Real-Time Implementation of a Novel Design Approach for Sub-GHz Long-Range Antenna for Smart Internet of Things Communication

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Introduction
Long-range technology is a wireless technology developed to meet the requirements for longer coverage, low-power devices, and deep indoor penetration.Overall, this technology is a boon for the IoT era because it helps to increase network coverage, battery life, and spectrum efciency of devices.Because the LoRa device can communicate with the LoRa module through its interface, it does not require a network for connectivity.As a result, LoRa is benefcial for rural, forest, and mountain areas to track nature where there is no Internet network.LoRa technology is in high demand for monitoring, tracking, and sensing [1].Tese characteristics of LoRa allow it to be used in a variety of applications, as shown in Figure 1 [2].However, as shown in Figure 1, there are some applications, where very small data must be communicated, as in sensor data communication.For example, temperature and humidity sensors are used in smart agriculture; light and gas detector sensors are used in smart homes; light sensors are used in smart street lighting systems and location tracking systems; bin full sensors are used in waste management systems; vacant space sensors are used in smart parking systems; and many more sensors' data are used for automation.
In India, the LoRa communication spectrum is 865.867MHz [3].Te LoRa frequency in Europe is 868 MHz, while in the United States, it is 915 MHz, with a bandwidth of 125 kHz for both up and down frequencies.LoRa technology makes it possible to talk over long distances while using very little power.Tis flls a traditional gap in sensor networks.It also works well with other technologies because it operates in an unlicensed band and is suited for both indoor and outdoor use.Because LoRa is based on the open-source Lora WAN protocol, it is a globally accessible network [4].Sensors, boards, the processing unit, the LoRa module (Arduino), and antennas are all included in the smart devices.Several sensors are used in automation and smart communication for data collection, processing, and management.Because there is no generally established standard for smart long-range communication, it is a chance to adapt and modify existing high-accuracy protocols, such as SIG-FOX and LoRa, which are also used by the industry for longrange communication [5].However, Wi-Fi is another commonly used standard for smart communication.It consumes a lot of power and has a maximum coverage range of 40-45 m, so long-range communication and Wi-Fi interbuilding are not possible.So, the antenna design focuses on miniaturization without reducing the size of smart sensors.Tis makes it possible to build a small, compact device that can use LoRa technology to transfer data while using very little power.
Various issues with IoT antennas used for diferent frequencies are discussed in the literature, with some focusing on tiny and low-profle antennas.Table 1 compares the suggested design to their prototype to validate the benefts of the designed antenna in terms of performance, size, bandwidth, and design complexity.Te monopole antenna is shown in [17][18][19] for wideband applications.In the literature, some meandering antennas are designed for IoT applications [20].In [21,22], slots, slits, and inductive loading with vias achieve more than 90% miniaturization, and the design is usable in the 805-835 MHz frequency range.Most PIFA antennas are conceived and developed for L-band communication.Because it can operate digitally and provide low-power communications, the PIFA structure is a viable choice for sub-GHz range applications.Folded dipole approaches and meandering techniques are used to reduce size [23][24][25], and meander line antennas outperform PIFA in terms of bandwidth and radiation efciency [26,27].In some literature, the metamaterial concept is also used to reduce the size of an antenna, increase the bandwidth of an antenna, and improve the radiation of an antenna [6,28].Te metamaterial concept is useful in creating a compact antenna, but design complexity is also increased as compared to the monopole design [29].Te surface waves always play a role in the gain and efciency of the antenna.Te surface reduction techniques are also studied and taken into account as per the literature [30].Only one type of monopole antenna, the meander line antenna, gives good efciency with moderate sensitivity [31].So, the meandering approach to building a small antenna for LoRa communication is sorted after all references.
In this article, a meandered monopole antenna is designed which has a dipole-shaped ground, and feed is given by a rectangular stub between the ground and patch.Te proposed design can be operated for LoRa applications in the 868 MHz spectrum.Te antenna is designed and fabricated on a fame retardant (FR4) substrate with ε r � 4.4.Te designed antenna is fabricated and measurements are taken to cross-check the results with the simulated one.
Te novelty of the proposed work can be summarised as follows: (1) To overcome the issues of coverage for LoRa devices, the proposed antenna has better coverage than the previously designed antenna and conventional monopole antenna.Te proposed antenna meets the requirements for LoRa communication.(2) Te design is also simple, so there is ease of fabrication and the small-size tends to the placement of this antenna at the LoRa module, which will decrease the size of the communication module of the LoRa device.(3) A compact meandered antenna is proposed for LoRa applications.Te work cited in [31][32][33][34][35] covered the L bands, but they are not suitable for LoRa communication.(4) While optimizing the size of the antenna, a unique meandered design is implemented with the dipoleshaped ground.As per the author's information, such a unique design with a compact size, low cost, and good gain is not achieved in any work.
Te proposed antenna design techniques, strategy, and theoretical analysis are discussed in Section 2, and the simulated and measured results are mentioned in Section 3. Section 4 deals with the real-time implementation of the LoRa antenna in diferent environments.In Section 5, the article is concluded.International Journal of Antennas and Propagation

Details of Proposed Design
Te major challenges for LoRa antennas are the requirements for small-diameter planar antennas that provide long-distance communication with better power management.In this section, the detailed antenna geometry is discussed, its equivalent circuit is presented, and the design strategy behind optimization is discussed.Te designed structure was simulated in antenna simulator software, which is based on the FEM method.

