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Review

Overview of Reconfigurable Antenna Systems for IoT Devices

by
Elena García
1,
Aurora Andújar
1 and
Jaume Anguera
1,2,*
1
Ignion, 08174 Barcelona, Spain
2
Research Group on Smart Society, La Salle Engineering, Universitat Ramon Llull, 08022 Barcelona, Spain
*
Author to whom correspondence should be addressed.
Electronics 2024, 13(20), 3988; https://doi.org/10.3390/electronics13203988
Submission received: 29 June 2024 / Revised: 2 August 2024 / Accepted: 6 August 2024 / Published: 10 October 2024
(This article belongs to the Special Issue Smart Antennas and Systems for 5G and Beyond: Latest Advances and Prospects)
Browse Figures
Versions Notes

Abstract

:
The proliferation of Internet of Things (IoT) devices, such as trackers and sensors, necessitates a delicate balance between device miniaturization and performance. This extends to the antenna system, which must be both efficient and multiband operational while fitting within space-constrained electronic enclosures. Traditional antennas, however, struggle to meet these miniaturization demands. Reconfigurable antennas have emerged as a promising solution for adapting their frequency, radiation pattern, or polarization in response to changing requirements, making them ideal for IoT applications. Among various reconfiguration techniques (electrical, mechanical, optical, and material-based), electrical reconfiguration reigns supreme for IoT applications. Its suitability for compact devices, cost-effectiveness, and relative simplicity make it the preferred choice. This paper reviews various approaches to realizing IoT reconfigurable antennas, with a focus on electrical reconfiguration techniques. It categorizes these techniques based on their implementation, including PIN diodes, digital tunable capacitors (DTCs), varactor diodes, and RF switches. It also explores the challenges associated with the development and characterization of IoT reconfigurable antennas, evaluates the strengths and limitations of existing methods, and identifies open challenges for future research. Importantly, the growing trend towards smaller IoT devices has led to the development of antenna boosters. These components, combined with advanced reconfiguration techniques, offer new opportunities for enhancing antenna performance while maintaining a compact footprint.

1. Introduction

The relentless march of technology is reshaping our world, with the Internet of Things (IoT) leading at the forefront of this transformation. The IoT, a vast network of interconnected devices collecting and sharing data, exemplifies this evolution.
At the heart of reliable communication within IoT systems lies the antenna, the crucial gateway for data transfer. From industrial automation to agricultural monitoring, antennas ensure seamless communication between devices, sensors, and the central network across diverse applications. However, traditional single-band antennas struggle to keep up with the demands of contemporary systems. While multiband antennas have emerged as a solution for operating across multiple frequencies, challenges persist due to the limited space available within these devices and their intricate internal electronics.
This is where reconfigurable antennas emerge as a compelling solution, addressing these challenges with their inherent adaptability. As a difference, reconfigurable antennas can dynamically adjust their properties, offering greater versatility. These structures can switch between various states, altering characteristics such as operating frequency, radiation pattern, and polarization. This inherent flexibility allows them to excel in the dynamic world of the IoT [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21].
The growth of the field of reconfigurable antennas has garnered significant attention in recent years, driven by the increasing demand for wireless communication solutions. As depicted in Figure 1, the number of published papers in IEEE and MDPI on reconfigurable antennas for IoT applications has shown a substantial growth trajectory since 2013, highlighting the increasing research interest in this area. This means that there is potential for reconfigurable antennas to address the unique challenges posed by miniaturization and multiband operation requirements in modern IoT devices.
This review selected articles from scientific databases [22,23] using keywords such as “IoT”, “antennas”, “reconfigurable”, “tunable”, and combinations thereof. The search results were sorted by publication date, from most recent to oldest, to prioritize novelty and current research trends. Citation count was not a primary selection criterion, as recent articles may have limited citations but represent the forefront of ongoing research.
Reconfigurable antennas achieve their adaptability through various techniques, all capable of modifying operating frequency, radiation pattern, or polarization. Table 1 provides a comparative overview of four commonly employed reconfiguration techniques. Notably, electrical reconfiguration emerges as the preferred choice for IoT applications due to several key advantages. Firstly, it boasts low power consumption, a crucial consideration for battery-powered IoT devices. Secondly, the miniaturization of electrical components enables compact antenna designs, addressing the space constraints inherent in many IoT devices. Finally, electrical reconfiguration offers a relatively low level of complexity when integrated into the overall antenna design.
This paper delves into electrical reconfiguration techniques (Figure 2) for IoT applications. For a comprehensive overview of mechanical, optical, and materials-based reconfiguration, refer to [24,25].
The following sections will explore these methods in detail. PIN diodes are explained in Section 2, followed by descriptions of digital tunable capacitors (DTCs) in Section 3 and varactor diodes in Section 4. Section 5 will cover RF switches. A comparative discussion of these techniques is presented in Section 6, and the paper concludes with key findings in Section 7.

