Introduction

The evolution of wireless communication systems has led to the widespread use of low power electronic devices, particularly in applications such as the Internet of Things (IoT) and wireless sensor networks1,2. The replacement of batteries in such devices is both time consuming and costly, and it carries an additional risk of environmental contamination resulting from improper disposal practices3. As a result, the increasing interest in harvesting ambient energy from multiple sources including solar power, wind flow, mechanical vibrations, heat differentials, and radio frequency (RF) emissions reflects its growing potential as a reliable solution for long term, sustainable power supply in low energy electronics4,5,6. RFEH distinguishes itself from other energy harvesting approaches by offering a reliable and sustainable power source, making it a compelling solution for long term energy needs. The continued evolution of wireless communication systems has resulted in a growing density of ambient RF signals originating from sources such as cellular infrastructure and wireless local area networks (WLANs),7,8,9. This surge in available RF energy has improved the viability of RF energy harvesting as an alternative means of powering low power electronic devices,10,11.

In practical implementations involving continuously operating sensors, RFEH serves as an important mechanism for transforming incident EM waves into usable electrical power12,13,14. Although Wi-Fi at 2.45 GHz remains the dominant RF in indoor environments15,16. Ambient RF energy is also abundantly available across a wide spectrum of frequency bands. These include LTE at 600 and 700 MHz, CDMA at 800 MHz, GSM at 900 MHz, GSM/4G at 1800 MHz, and signals from the Digital Terrestrial Television (DTT) band, making such energy accessible in both indoor and outdoor scenarios17, . The insight presented herein originates from a thorough investigation of documented power spectrum analyses and empirical measurement findings in existing literature, where the performance and viability of multiple frequency bands were examined under both indoor and outdoor propagation conditions18,19. Energy harvesting circuits are essential for delivering sufficient electrical power to support the operation of low power devices within IoT networks7,8.

The antenna serves as a fundamental component in RFEH and WPTs, where it is responsible for efficiently transmitting and receiving RF signals essential for power conversion and delivery20. The ambient environment is rich in RF signals exhibiting diverse polarization, which can be harnessed as a viable energy source for the operation of self powered devices without reliance on conventional batteries21,22. In recent years, researchers have increasingly concentrated on designing wideband antennas equipped with selective band rejection functionalities, aimed at mitigating interference caused by coexisting narrowband signals in modern wireless applications23. Exhibiting a wideband characteristic provides a significant advantage in ambient RFEH applications, allowing the antenna to function across various prevalent RF frequency bands and thereby improving its ability to capture and accumulate adequate energy5. In pursuit of the targeted performance metrics, multiple design modifications have been employed, including the introduction of slits and slots of varying dimensions across the radiating structure, the feed line, and the ground plane, to enhance EM behavior and improve antenna characteristics24,25,26. The use of \(\lambda\)/2 and \(\lambda\)/4 open-ended slit configurations8,12, in conjunction with defected ground structures (DGS)27,28, has been explored in the literature for enhancing performance across various wireless communication systems. In29, the researchers developed a circular monopole antenna incorporating an annular-ring structure, specifically designed to function efficiently at a center frequency of 5.80 GHz. Compared to a typical monopole antenna, the presented design exhibits a 12.8% enhancement in BW and delivers a realized gain of 5.70 dBi at a comparatively higher operating frequency, thereby demonstrating improved performance characteristics suitable for high frequency applications. This paper30 introduces the design and fabrication of a compact, printed wide-slot antenna incorporating a modified L-shaped microstrip feed line, tailored to support broadband wireless communication systems. The proposed antenna structure is realized on an FR-4 dielectric substrate characterized by a relative permittivity of 4.4. It demonstrates a broad BW of 3.51 GHz, spanning frequencies between 1.21 GHz and 4.72 GHz. Proposed31 an innovative back-to-back microstrip antenna configuration that integrates broadband omnidirectional wireless energy harvesting (2.03–4.08 GHz) with high-directivity WPT at 5.8 GHz, all within a compact design. The antenna demonstrated a measured BW exceeding 67%, and its harvesting performance remained consistent at incidence angles up to \(\pm 60^0\), indicating strong angular robustness. Nevertheless, the study reported limitations including low harvested power levels and limited characterization of the radiation pattern at 5.8 GHz. In this work32, the authors introduced a compact coplanar waveguide (CPW)-fed bow-tie slot antenna designed to support wideband operation for IoT-integrated smart healthcare applications. The antenna is optimized to perform efficiently across the 1.41–1.86 GHz, which plays a vital role in enabling reliable WPT in IoT Sensor nodes. An improved broadband circularly polarized (CP) antenna was introduced in33, featuring a DGS and a uniquely shaped radiating patch. The design enhances axial ratio BW and impedance matching across a broad frequency spectrum while maintaining a compact form factor without degrading performance. The fabricated antenna underwent experimental validation, exhibiting a broad axial ratio BW of approximately 81.5% across the frequency range. A peak gain of 4.2 dBi was recorded, and the antenna preserved consistent radiation characteristics throughout the entire operating band. In34, the researchers introduced a compact multiband slot antenna designed specifically for wireless communication applications, covering GPS at 1.575 GHz, WiMAX at 2.4/3.5 GHz, and WLAN at 5.2/5.8 GHz. The antenna structure features a modified rectangular slot integrated into a microstrip-fed ground plane. To improve impedance matching and frequency selectivity, a T-shaped radiating stub is incorporated into the design, contributing to enhanced performance across the targeted frequency bands. Despite the antenna’s capability to operate across multiple frequency bands with a compact form factor, the relatively low gain observed in specific bands may hinder its performance in scenarios that require robust signal propagation. This study35, presents a CPW fed monopole antenna fabricated on an FR4 substrate, exhibiting quad-band resonance at essential wireless communication frequencies. The key contribution lies in the integration of multiple frequency bands within a single radiating structure, without any increase in the overall antenna footprint. Nevertheless, the Antenna exhibits limited BW at some resonant frequencies.