Design Strategy.
Te technical design strategy for the proposed antenna is briefy described in this section.It is critical to determine the antenna type, material, and thickness before beginning the design process.Because of its excellent performance, the planar meander line antenna was prioritized, and FR4 epoxy substrate material was used to reduce the antenna's fabrication cost.After that, the appropriate electromagnetic (EM) software was chosen.Te antenna is designed and simulated using version 15.0 of the high-frequency structure simulator (HFSS).Utilizing the transmission line model equations, the proposed antenna is constructed.After selecting the operating frequency and application for which the antenna is needed, the antenna is designed using a reverse analysis process.Te decided dimension is optimized to the required frequency band.Te frst step is to select the antenna technology such as meandered or MIMO, fractal, or metamaterial, as per the available space on the Lora module or on-chip [32][33][34][35].After that, the proposed antenna is designed, and the dimensions are described as a variable so that it will count for parametric study.Te dimensions and design are modifed based on the parametric results to achieve the required frequency band and antenna performance.Te antenna is designed using the technical strategy depicted in Figure 2.

Antenna Geometry.
Te proposed meandered monopole antenna consists of the meandered monopole and dipole ground.Te feed is provided using the rectangular stub between them.Te proposed antenna's design is based on the coplanar waveguide (CPW) feed structure, which has been further enhanced through optimizations involving the dipole ground and rectangular monopole structures.Tese optimizations have led to the development of a meandering structure with a stub.Te antenna is fabricated on a 1.6 mm thick FR4 substrate for mechanical support and as its foundation.Te dimensions of the antenna have been carefully adjusted through a parametric study, resulting in an optimized size of 55 mm × 55 mm (0.16λ × 0.16λ × 0.005λ).Te design primarily comprises a folded meandered line, a dipole structure ground, and a stub for the feed.
Te proposed antenna design dimensions are the following: , and r � 1 mm.Te proposed antenna has both the resonating patch and ground on the top of the substrate with stub as mentioned in Figures 3(a) and 3(b) showing the ground (green), patch (patch), and feed structure (black).In the parametric analysis, the meander structure has been examined, and the results are depicted in Figures 4 and 5. Figure 6 presents the S11(dB) graph, which demonstrates the impact of varying the feed line width on the antenna's performance.In addition, Figure 7 illustrates the S11(dB) graph, which showcases how changes in the length of the dipole structure infuence the antenna's behavior.Te −10 dB impedance bandwidth of the proposed antenna is 11 MHz from 860 to 871 MHz, which covers the LoRa spectrum.Te gain for the complete band is more than 0.5 dB.

Meander Line Teory and Equivalent Circuit Diagram.
An electrically small antenna is defned as an antenna whose maximum dimension is less than one-tenth of a wavelength [36].A meander line antenna consists of standing and At the same meander, the split increases, and the resonance frequency decreases [37].A spiral antenna is a development of the fundamental folding antenna and has frequencies signifcantly lower than single-element antenna resonances of identical length.Te efciency of meander line antennas surpasses that of traditional half and quarter-wavelength antennas.Te efectiveness of these antennas relies on the reduction factor of their size, determined by the number of meandering elements per wavelength and the spacing between rectangular loops.In the proposed design, the spacing between the lines is set at 0.7 mm, while the width measures 0.8 mm.By ensuring smooth edges, the impedance matching and gain of the antenna are signifcantly enhanced.Te meander line antenna consists of both vertical and horizontal components, each playing a distinct role.Te vertical meander line functions as an inductor, while the horizontal one serves as a capacitor.A dipole patch antenna that behaves as ground typically consists of a conductive patch (often rectangular or circular) placed over a ground plane with a feedline connected to the patch.Te equivalent circuit represents the antenna's impedance, resonance, and radiation characteristics.A meandered structure with two parallel stubs on both sides is a common confguration used in   International Journal of Antennas and Propagation microwave and RF (radio frequency) engineering for various applications, such as impedance matching, fltering, or tuning.To represent this structure with an equivalent circuit, a combination of lumped elements (resistors, capacitors, and inductors) is used to model its electrical behavior.Together, they contribute to the overall performance and characteristics of the antenna.By implementing the meander line design, the antenna achieves improved radiation efciency compared to traditional half and quarter-wavelength antennas.Tis enhancement stems from the careful arrangement of the meandering elements and the optimization of the dimensions, specifcally the spacing and width between the lines.Te smooth edges play a crucial role in minimizing impedance mismatches, resulting in improved performance metrics, such as gain and radiation patterns.
Furthermore, the combination of the inductive and capacitive elements in the vertical and horizontal meander lines allows for better control over the antenna's electrical characteristics.Tis arrangement enables the antenna to efectively transmit and receive signals within the desired frequency range.Te utilization of these elements ensures optimal utilization of the available space and enhances the overall performance of the antenna system.Te proposed meander line antenna design ofers enhanced radiation effciency when compared to traditional half-and quarterwavelength antennas.Trough careful optimization of the meandering elements and dimensions, along with the utilization of inductive and capacitive components, the antenna achieves improved impedance matching, gain, and overall performance.Te equivalent circuit diagram of the basic meander line is shown in Figure 8, where L and C are lumped elements.Te equivalent circuit diagram of the proposed design is mentioned in Figure 9.