2. PIN Diode

The PIN diode reconfiguration technique is one of the most prevalent methods for reconfigurable antennas due to its design simplicity [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50]. These are semiconductor devices that function as variable resistors. When a voltage is applied in a forward direction (forward bias), they have very low resistance, allowing current to flow freely. This effectively “turns on” the PIN diode and alters the current path within the antenna. Conversely, a reverse bias creates a high-resistance state, essentially “turning off” the PIN diode and blocking current flow. This switching ability allows for the selective connection and disconnection of antenna elements, thereby modifying the overall antenna shape and electrical properties and achieving reconfiguration.
In [51], the paper introduces a compact 25 mm × 25 mm reconfigurable patch antenna designed to operate in multiple frequency bands. The antenna exhibits resonance at approximately 4.45 GHz with an impedance bandwidth spanning 4.07 GHz to 5.2 GHz and a reflection coefficient of around −13.97 dB. Additionally, it resonates at 8.2 GHz with an S11 value of −16.08 dB within the operational band of 8.05 GHz to 9.09 GHz.
The antenna’s reconfigurability is achieved through the integration of three PIN diodes. These semiconductor switches alter the antenna’s physical configuration when activated by applying a DC voltage bias, enabling its operation in four distinct frequency ranges: 2.4–2.5 GHz, 3.9–5.2 GHz, 6.9–7.2 GHz, and 7.14–9.1 GHz. The specific PIN diode model employed in [52] is a silicon-based component optimized for microwave antenna switching applications (10 MHz to 10 GHz). In its conductive state (forward-biased), the diode presents a low resistance of approximately 3 ohms, facilitating efficient current flow due to its high forward current handling capacity of 100 mA. Conversely, when in the off state (reverse-biased), the diode’s low capacitance of 0.15 pF effectively minimizes signal leakage.
The paper [53] presents a reconfigurable 41 mm × 44 mm antenna designed for a healthcare monitoring system within the Internet of Things (IoT) framework (Figure 3). The antenna operates in dual bands, targeting the 2.4 GHz and 5.8 GHz ISM bands. A PIN diode [54] facilitates frequency switching (Figure 4), controlled by a dedicated biasing circuit. In its on state, the diode exhibits a series resistance of 1.05 Ω and an inductance of 0.7 nH. Conversely, the off state presents the diode in a shunt configuration with a resistance of 2 kΩ and a capacitance of 0.18 pF.
Current distribution analysis reveals that the main radiating patch primarily contributes to the 5.8 GHz resonant frequency, with current concentration around the transmission line and patch edges. At 2.4 GHz, the current concentrates along the inverted U-slot. When the PIN diode is on, the maximum current occurs at the slot edges for 2.4 GHz operation. Conversely, with the diode off at 5.8 GHz, the current concentrates around the main radiating patch center.
The biasing circuit (Figure 5) controls the PIN diode’s state. It employs a simple topology consisting of two choke inductors (Lp) and two decoupling capacitors (Cb). The choke inductors, placed in series with the power supply (VCC) and ground (GND), effectively block high-frequency RF signals from entering the biasing circuitry while allowing DC bias to pass. The decoupling capacitors, connected in parallel with the choke inductors, provide a low-impedance path for high-frequency noise, preventing its coupling into the bias lines. This configuration ensures that the PIN diode receives the appropriate bias voltage without interfering with the antenna’s RF performance.
It is documented that PIN diodes used for antenna reconfiguration can introduce nonlinearities, particularly at high power levels. This phenomenon arises from the interaction between the diode’s internal capacitance and the RF signal. These nonlinearities manifest as unwanted harmonics and intermodulation distortion, potentially interfering with nearby systems. In Figure 6, the S11 parameter of this antenna is shown, and when the antenna is turned off, it functions at a single 5.8 GHz frequency band.
Reference [55] describes a novel multiband Multiple-Input Multiple-Output (MIMO) antenna design for 5G sub-6 GHz applications. The antenna utilizes two planar inverted-F antenna (PIFA) elements with two PIN diodes [52] and biasing circuits to achieve radiation pattern reconfiguration. Both PIFA elements operate in three bands (220 MHz to 2330 MHz and 0.8 GHz to 6 GHz), covering cellular standards from Global System for Mobile Communications (GSM) to 5G New Radio (NR).
When activated, PIFA 1 resonates at 0.94 GHz (260 MHz bandwidth) and additionally at 3.95 GHz and 4.75 GHz (combined 2330 MHz bandwidth). PIFA 2 exhibits resonances at 1.85 GHz (510 MHz bandwidth), 2.55 GHz (220 MHz bandwidth), and 4.45 GHz (2000 MHz bandwidth). Mutual coupling between ports 1 and 2 remains below 10 dB at their respective resonances.

