 465–469, doi: 10.1109/WPW54272.2022.9854040.
[18] L. Mayer, “Antenna design for future multi-standard and multi-frequency RFID systems,” Ph.D. thesis, Vienna Univ. Technol., Vienna, Austria, 2009.
[19] K. Fotopoulou and B. W. Flynn, “Wireless power transfer in loosely coupled links: Coil misalignment model,” IEEE Trans. Magn., vol. 47, no. 2, pp. 416–430, Feb. 2011, doi: 10.1109/TMAG.2010.2093534.
[20] “AN11019: CLRC663, MFRC630, MFRC631, SLRC610 antenna design guide,” NXP Semiconductors, Eindhoven, The Netherlands, Jun. 2018. [Online] . Available: http://www.nxp.com/
[21] “AN12342: Card coil design guide for MIFARE DESFire light,” NXP Semiconductors, Eindhoven, The Netherlands, 2019. [Online] . Available: https://www.nxp.com/docs/en/application-note/AN12342.pdf
[22] X. Qing and Z. N. Chen, “Characteristics of a metal-backed loop antenna and its application to a high-frequency RFID smart shelf,” IEEE Antennas Propag. Mag., vol. 51, no. 2, pp. 26–38, Apr. 2009, doi: 10.1109/MAP.2009.5162014.
[23] X. Qing and Z. N. Chen, “Proximity effects of metallic environments on high frequency RFID reader antenna: Study and applications,” IEEE Trans. Antennas Propag., vol. 55, no. 11, pp. 3105–3111, Nov. 2007, doi: 10.1109/TAP.2007.908575.
[24] C. Zhang and Y. Xie, “The closed-form solution of frequency shift for an HF RFID coil antenna in metallic environments,” IEEE Internet Things J., vol. 5, no. 5, pp. 3927–3941, Oct. 2018, doi: 10.1109/JIOT.2018.2854297.
[25] B. J. B. Deutschmann, L. Goertschacher, P. Priller, and J. Grosinger, “Efficient assessment of the impact of metallic obstacles on the wireless power transfer in loosely coupled links,” in Proc. 49th Eur. Microw. Conf. (EuMC), 2019, pp. 579–582, doi: 10.23919/EuMC.2019.8910739.
[26] S. Bovelli, F. Neubauer, and C. Heller, “A novel antenna design for passive RFID transponders on metal surfaces,” in Proc. Eur. Microw. Conf., 2006, pp. 580–582, doi: 10.1109/EUMC.2006.281458.
[27] S. Bovelli, F. Neubauer, and C. Heller, “Mount-on-metal RFID transponders for automatic identification of containers,” in Proc. Eur. Microw. Conf., 2006, pp. 726–728, doi: 10.1109/EUMC.2006.281004.
[28] K. D’hoe, A. Van Nieuwenhuyse, G. Ottoy, J.-P. Goemaere, and L. De Strycker, “A new low-cost HF RFID loop antenna concept for metallic environments,” in Proc. 16th Int. Conf. Syst., Signals, Image Process., 2009, pp. 1–5, doi: 10.1109/IWSSIP.2009.5367785.
[29] K. D’hoe et al., “Influence of different types of metal plates on a high frequency RFID loop antenna: Study and design,” Adv. Elect. Comput. Eng., vol. 9, no. 2, pp. 3–8, Jun. 2009, doi: 10.4316/AECE.2009.02001.
[30] M. Gebhart, R. Neubauer, M. Stark, and D. Warnez, “Design of 13.56 MHz smartcard stickers with ferrite for payment and authentication,” in Proc. 3rd Int. Workshop Near Field Commun., 2011, pp. 59–64, doi: 10.1109/NFC.2011.14.
[31] A.-I. Petrariu, “13.56 MHz RFID multi-turn antenna for metallic environments,” in Proc. Eur. Conf. Use Modern Inf. Commun. Technol. (ECUMICT), 2012, pp. 1–10.