Simulations and Measurements Results
Tis section provides a detailed explanation of the fabricated antenna, along with the simulation and measurement results.In addition, the design includes the development of a connecting cable and the implementation of impedancematching techniques.Te efectiveness of impedance matching is primarily infuenced by the width of the meandered line and ground plane.Figure 10 showcases the top 10(a) and bottom 10(b) of fabricated antennas.To assess the performance of the fabricated antenna, the S11(dB) parameters are measured using a network analyzer model number ZNLE6, as depicted in Figure 11.Te return losses, both simulated and measured, are displayed in Figure 12.Notably, the antenna's bandwidth spans from 860 MHz to 871 MHz, encompassing the 868 MHz bands associated with LoRa technology.
Comparing the S11(dB) return loss graph to the simulated results, it is evident that they align closely.However, some variations in resonance may arise due to potential mismatches at the port connectors.Te importance of impedance matching through the meandered line and ground plane width is emphasized.Te fgures included illustrating the simulated and fabricated antennas, as well as the measured S11(dB) parameters and return losses.Te observed bandwidth of the antenna is confrmed to cover the desired LoRa frequency range.While the S11(dB) return loss closely matches the simulated results, slight variations in resonance are attributed to potential mismatching at the port connectors.Te simulated gain band is also mentioned in Figure 13(a), which represents the copolarization and cross-polarisation band, gain at the operating band is more than 0.5 dB, and as mentioned in Figure 13(b), the efciency of the proposed antenna is more than 75% for the complete operating band.Figure 14 shows the surface current on a radiating patch at 868 MHz.From the fgure, it can be concluded that the surface current on a meandered line is opposite in direction and will cancel out to some extent, and  6 International Journal of Antennas and Propagation the resultant current will be at an angle of 36 degrees from the Y-axis, which can be verifed by looking into the peak of the radiation pattern.
To validate the advantages of the newly developed antenna, a comparative analysis was conducted with an existing design operating in the same frequency band, International Journal of Antennas and Propagation specifcally for LoRa applications.Tis comprehensive evaluation considered various parameters, including antenna dimensions, operating bands, bandwidth, radiation efciency, gain, substrate material, and design complexity.Te results of this comparison are tabulated in Table 1.One crucial aspect of the antenna's performance is its radiation pattern, which helps ascertain its directionality.To determine the gain of the proposed design accurately, a reference technique was employed.Tis involved placing the antenna, under test, in an anechoic chamber at a precise distance of 5.2 meters from a reference antenna, which was a ridge horn antenna.Figure 15 illustrates this anechoic chamber setup.For the analysis, the radiation pattern was evaluated at a specifc frequency of 868 MHz.Te pattern was then characterized in both the E-plane and H-plane, which are depicted in Figure 16.Te fndings from the radiation pattern examination revealed that the maximum radiation occurred at an angle of 36 degrees in the theta plane.Tis phenomenon can be attributed to the direction in which the feld is emitted from the antenna.Furthermore, the radiation pattern was efectively modeled using the designed antenna, as shown in Figure 17.Te accuracy of this modeling process ensures that the antenna's behavior and performance can be predicted with reliability.
Te proposed antenna's strengths were substantiated by conducting a thorough comparison with an existing design within the same frequency band and for LoRa applications.Tis analysis considered multiple critical factors, providing a comprehensive evaluation of the antenna's superiority.Moreover, the radiation pattern was studied in detail, enabling a precise understanding of the antenna's directionality and radiation characteristics.Te gain measurement, achieved through the reference technique, contributed to the accurate assessment of the antenna's performance.Te successful modeling of the radiation pattern with the designed antenna further reinforced the credibility of the study's outcomes.Te meticulous evaluation and analysis undertaken throughout this study contribute signifcantly to the advancement of antenna technology and its applications in the feld of wireless communication, particularly in LoRa-based systems operating in the 868 MHz frequency range.