Numerous reported antenna designs suffer from inadequate impedance BW, preventing effective coverage of multiple RF sources. Additionally, many of these antennas demonstrate restricted gain and reduced efficiency when evaluated under practical deployment conditions. In addition, the lack of effective structural optimization has contributed to diminished power conversion performance, especially under operating conditions where multiple interfering signals are present. In order to mitigate the existing limitations observed in previous designs, this research work introduces an approach that enables wideband performance in a highly compact configuration, thereby addressing the fundamental limitations of conventional slot antennas, which typically require a compromise between achievable BW and antenna miniaturization. In the proposed design, an inverted T-shaped stub and a pair of E-shaped stubs are integrated into a rectangular slot, a configuration that supports wideband performance while simultaneously improving the uniformity and strength of the surface current distribution. This structural modification markedly improves impedance matching and operational BW while preserving the original physical footprint of the design. Furthermore, the antenna was experimentally characterized in an anechoic chamber and subsequently integrated with a rectifier circuit, thereby validating its practical feasibility for ambient RFEH in IoT sensor network applications. The main contributions are summarized as follows:

  1. (1)

    Wideband performance: The proposed antenna achieves a measured impedance BW of 1.05 GHz (0.84–1.89 GHz) and a peak realized gain of 4.86 dBi, offering superior size-to-performance ratio compared to existing designs.

  2. (2)

    Compact structure: The antenna maintains a compact footprint of 0.65\(\lambda _g\) \(\times\) 0.49\(\lambda _g\), making it suitable for integration in space constrained IoT and RFEH applications.

  3. (3)

    Innovative design approach: The integration of an inverted T-shaped stub and dual E-shaped stubs within a rectangular slot introduces multiple resonant modes and enhances surface current distribution, significantly improving impedance matching and BW.

  4. (4)

    Experimental validation: The proposed design was fabricated and experimentally validated in an anechoic chamber, demonstrating strong agreement between simulated and measured results

  5. (5)

    Energy harvesting integration: The antenna was successfully integrated with a rectifier circuit, confirming its capability for ambient RFEH under realistic conditions.