3. Digital Tunable Capacitors

Digital tunable capacitors (DTCs) are electronically controlled capacitors that can adjust their capacitance by changing the material properties between the plates, allowing them to fine-tune antenna impedance for better signal transfer and slightly adjust resonant frequency for wider operating ranges [56,57,58,59,60,61,62,63,64,65,66,67]. This makes them compact, fast-switching alternatives to traditional mechanically tuned capacitors, which are found to be useful in mobile phones, wearables, and radio equipment.
Controlling DTCs involves sending electronic signals. There are two main methods. The first is the General-Purpose Input/Output (GPIO). This established approach leverages a microcontroller’s digital pins, facilitating basic on/off control. However, it lacks the ability to send complex commands or retrieve switch diagnostics, limiting its functionality for intricate applications. On the other hand, there is the Mobile Industry Processor Interface (MIPI) RF Front-End (RFFE) Control Interface. This standardized protocol, prevalent in mobile devices, offers a more sophisticated approach. MIPI RFFE empowers intricate control by enabling the transmission of complex commands, the retrieval of switch status data (temperature, diagnostics), and the configuration of switch parameters. While this enhanced functionality makes MIPI RFFE ideal for high-performance applications, it necessitates a more complex implementation compared to GPIO.
The paper [68] presents a reconfigurable coplanar-fed single-arm bowtie antenna measuring 70 mm × 48 mm for IoT applications at multiple frequencies: 868 MHz, 915 MHz, 1 GHz, 1.8 GHz, and 2.1 GHz. The antenna achieves frequency reconfigurability through a digitally tunable capacitor (DTC) [69]. This DTC is placed in a shunt position on the feeding line to dynamically tune the antenna’s input impedance. The 5-bit digital control of the DTC allows for 32 capacitance states. The DTC model incorporates its constant inductance of 0.7 nH and variable capacitance and resistance based on the control input. In Figure 7, S11 in the different states of the DTC can be seen.
The antenna’s inherent resonance occurs at 2.1 GHz. Two ground sections optimize the radiation behavior by providing a balanced current return path, while the feeding mechanism supports the removal of one bowtie arm, constructively contributing to the remaining arm’s radiation. To further adjust the input impedance in conjunction with DTC configurations, three slot arrays are etched into the bowtie arm.
Article [70] introduces a compact, reconfigurable 28 mm × 8 mm × 8 mm antenna (Figure 8) that utilizes a DTC. Mounted on a 120 mm × 200 mm ground plane, the antenna operates within the white-space frequency band, ranging from 470 MHz to 700 MHz. The antenna’s equivalent circuit (Figure 9) comprises a matching circuit, a radiating element, and the DTC. The antenna’s resonance frequency inversely correlates with the DTC’s capacitance value. A DTC positioned at the monopole’s apex enables frequency tuning. The design prioritizes consistent performance across all states by optimizing impedance for mid-range DTC capacitance values, with slightly reduced performance at the higher and lower ends.
In contrast to traditional varactor diodes that necessitate complex tuning voltage circuits, the DTC offers a simplified control scheme. It leverages a serial interface (I2C signals) for the digital adjustment of its capacitance across 32 selectable states. The capacitance range of the DTC depends on the connection configuration (series or shunt). In a series configuration, it varies from 0.38 pF to 4.32 pF, while the shunt configuration offers a range of 0.9 pF to 4.6 pF. The reflection coefficient (S11) shows the accuracy of the resulting tunning (Figure 10).
Reference [71] proposes an LoRaWAN 28 mm × 8 mm × 8 mm (1792 mm3) meandered inverted-F antenna with a compact design of 18 mm × 20 mm and an elongated ground plane of 750 mm × 18 mm. Due to the narrow-band performance of the antenna system, a DTC is employed to achieve the required operational bandwidth from 820 MHz to 960 MHz. This approach allows for impedance matching across the band by electronically adjusting the capacitance value from 0.5 pF to 4 pF. The antenna delivers a realized gain of 1 dBi within the range of operating frequencies.

4. Varactor Diodes

Varactor diodes are a type of semiconductor diode exhibiting a voltage-dependent capacitance. This property allows for the continuous tuning of their capacitance through an applied bias voltage. By adjusting the bias voltage, the varactor diode’s capacitance changes, which in turn affects the antenna’s resonant frequency and enables its operation at different frequencies. However, varactor diodes typically require high biasing voltages (up to 20 V) and have limitations in terms of their power handling capability [72,73].
The paper [74] introduces a compact 980 mm3 frequency-reconfigurable dual-band planar inverted-F antenna (PIFA) designed specifically for integration into battery-powered Internet of Things (IoT) and narrow-band IoT (NB-IoT) devices (Figure 11). The antenna is designed to be adapted on the frequency bands of 703 MHz and 880 MHz. However, the obtained bandwidths are insufficient. To address this, two varactor diodes [75] placed on the 50 Ω printed line are integrated. The reconfigurable antenna architecture operates in three frequency bands: 703–803 MHz, 791–862 MHz, and 880–960 MHz. These diodes offer a wide capacitance range (3.11 pF to 69.45 pF) depending on the applied voltage. This variation in capacitance allows for adjusting the resonant frequencies of each radiating element within the PIFA design. The electrical equivalent circuit model of the varactor diode has three key elements, a variable capacitance, the parasitic capacitance, and the series resistance and inductance, which are the internal losses (0.25 Ω) and physical properties (0.6 nH) of the varactor diode (Figure 12).
Varactor diodes necessitate reverse polarization for operation, with capacitance directly influenced by the applied DC voltage. A parallel LC circuit integrated into the antenna feed provides the necessary DC bias while effectively separating incoming DC and RF signals at the input port. This configuration, employing a single port for both antenna feeding and varactor diode biasing, is enabled by the decoupling LC block. The self-inductance acts as a short circuit for DC signals, while the capacitance serves the same purpose for RF signals. To isolate the parasitic element from the DC voltage, a capacitor is included. As illustrated (Figure 13), the 50 Ω printed lines incorporate all components essential for antenna reconfiguration. In Figure 14, there is proof of the frequency reconfigurability by the reflection coefficient.
Reference [76] presents a frequency-reconfigurable antenna designed as a vertical Self-Resonating Radiator (SRR) with a footprint of 80 mm × 80 mm × 15 mm (Figure 15). To achieve frequency reconfigurability, the fixed capacitor is replaced by a tunable capacitor. Two varactor diodes [77] are connected in series with a back-to-back configuration across a gap in the radiating element. A lumped inductor of 56 nH is integrated within the DC bias circuit to isolate the RF signal. The antenna’s operating frequency is tuned by adjusting the biasing voltage. With four states of the varactor, it is adapted to 850 MHz and from 900 MHz to 960 MHz (Figure 16).
The paper [78] proposes a compact, dual-band frequency-tunable antenna system designed for mobile handsets. The system integrates a dual-band PIFA with a parasitic element. The parasitic element’s first resonant frequency is electronically tunable between 700 MHz and 900 MHz using a varactor diode [79]. Additionally, the system incorporates a single-band PIFA specifically designed for the LTE 2500 band. To ensure proper biasing of the varactor diode by the feeding pin, a 10 nF DC blocking capacitor is employed on the PIFA. This capacitor isolates the feeding strip from the short circuit, enabling efficient DC current flow to the diode. Furthermore, a parallel decoupling LC circuit (L = 57 nH, C = 47 pF) is connected to the feeding line. This circuit isolates the DC biasing signal from the incoming RF signal at the antenna port. The varactor diode’s capacitance can be adjusted by varying the DC biasing voltage. The capacitance ranges from 0.35 pF at 5 V to 1.05 pF at 12 V.