[32] A.-I. Petrariu, V. Popa, V.-G. Gaitan, and I. Finis, “Test results for HF RFID antenna system tuning in metal environment,” in Proc. 13th Int. Carpathian Control Conf. (ICCC), 2012, pp. 543–546, doi: 10.1109/CarpathianCC.2012.6228704.
[33] W. Pachler et al., “A silver inkjet printed ferrite NFC antenna,” in Proc. Loughborough Antennas Propag. Conf. (LAPC), 2014, pp. 95–99, doi: 10.1109/LAPC.2014.6996329.
[34] H. Saghlatoon and P. Mousavi, “A novel booster antenna on flexible substrates for metal proximity NFC applications,” in Proc. IEEE Int. Symp. Antennas Propag. USNC/URSI Nat. Radio Sci. Meeting, 2015, pp. 1768–1769, doi: 10.1109/APS.2015.7305273.
[35] H. Saghlatoon, R. Mirzavand Boroujeni, M. M. Honari, and P. Mousavi, “Low-cost inkjet printed passive booster for increasing the magnetic coupling in proximity of metal object for NFC systems,” IEEE Microw. Wireless Compon. Lett., vol. 26, no. 12, pp. 996–998, Dec. 2016, doi: 10.1109/LMWC.2016.2623243.
[36] H. Saghlatoon, R. Mirzavand, M. M. Honari, and P. Mousavi, “Investigation on passive booster for improving magnetic coupling of metal mounted proximity range HF RFIDs,” IEEE Trans. Microw. Theory Techn., vol. 65, no. 9, pp. 3401–3408, Sep. 2017, doi: 10.1109/TMTT.2017.2676095.
[37] L. J. Goertschacher, F. Amtmann, U. Muehlmann, E. Merlin, P. Priller, and J. Grosinger, “Passive HF RFID repeater for communicating with tags in metal housings,” IEEE Antennas Wireless Propag. Lett., vol. 19, no. 9, pp. 1625–1629, Sep. 2020, doi: 10.1109/LAWP.2020.3012202.
[38] I. Ortego, N. Sanchez, J. Garcia, F. Casado, D. Valderas, and J. I. Sancho, “Inkjet printed planar coil antenna analysis for NFC technology applications,” Int. J. Antennas Propag., vol. 2012, Mar. 2012, Art. no. 486565, doi: 10.1155/2012/486565.
[39] A. Finocchiaro, G. Ferla, G. Girlando, F. Carrara, and G. Palmisano, “A 900-MHz RFID system with TAG-antenna magnetically-coupled to the die,” in Proc. IEEE Radio Freq. Integr. Circuits Symp., 2008, pp. 281–284, doi: 10.1109/RFIC.2008.4561436.
[40] J. R. Lopera et al., “Adaptive NFC WPT System Implementing Neural Network-Based Impedance Matching with Bypass Functionality,” in Proc. IEEE Int. Microw. Symp., submitted.
[41] M. Wagih et al., “Microwave-enabled wearables: Underpinning technologies, integration platforms, and next-generation roadmap,” IEEE J. Microw., vol. 3, no. 1, pp. 193–226, Jan. 2023, doi: 10.1109/JMW.2022.3223254.
[42] S. Jeong, T.-H. Lin, and M. M. Tentzeris, “A real-time range-adaptive impedance matching utilizing a machine learning strategy based on neural networks for wireless power transfer systems,” IEEE Trans. Microw. Theory Techn., vol. 67, no. 12, pp. 5340–5347, Dec. 2019, doi: 10.1109/TMTT.2019.2938753.
[43] W. Pachler et al., “A novel 3D packaging concept for RF powered sensor grains,” in Proc. IEEE 64th Electron. Compon. Technol. Conf. (ECTC), 2014, pp. 1183–1188, doi: 10.1109/ECTC.2014.6897440.
[44] T. Meister, K. Ishida, C. Carta, N. Muenzenrieder, and F. Ellinger, “Flexible electronics for wireless communication: A technology and circuit design review with an application example,” IEEE Microw. Mag., vol. 23, no. 4, pp. 24–44, Apr. 2022, doi: 10.1109/MMM.2021.3136684.
Digital Object Identifier 10.1109/MMM.2022.3233508