Implementations
Te fnal circuit design and assembled version can be seen in Figure 18, which depicts all pertinent components, including the proposed antenna, LoRa radio module, microcontroller, sensor antenna, and other passive components.Two sets are prepared so that one is used as a receiver and the other as a LoRa transmitter for the real-time LoRa connectivity experiments.Te input signal is digital data using the Arduino module.Te message was sent from transmitter to receiver and vice versa using Arduino software after traveling a certain distance in the diferent scenarios as mentioned in Section 4 of the manuscript.Te maximum transmitted power is limited to 14 dBm.Te radio module used is EByte E32-868T30S, SX1276, 868 MHz, 30 dBm, as shown in Figure 19.Terefore, a series of feld experiments were conducted to evaluate the performance of a proposed antenna in diferent communication scenarios.Te dimensions of the printed circuit board are 70 mm × 60 mm.Te connection between the Arduino and the LoRa module is shown in Figure 20.
Te antenna design has been efectively utilized with LoRa technology, enabling the transmission of data over considerable distances.In ideal conditions with a clear line of sight, the antenna achieved a remarkable communication range of up to 8 km.Even in urban settings, where obstacles and interference are more prevalent, it managed to maintain a reliable connection for distances exceeding 1 km.Moreover, within building areas, the antenna provided a satisfactory coverage of approximately 250 m.To validate its performance, the receiver was mounted on the roof of a vehicle on an express highway, while the transmitter was positioned in a line-of-sight environment.Te communication remained    International Journal of Antennas and Propagation consistently stable and connected over the entire 8 km range, as demonstrated in Figure 21.
Moving on to the second experiment, the receiver, equipped with a reference antenna, was placed on a vehicle, while the transmitter was located inside another moving car within an urban setting.Te proposed antenna was used as the transmitter in this scenario, as illustrated in Figure 22.Te results indicated that the communication range achieved with the proposed antenna was approximately 1.3 km.However, when the reference antenna was employed instead, the range was limited to only 900 m.Tis suggests that the proposed antenna provides superior coverage than the reference antenna.
In the third experiment, the transmitter was positioned on the balcony of a building, and the receiver antenna was moved to a neighboring building, as shown in Figure 23.Using the proposed antenna, the two LoRa devices were able to establish communication each other within a range

Conclusion
In this article, a meandered-dipole planar antenna is proposed for a long-range application, and the antenna is implemented in a real-time environment to verify its performance.Te antenna latency is reported to be less than the standard monopole antenna used for LoRa module devices of the brand Arduino.Te fabricated antenna results are in good agreement with the designed antenna.Te achieved gain at the operating band is more than 0.5 dB and the efciency of the proposed antenna is more than 75% for the complete operating band from 861 MHz to 871 MHz.Te designed antenna is successfully implemented with LoRa connectivity and communicates the data up to 8 km in line-of-sight communication, more than 1 km in urban environments, and approximately 250 m of connectivity in building areas.Te characteristics of the antenna were analyzed and found suitable for the LoRa applications.Future studies can focus on increasing the range of LoRa in high-trafc areas by adding more gateways along its route.Te 433 MHz frequency can be combined with the LoRa antenna to make it multiband and universal in India.In this case, the LoRa antenna is only used to send data, but in the future, it could be used with many sensors to make the environment smarter, such as smart lighting or a smart agriculture system.Te bandwidth can also be improved by adding meta surface ground structure to both antennas.

Figure 1 :
Figure 1: Diferent felds where the LoRa can play a major role in sensor data communication [2].

Figure 4 :
Figure 4: S11(dB) graph with the variation of the width of the meander (W 1 ).

Figure 6 :
Figure 6: S11(dB) graph with the variation of the width of the feed line (W 8 ).

Figure 7 :
Figure 7: S11(dB) graph with the variation of length of a ground dipole (L 4 ).

Figure 14 :
Figure 14: HFSS simulated profle for surface charge density on radiating patch at the frequency 868 MHz.

Figure 15 :Figure 16 :Figure 17 :Figure 18 :
Figure 15: Te fabricated antenna is placed in an anechoic chamber for gain and pattern measurement: (a) the antenna is placed in an anechoic chamber and (b) horn and fabricated antenna at 5.2 m distance.

Figure 21 :Figure 22 :
Figure 21: First experiment-connectivity for line of sight: (a) receiver set up and (b) map where the experiment (Tx-transmitter and Rx-receiver) performed.

Figure 23 :
Figure 23: Tird experiment-setup for in-building connectivity: (a) receiver set up and (b) map where the experiment (Tx-transmitter and Rx-receiver) performed.

Table 1 :
Comparison of the proposed antenna with other antennas published in recent years.