5. RF Switches: MEMS

Radio Frequency (RF) switches play a crucial role in controlling the flow of high-frequency signals. These are often referred to as microwave switches, which function as electronic gates, selectively routing or blocking signals based on an external control signal. There are two primary categories of RF switches: semiconductor switches and Microelectromechanical System (MEMS) switches.
Semiconductor switches utilize semiconductor devices like PIN diodes and Field-Effect Transistors (FETs) to modulate the flow of RF signals. By applying a voltage across the control terminals of the device, the switch changes its state, either presenting a low impedance (on state) for signal transmission or a high impedance (off state) for signal blocking. Renowned for their exceptionally fast switching speeds and mature technology, semiconductor switches offer predictable behavior. However, they are limited by their lower linearity, higher power consumption, and susceptibility to crosstalk.
Microelectromechanical System (MEMSs) switches employ a different approach. They rely on microscopic mechanical structures, such as levers or membranes, that physically move to establish or break conductive paths for the RF signal. MEMS switches offer several advantages over their semiconductor counterparts. They typically exhibit lower insertion loss, reducing signal attenuation, and superior port isolation, minimizing unwanted signal leakage. Additionally, MEMS switches boast higher linearity and lower power consumption. Thanks to advancements in MEMS technology, these switches have become increasingly miniaturized, making them ideal for space-constrained applications like IoT devices. However, MEMS switches are not without their challenges. Repeatability and reproducibility can vary across different fabrication batches, and switching speeds are generally slower compared to semiconductor options. For more information about the differences in RF switches, refer to [80].
Despite these limitations, the combination of their low power consumption, high linearity, and small size makes MEMS switches a compelling choice for signal routing in resource-constrained IoT devices [81,82,83,84,85,86,87,88,89,90,91,92,93,94].
Controlling MEMS switches, akin to DTCs, necessitates electronic signaling. As with DTCs, two primary methods are available: General-Purpose Input/Output (GPIO) and the Mobile Industry Processor Interface (MIPI) Radio Frequency Front-End (RFFE) Control Interface. As previously detailed in the DTC section, GPIO employs a microcontroller’s digital pins for fundamental on/off control. Conversely, the MIPI RFFE Control Interface exerts control over switch states by transmitting a clock signal and data representing distinct switch positions.
The design presented in [95] is a reconfigurable antenna for operation across three distinct frequency bands: a lower band (690–960 MHz), a mid-band (1700–2700 MHz), and an upper band (3.4–3.6 GHz). The compact antenna utilizes a metal rim measuring 146 mm × 5 mm mounted on a 143 mm × 74 mm printed circuit board (PCB). A single-pole 4-throw (SP4T) MEMS switch [96] with low loss and high isolation is employed to specifically enhance the bandwidth of the lower band. Series capacitors (RF1–RF4) with progressively increasing values from 0.4 pF to 1.0 pF are incorporated into each channel (Figure 17). The MIPI RFFE Control Interface provides independent control over the on/off state of each RF channel. In Figure 18, S11 is observed in different states.
A different antenna approach is based on antenna booster technology, which utilizes a tiny, non-resonant antenna component strategically placed on the existing ground plane of a wireless device. This antenna booster excites the ground plane to radiate electromagnetic waves, transforming it into the main radiator. This approach is particularly advantageous because the ground plane size is often comparable to the relevant wavelengths, allowing for a sufficient bandwidth and efficiency. The three key elements of this technology are as follows: First, the antenna booster element, as it acts as an exciting element on the device’s ground plane. It excites specific current distributions on the ground plane, enabling it to radiate efficiently. Second, the matching network is typically comprised of Surface-Mount Devices (SMDs) like capacitors and inductors. By selecting and configuring these components, the impedance of the antenna system can be tuned to achieve optimal performance at the desired frequency band. And third is the ground plane, where its size significantly impacts bandwidth and efficiency. Larger ground planes tend to offer wider bandwidths and higher efficiency due to their ability to support a wider range of current distributions. However, this presents a challenge for miniaturization in compact IoT devices.
In this regard, reference [57] presents a compact PCB design measuring 50 mm × 50 mm. It integrates an antenna booster element with dimensions of 30 mm × 3 mm × 1 mm (height) and a dedicated clearance area of 12 mm × 40 mm (Figure 19) [97]. This design achieves multiband operation, covering cellular bands from 698 MHz to 960 MHz and 1710 MHz to 2200 MHz and additionally supporting GPS functionality at 1575 MHz.
Two MEMS SP8T GPIO switches facilitate switching between these multiple bands. To achieve impedance matching across these diverse frequencies, the authors employed six dedicated matching networks (Figure 20). Each network utilizes Surface-Mount Device (SMD) components in a simple L-type configuration to optimize the antenna’s performance for a specific cellular band or GPS operation. Using six states of the switches, frequency coverage is achieved (Figure 21).
In [98], a reconfigurable antenna architecture on a compact PCB measuring 70 mm × 65 mm is proposed [99]. It incorporates a 30 mm × 3 mm × 1 mm (height) antenna booster element and a dedicated clearance area of 15 mm × 45 mm. The antenna booster technology is the same as explained previously. This design leverages a MEMS SP4T MIPI RFFE switch [100] to achieve multiband operation with up to 65 useful states (Figure 22 and Figure 23). However, only seven switch states are utilized to facilitate multiband LTE functionality, covering cellular bands from 698 MHz to 960 MHz and 1710 MHz to 2170 MHz (Figure 24). The matching networks for each RF output port were optimized by carefully selecting the values of series-connected components to achieve proper impedance matching across these frequency bands.
The reconfigurable antenna architecture detailed in [98] was successfully integrated into a multi-sensor cellular IoT prototyping platform for the market [101,102]. This device incorporates an RF/MCU chip based on Nordic’s nRF9151 low-power System-in-Package (SiP). Featuring an ARM Cortex-M33 microcontroller, the chip integrates LTE-M, narrow-band IoT, DECT NR+ modem, and GNSS capabilities. For detailed specifications, refer to [102]. This demonstrates that the architecture is not only novel (Figure 25) but also holds practical value for real-world applications.
As illustrated in Figure 26, the antenna exhibits a quasi-isotropic radiation pattern with a directivity of approximately 3 dBi. This is particularly beneficial for IoT devices where the incoming signal direction and device orientation are often random, a common scenario in IoT communication. The directivity is D = 3.1 dBi at 800 MHz and D = 3.2 dBi at 2 GHz. These low-directive radiation patterns are advantageous for wireless devices, as they accommodate the unknown direction of incoming signals and the varying orientations of the devices.