Antenna design and analysis

The geometry of the proposed wideband slot antenna is illustrated in Fig. 1. It features a rectangular slot measuring 68 mm \(\times\) 44 mm etched on one surface of the dielectric substrate. The slot is modified by incorporating an inverted T-shaped stub along its upper edge, accompanied by two E-shaped stubs positioned on the left-hand (LH) and right-hand (RH) sides of the slot. To achieve a compact footprint, the inverted T-shaped stub incorporates horizontal strips folded symmetrically on both ends. The rectangular slot is excited using a T-shaped feed patch, while the microstrip line is positioned on the opposite side of the substrate to facilitate effective coupling. The feed line is designed with a width of \(W_{f}\) = 1.6 mm to maintain a characteristic impedance of 50 \(\Omega\). The feed line is symmetrically positioned on a large ground plane, which may obstruct the integration of additional electronic components on the printed circuit board (PCB). Alternatively, the feed line can be designed with a \(90^{0}\) bend or placed asymmetrically along the ground plane, thereby creating additional space for the placement of other circuit components. As most existing designs position the feed lines symmetrically on the ground plane for consistency and ease of comparison36,37,38, our design adopts a similar configuration by placing the feed line symmetrically as well. To enhance impedance matching, a step is introduced on the lower section of the T-shaped feed patch at both the LHS and RHS. The proposed antenna achieves a wide impedance BW at the targeted \(f_{o}\), making it suitable for efficient RFEH applications. This antenna design utilizes an FR-4 substrate, which exhibits a dielectric constant \(\varepsilon _{r}\) = 4.4, and a loss tangent of 0.02. The EM characteristics of the substrate were systematically evaluated and accounted for throughout the simulation and fabrication processes. The final dimensions of the proposed wideband antenna, utilized for fabrication of the prototype is depicted in Fig. 1, which was subsequently used for measurement.

Fig. 1
Fig. 1
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Complete view of the wideband slot antenna. The parameters were systematically optimized according to the values outlined below: \(\hbox {L}_{s}\) = 76, \(\hbox {L}_{p}\) = 68, \(\hbox {L}_{1}\) = \(\hbox {L}_{2}\) = 36, \(\hbox {L}_{3}\) = \(\hbox {L}_{4}\) = 32, \(\hbox {L}_{5}\) = 12.5, \(\hbox {L}_{6}\) = 11.5, \(\hbox {L}_{7}\) = 15.4, \(\hbox {L}_{8}\) = 2.83, \(\hbox {L}_{9}\) = 12, \(\hbox {L}_{f}\) = 49.6, \(\hbox {W}_{f}\) = 1.6, \(\hbox {W}_{p}\) = 44, \(\hbox {W}_{s}\) = 100, a = 4.3, b = 5, c = 4, d = 3, e = 4, h = 1 : (All units are in mm).

Fig. 2
Fig. 2
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A structured design methodology of the proposed wideband antenna.

Fig. 3
Fig. 3
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Parametric analysis on \(L_{1}\).

Fig. 4
Fig. 4
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Parametric analysis on \(L_{3}\).

The geometry of the design results from transforming a rectangular patch into four different configurations. The first configuration included only the T-shaped feed patch, the second added E-shaped stub to the LHS of feed followed by integrating inverted T-shaped stub and finally, the fourth scenario combined all elements into a complete structure. These were labeled as Ant-I, Ant-II, Ant-III, and the Proposed Antenna, respectively as shown in Fig. 2.

The results confirmed that the integrated contribution of each design element plays an important role in achieving the observed wideband characteristics. In the configuration of Ant-I, where only the T-shaped feed patch is embedded within the slot, the antenna exhibits limited impedance matching across a narrow frequency band. This constrained resonant response highlights inadequate EM coupling, thereby restricting its capability for wideband operation. The integration of an E-shaped stub in Ant-II led to a significant improvement in impedance matching and a noticeable extension of the operational BW. This enhancement is primarily attributed to the increased surface current distribution along the slot edges. Compared to Ant-I, Ant-II demonstrates deeper nulls and wider frequency coverage, suggesting the onset of resonant mode merging. When the inverted T-shaped stub is employed in Ant-III, the antenna exhibits a significantly enhanced BW performance. An enhanced impedance match is observed in the mid-frequency range, thereby extending the region where the reflection coefficient remains below – 10 dB. The proposed configuration exhibits enhanced surface current distribution and introduces supplementary resonant modes, thereby facilitating a smoother transition between operating states. The proposed antenna, incorporating a rectangular slot, an inverted T-shaped stub, dual E-shaped stubs, and an extended T-feed patch, exhibits superior wideband performance through the synergistic integration of its structural elements. The measured reflection coefficient \(S_{11}\) remains consistently below – 10 dB across the frequency range.