6. Discussion

Table 2 presents a comparative analysis of the antenna dimensions for the various reconfigurable antenna designs discussed. The table encompasses key parameters such as the reconfiguration technique employed, the overall antenna size, the operational frequency range, and the multiband compatibility.
A notable distinction among the compared antennas is between resonant and non-resonant designs. The latter category, exemplified by the antenna booster elements detailed in [56], stands out for its compact size, making it particularly suitable for IoT devices. These non-resonant antennas often demonstrate multiband capabilities, contrasting with the smaller, single-band antenna presented in [71].
Selecting the optimal electrical reconfigurable antenna technology (varactors, PIN diodes, digital tunable capacitors, or RF MEMS switches) hinges on the application’s specific requirements. Table 3 provides a comparison of these technologies. Interestingly, research suggests that PIN diodes are the most prevalent choice, followed by the DTC, varactors, and, lastly, RF MEMS switches (Figure 27). This preference for PIN diodes might be attributed to their simpler implementation and potentially lower cost compared to other options.
An interesting dichotomy emerges when comparing research trends and industry practices in reconfigurable antenna design for IoT applications. While academic publications often favor PIN diodes for their reconfigurability, companies developing IoT antennas seem to be shifting towards RF MEMS switches and digital tunable capacitors (DTCs) [104,105,106,107,108]. This divergence primarily stems from the power consumption and voltage limitations of varactors and PIN diodes in low-power IoT environments.
While varactors and PIN diodes offer reconfigurability for antenna design, they introduce challenges in meeting power and voltage constraints. Varactors, despite their wide tuning range, might require higher voltages for significant capacitance changes. PIN diodes, although efficient when conducting current due to their low forward voltage drop, necessitate a reverse bias voltage to turn off, which can exceed 50 V in some cases.
Generating this higher voltage in low-power IoT settings often requires additional circuitry, further increasing power consumption. For instance, a typical IoT application operates at 3 V. While a voltage regulator with this input voltage can provide voltages from 3 V to 24 V [109], its power dissipation can be as high as 400 mW, introducing unnecessary complexity and power consumption. Therefore, for practical IoT applications, the industry prioritizes power efficiency and circuit simplicity. This explains the growing preference for solutions like MEMS switches and DTCs.
The choice between a DTC and an RF switch hinges on the required reconfiguration range. DTCs offer discrete capacitance values, limiting the difference between the minimum and maximum capacitance achievable. Conversely, RF switches provide greater flexibility for achieving a wider range. Each branch of an RF switch can be connected to various components like inductors, capacitors, or resistors, enabling the creation of custom frequency responses.

7. Conclusions

Reconfigurable antennas have emerged as a powerful solution for the challenges faced by antennas in the ever-growing world of the Internet of Things (IoT). These challenges include miniaturization, as traditional antennas struggle to fit within the space constraints of compact IoT devices. And multiband operation, as single-band antennas limit functionality, while traditional multiband antennas can still be bulky. To overcome this drawback, antenna booster technology and multiband operation with electrically small non-resonant elements called antenna boosters have been developed. Thanks to the addition of a matching network, the frequency bands of operation can be selected. Furthermore, with the addition of tunability in the matching network, the flexibility increases, allowing for more frequency bands. This is particularly interesting when the number of bands is large and/or the size of the ground plane is small in terms of the wavelength. When the ground plane length is less than ~0.2λ, it is challenging to obtain the bandwidth and to include space for the antenna element. Therefore, reconfigurability with an antenna booster combines the advantages of tunability and the small size of the antenna booster elements.
It is highlighted that electrical reconfiguration is the preferred method for IoT applications due to its low power consumption, which is crucial for battery-powered devices, miniaturization potential, and the simplicity of integration into the antenna design. These techniques include PIN diodes, digital tunable capacitors (DTCs), varactor diodes, and RF switches. It is important to select the appropriate electrical reconfiguration technique based on specific application requirements, particularly focusing on power consumption in the context of IoT devices. Interestingly, a gap exists between research and industry trends. While research often favors PIN diodes for their reconfigurability, industry is shifting towards RF MEMS switches and DTCs. This can be attributed to the power consumption limitations of PIN diodes and varactors in low-power IoT environments.
Overall, antenna boosters significantly enhance reconfigurable antennas in IoT devices by providing compact, versatile, and cost-effective solutions that meet the stringent demands of modern applications. They ensure efficient multiband operation and improved performance while occupying minimal space. This integration simplifies antenna design, making antenna boosters crucial for the rapid development and deployment of IoT devices.