A thorough parametric analysis was carried out using HFSS. The return loss was evaluated across three structural variants: \(L_{1}\), \(L_{3}\), and \(L_{5}\). It was observed that the integration of these structural components substantially broadens the antenna’s impedance BW, mainly as a result of multiple resonant pathways and effective mode coupling39,40. Figure 3 demonstrates that varying \(L_{1}\) within the range of 31–36 mm leads to a noticeable reduction in the lower resonance frequency. This phenomenon supports classical slot antenna behavior, where a longer slot effectively elongates the current path, thus decreasing the natural resonance. An empirical expression describing this variation in the lower resonant mode is given as:

$$\begin{aligned} f_{1} = \frac{c}{2\ell _{\text {eq}}\, (\varepsilon _{\text {eff}})^{1/2}}, \quad \text {where} \quad \ell _{\text {eq}} = L + W \end{aligned}$$
(1)

Here, \(\varepsilon _{\text {eff}}\) represents the effective dielectric constant, which is approximated as \(\varepsilon _{\text {eff}}\) = (\(\varepsilon _{r}\) + 1)/2, where \(\varepsilon _{r}\) denotes the relative permittivity of the substrate41. The parameter c indicates the speed of light in free space, while L and W correspond to the length and width of the rectangular slot, respectively. Due to its strong impact on the resonant frequency \(f_{1}\), the length \(L_{1}\) plays a key role in setting the antenna’s lower frequency boundary. When \(L_{1}\) is tuned to 36 mm, the impedance matching noticeably improves, starting from 0.84 GHz. Also, from the equation:

$$\begin{aligned} \lambda _{g} = \frac{c}{f \sqrt{\varepsilon _{\textrm{eff}}}} \end{aligned}$$
(2)

The guided wavelength is used for the calculation of the lengnth \(L_{1}\). Increase in the effective length lowered the resonance while decreasing \(L_{1}\) raises it. In an optimally designed wide slot, the length \(L_1\) is calculated as:

$$\begin{aligned} L_1 = \frac{\lambda _g}{2} = 36 mm \end{aligned}$$
(3)
Fig. 5
Fig. 5
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Parametric analysis on \(L_{5}\).

Figure 4 illustrates the influence of varying \(L_{3}\), which corresponds to the vertical section of the inverted T-stub. Increasing \(L_{3}\) extends the effective electrical length that supports the first higher-order slot resonance, causing a shift of this mode toward lower frequencies and promoting stronger coupling with the fundamental mode generated by the main rectangular slot. As a result, the interaction between these modes produces a wider continuous – 10 dB impedance BW. An increase in \(L_{3}\) from 27 to 32 mm leads to a notable improvement in impedance BW and a reduction in return loss. The influence of \(L_{3}\) on resonant mode convergence highlights its contribution to achieving a broader impedance BW, aligning with widely adopted principles in wideband antenna design. In terms of equivalent circuit interpretation, the extended \(L_{3}\) increases the shunt susceptible component near the feed slot interface, which facilitates improved real-part impedance matching across the mid-band frequencies, in conjunction with the contribution of the E-shaped stubs. Figure 5 demonstrates that modifying \(L_{5}\) induces minor changes in the high-frequency band of the system’s response. Although the modifications appear minimal, they clearly indicate that the E-shaped stubs function as auxiliary resonators, significantly affecting the mid-to-upper frequency band transitions. This observation aligns with the experimental results presented in34, where the inclusion of E-shaped stubs was shown to improve resonance consistency across adjacent bands. The dual E-shaped stubs function as auxiliary quarter wavelength \(\lambda\)/4 resonators that introduce additional current paths along the vertical arms of the structure. These paths facilitate the merging of adjacent resonant modes, thereby smoothing the \(S_{11}\) response between resonances and flattening the impedance transition across the mid to upper frequency bands. Collectively, these effects result in a significant enhancement of the – 10 dB impedance BW while maintaining a compact physical footprint. This behavior is consistent with the surface current redistribution observed in Fig. 13, particularly at 1.6 GHz, where strong current concentrations appear on the E-shaped stubs. Furthermore, the observed mid-band performance improvement aligns with the L5 parametric sweep, which effectively adjusts the electrical length of the E-stubs, as illustrated in Fig. 4. The vertical portion of the E-stub is the dominant length setting this resonance express as:

$$\begin{aligned} f_2 = \frac{c}{4 L_{\textrm{stub}} \sqrt{\varepsilon _{\textrm{eff}}}} \end{aligned}$$
(4)

where \(L_{stub}\) is the effective length of the E-shaped stub.

Fig. 6
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Simulated and measured: reflection coefficient (\(|S_{11}|\)) of the Antenna as a function of frequency.

Fig. 7
Fig. 7
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Simulated and measured: peak realized gain, of the antenna as a function of frequency.