Author Contributions

Conceptualization, E.G., J.A. and A.A.; methodology, E.G.; software, E.G.; validation, E.G., J.A. and A.A.; formal analysis, E.G.; investigation, E.G.; resources, E.G.; data curation, E.G.; writing—original draft preparation, E.G.; writing—review and editing, E.G., J.A. and A.A.; visualization, E.G.; supervision, J.A.; project administration, E.G.; funding acquisition, J.A. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Industrial Doctorate Plan of the Secretariat of Universities and Research of the Department of Business and Knowledge of the Generalitat of Catalonia (Reference 2021 DI 23).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Author E.G., J.A. and A.A were employed by the company Ignion. All authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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  104. Johanson Technology. IoT Multiband Antenna Solution. 2022. Available online: https://www.johansontechnology.com/downloads/iot-multiband-antenna-solution.pdf (accessed on 12 June 2024).
  105. Ignion. AN_NN03-310_Thingy91. 2023. Available online: https://ignion.io/files/AN_NN03-310_Thingy91.pdf (accessed on 12 June 2024).
  106. Taoglas. PCS.50.A. 2022. Available online: https://www.taoglas.com/datasheets/PCS.50.A.pdf (accessed on 12 June 2024).
  107. Kyocera AVX. EC646-01. 2023. Available online: https://datasheets.kyocera-avx.com/1004795-EC646-01.pdf (accessed on 12 June 2024).
  108. Abracon. Tunable Antenna for Global NB-IoT and LTE Coverage. 2020. Available online: https://abracon.com/uploads/resources/Tunable-Antenna-for-Global-NB-IoT-and-LTE-Coverage.pdf (accessed on 18 June 2024).
  109. LM2735-Q1 520-kHz and 1.6-MHz Space-Efficient Boost and SEPIC DC/DC Regulator. Available online: https://www.ti.com/lit/ds/symlink/lm2735-q1.pdf?ts=1719354050399&ref_url=https%253A%252F%252Feu.mouser.com%252F (accessed on 26 June 2024).
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Figure 1. Number of reconfigurable antenna papers published. Data obtained from scientific databases [22,23], using the following keywords: antenna, IoT, reconfigurable, and tunable/tuneable.
Figure 1. Number of reconfigurable antenna papers published. Data obtained from scientific databases [22,23], using the following keywords: antenna, IoT, reconfigurable, and tunable/tuneable.
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Figure 2. Schematic of reconfigurable antenna techniques, with a focus on those relevant to the IoT, which will be discussed in detail.
Figure 2. Schematic of reconfigurable antenna techniques, with a focus on those relevant to the IoT, which will be discussed in detail.
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Figure 3. Design of a reconfigurable 41 mm × 44 mm antenna with a PIN diode [53].
Figure 3. Design of a reconfigurable 41 mm × 44 mm antenna with a PIN diode [53].
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Figure 4. PIN diode’s lumped elements in (a) on state (b) off state presented in [53].
Figure 4. PIN diode’s lumped elements in (a) on state (b) off state presented in [53].
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Figure 5. Detailed biasing circuit [53]. Lp represents the choke inductors, creating a high impedance for the operation frequencies. Cb represents the DB blocks, allowing the DC circuit to connect only to the VCC, the LP, the PIN diode, the Lp, and the ground.
Figure 5. Detailed biasing circuit [53]. Lp represents the choke inductors, creating a high impedance for the operation frequencies. Cb represents the DB blocks, allowing the DC circuit to connect only to the VCC, the LP, the PIN diode, the Lp, and the ground.
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Figure 6. S11 parameter of the antenna with the PIN diode [53].
Figure 6. S11 parameter of the antenna with the PIN diode [53].
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Figure 7. S11 of the different states of the DCT with the antenna in [68].
Figure 7. S11 of the different states of the DCT with the antenna in [68].
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Figure 8. Antenna geometry with a DTC [70].
Figure 8. Antenna geometry with a DTC [70].
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Figure 9. Equivalent circuit of the antenna [70].
Figure 9. Equivalent circuit of the antenna [70].
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Figure 10. Measured reflection coefficient of the antenna proposed in [70].
Figure 10. Measured reflection coefficient of the antenna proposed in [70].
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Figure 11. Prototype of the reconfigurable antenna [74].
Figure 11. Prototype of the reconfigurable antenna [74].
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Figure 12. The varactor diode equivalent circuit [75].
Figure 12. The varactor diode equivalent circuit [75].
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Figure 13. Feed and short pins connected to printed lines with all the components for the reconfiguration [74].
Figure 13. Feed and short pins connected to printed lines with all the components for the reconfiguration [74].
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Figure 14. S11 parameter of the reconfigurable antenna for C1 = 60 pF and C2 from 8 to 60 pF and for C1 from 8 to 60 pF and C2 = 60 pF [74].
Figure 14. S11 parameter of the reconfigurable antenna for C1 = 60 pF and C2 from 8 to 60 pF and for C1 from 8 to 60 pF and C2 = 60 pF [74].
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Figure 15. Configuration of the proposed antenna: (a) 3D view, and (b) top view. The detail DC bias circuit and circuit model of the varactor is displayed in the inset. [76].
Figure 15. Configuration of the proposed antenna: (a) 3D view, and (b) top view. The detail DC bias circuit and circuit model of the varactor is displayed in the inset. [76].
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Figure 16. Measured and simulated S11 of the different states [76].
Figure 16. Measured and simulated S11 of the different states [76].
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Figure 17. The configuration of the SP4T switch [95].
Figure 17. The configuration of the SP4T switch [95].
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Figure 18. Measured reflection coefficient results of the antenna at different states [95].
Figure 18. Measured reflection coefficient results of the antenna at different states [95].
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Figure 19. PCB with antenna booster element in clearance area for cellular and GPS operation [57].
Figure 19. PCB with antenna booster element in clearance area for cellular and GPS operation [57].
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Figure 20. Reconfigurable antenna architecture with matching network system for multiband operation [57].
Figure 20. Reconfigurable antenna architecture with matching network system for multiband operation [57].
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Figure 21. S11 for states covering the desired frequency bands [57].
Figure 21. S11 for states covering the desired frequency bands [57].
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Figure 22. Prototype of the reconfigurable architecture integrating an antenna booster element [98].
Figure 22. Prototype of the reconfigurable architecture integrating an antenna booster element [98].
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Figure 23. RF SP4T switch architecture with all matching networks and an antenna booster element [98].
Figure 23. RF SP4T switch architecture with all matching networks and an antenna booster element [98].
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Figure 24. Measured S11 of the states when using the scheme shown in Figure 22 and Figure 23 [98].
Figure 24. Measured S11 of the states when using the scheme shown in Figure 22 and Figure 23 [98].
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Figure 25. Thingy 91 X by Nordic Semiconductor [102], a multi-sensor cellular IoT prototyping platform in the market integrating antenna booster technology from Ignion [103].
Figure 25. Thingy 91 X by Nordic Semiconductor [102], a multi-sensor cellular IoT prototyping platform in the market integrating antenna booster technology from Ignion [103].
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Figure 26. Measured radiation patterns showing the realized gain at 800 MHz (on the left) and 2 GHz (on the right) [98]. Radiation patterns are quasi-isotropic, which is convenient when the direction of the incoming signal is not known.
Figure 26. Measured radiation patterns showing the realized gain at 800 MHz (on the left) and 2 GHz (on the right) [98]. Radiation patterns are quasi-isotropic, which is convenient when the direction of the incoming signal is not known.
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Figure 27. Percentage of types of reconfiguration used based on publications in the IoT antenna domain.
Figure 27. Percentage of types of reconfiguration used based on publications in the IoT antenna domain.
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Table 1. Comparison of reconfiguration techniques.
Table 1. Comparison of reconfiguration techniques.
Reconfigurable Antenna TechniqueDescriptionSuitable for IoT DevicesSuitable for Small DevicesCostComplexity
ElectricalUses electrical components: RF switches and PIN/varactor diodes to change antenna properties.Yes: low-power consumption, compact design possible.Yes: electrical components can be miniaturized for small devices.Relatively low costLow to moderate: established technology with well-understood components.
MechanicalPhysically changes the shape of the antenna: motors, gears.No: bulky mechanisms and motors consume significant amounts of power.No: mechanisms are inherently bulky and not suitable for miniaturization.Moderate to high costHigh: moving parts require complex design and fabrication.
OpticalUses light to change the properties of the antenna: light-sensitive materials.Potentially: fast reconfiguration, but power consumption for light source needs evaluation.Potentially: miniaturization is possible depending on the light source, but research is ongoing.Potentially high cost: emerging technology, potentially complex light source setups.High: emerging technology with potentially complex light source setups and material control mechanisms.
MaterialUses materials that change properties in response to external stimuli: temperature, voltage, light.Potentially: low-power possibilities depending on the material, but research is ongoing.Potentially: material properties could be suitable for miniaturization, depending on the material.Cost varies: it depends on material properties and the complexity of control mechanisms.Moderate to high: depends on material properties and control mechanisms. Can range from relatively simple to complex.
Table 2. Comparison of the antenna dimensions in each discussed article. Multiband is defined here as featuring at least a first frequency region [f1, f2] and a second frequency region [f3, f4], where f3 >> f2, for example, 698–960 MHz and 1710–2170 MHz.
Table 2. Comparison of the antenna dimensions in each discussed article. Multiband is defined here as featuring at least a first frequency region [f1, f2] and a second frequency region [f3, f4], where f3 >> f2, for example, 698–960 MHz and 1710–2170 MHz.
ReferenceReconfigurationAntenna DimensionsFrequency of OperationMultiband Compatibility
[51]3 PIN Diodes625 mm22.38 GHz, 2.48 GHz, 3.98 GHz, 4 GHz, 4.45 GHz, 6.99 GHz, 7.2 GHz, 8.25 GHzYes
[53]1 PIN Diode1804 mm22.4 GHz and 5.8 GHzYes
[55]2 PIN Diode18,000 mm30.8 GHz to 6 GHzYes
[68]DTC3360 mm2868 MHz, 915 MHz, 1 GHz, 1.8 GHz and 2.1 GHzYes
[70]DTC1792 mm3470 to 700 MHzNo
[71]DTC360 mm2820 to 960 MHzNo
[74]2 Varactor Diodes980 mm3703–960 MHzNo
[76]2 Varactor Diodes9600 mm2840 MHz to 950 MHzNo
[78]2 Varactor Diodes1242 mm3700–900 MHz and 2500–2700 MHzYes
[95]1 SP4T Switch730 mm2690–960 MHz and 1700–2700 MHzYes
[57]2 SP8T Switches90 mm3698–960 MHz and 1710–2170 MHzYes
[98]1 SP4T Switch90 mm3698–960 MHz and 1710–2170 MHzYes
Table 3. Comparison of the different electrical reconfigurable antenna technologies.
Table 3. Comparison of the different electrical reconfigurable antenna technologies.
FeatureVaractorPIN DiodeDigital Tunable CapacitorRF Switch: MEMS
FunctionVariable capacitorResistance in on state (3 Ω [52] when the current is 10 mA), capacitance in off state (0.2 pF [52])Adjustable capacitance (circuit)Connects/disconnects a switch. Low resistance in on state and low capacitance in off state (0.8 Ω and 145 fF [101]).
ConstructionSimilar to diodes (designed for capacitance variation)Two doped regions (p-type and n-type)Array of capacitors and switchesMicroscopic mechanical switch
Control SignalContinuously variable voltageVoltage appliedDigital signal (binary)Digital signal (binary)
Tuning RangeFine-tuningWide rangeDiscrete values, typically for fine-tuningWide range
Current HandlingLimitedCan handle moderate currentsLimited (by capacitors)Can handle moderate currents
Voltage needed0 up to 22 V + extra circuit voltage needed to transform from typical 3 V batteries to these high voltages.0 up to 100 V + extra circuit voltage needed to transform from 3 V batteries to these high voltages.~3 V compatible with batteries in IoT devices.~3 V compatible with batteries in IoT devices.
Power Consumption>0.01 mW1 mW up to 110 mW~0.5 mW~0.5 mW
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García, E.; Andújar, A.; Anguera, J. Overview of Reconfigurable Antenna Systems for IoT Devices. Electronics 2024, 13, 3988. https://doi.org/10.3390/electronics13203988

AMA Style

García E, Andújar A, Anguera J. Overview of Reconfigurable Antenna Systems for IoT Devices. Electronics. 2024; 13(20):3988. https://doi.org/10.3390/electronics13203988

Chicago/Turabian Style

García, Elena, Aurora Andújar, and Jaume Anguera. 2024. "Overview of Reconfigurable Antenna Systems for IoT Devices" Electronics 13, no. 20: 3988. https://doi.org/10.3390/electronics13203988

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