Simulation and experimental verified results

Figure 6 illustrates the measured and simulated reflection coefficient \(|S_{11}|\) of the proposed wideband slot antenna across the operating frequency range. Both results remained within the acceptable limits of the target operating BW. The slight deviation observed in the measured response can be attributed to several factors, including misalignment during fabrication between the top and bottom layers, insertion loss associated with the SMA connector or feed cable, and signal degradation caused by soldering lead resistance. The proposed design exhibits a simulated impedance BW of 1.10 GHz ranging from 0.87 to 1.97 GHz, while the measured BW is 1.05 GHz spanning from 0.84 to 1.89 GHz, both defined at the − 10 dB. The results demonstrate that the target resonant frequency, \(f_{o}\), was achieved with a simulated and measured FBW of 77.5% and 77% respectively. Additionally, the measured \(|S_{11}|\) response closely aligns with the simulated data, indicating good agreement between measured and simulated performance. At 1.9 GHz, the antenna achieves a peak realized gain of 4.97 dBi under simulation and 4.86 dBi based on measured results, as presented in Fig. 7.

Fig. 8
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Experimental setup for validating the performance of the antenna with a Horn Antenna in an anechoic chamber.

Fig. 9
Fig. 9
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Measured and simulated radiation patterns of the antenna for \(\Phi =0^{\circ }\) and \(\Phi =90^{\circ }\). (a, b) at 0.9 GHz; (c, d) at 1.47 GHz; (e, f) at 1.6 GHz; (g, h) at 1.9 GHz.

Fig. 10
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Measurement setup for RFEH validation of the proposed antenna.

Fig. 11
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Integrated RF-rectifier circuit layout.

The antenna’s performance was evaluated through experimental measurements conducted in a fully lined anechoic chamber, ensuring minimal EM interference and signal reflection. As shown in Fig. 8, the antenna under test (AUT) was vertically mounted on a low profile, non-metallic tripod positioned at the center of a wooden turntable to maintain consistent rotational alignment. The measurement setup facilitated azimuthal rotation, thereby enabling a complete characterization of the far-field radiation pattern. A standard gain horn antenna was strategically placed at a suitable distance to function as the transmitting source, with both antennas properly aligned within the far-field region. The walls of the chamber were carefully lined with pyramidal RF absorbents to simulate conditions similar to a free space environment, thereby minimizing EM reflections during measurements.

The radiation pattern corresponding to frequencies of 0.9, 1.47, 1.6, and 1.9 GHz are measured in both E and H plane as shown in Fig. 9. The comparison between measured and simulated results reveals a close agreement, exhibiting only minimal variations that are within acceptable limits. Across its designated operating frequency range, the proposed antenna achieves a favorable front-to-back (F/B) ratio, indicating effective directional radiation performance. At 0.9 GHz, the simulated E-plane radiation pattern exhibits a dominant main lobe oriented at \(0^0\), achieving a peak gain of approximately 2.2 dBi. In contrast, the back lobe observed near \(180^0\) is suppressed to about – 10 dBi, yielding a F/B ratio of approximately 12.2 dB. At the same operating frequency, the H-plane radiation pattern exhibits a main lobe gain of approximately 2 dBi, while the back lobe level is around – 8 dBi. Consequently, the resulting F/B ratio is estimated to be about 10 dB. At 1.6 GHz, a marked enhancement in antenna performance is observed. The E-plane radiation pattern exhibits a forward gain of approximately 2 dBi, whereas the back lobe level is reduced to about – 12 dBi, resulting in a F/B of 14 dB. The behavior in H-plane follows a comparable pattern, achieving a F/B ratio greater than 13 dB. These results demonstrate that the antenna retains acceptable directional discrimination performance, even at the lower boundary of the operating frequency band.

Fig. 12
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Simulated and measured radiation efficiency of the antenna.

Fig. 13
Fig. 13
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Simulated surface current distribution of the antenna at (a) 0.9 GHz, (b) 1.47 GHz and (c) 1.6 GHz.

In order to assess the practical performance of the proposed RFEH system, a fully integrated rectenna was developed and subjected to experimental evaluation under real world ambient conditions. As shown in Fig. 10, the measurement setup comprised the antenna, a rectifier circuit, an RF spectrum analyzer and a digital multimeter. The antenna structure was precisely designed to enhance the efficient capture of incident EM energy. To enable the conversion of the received RF signals into a practical DC output, the antenna was integrated with a high-performance rectification circuit. The RF rectifier, previously subjected to extensive testing and validation as reported in42, demonstrates low insertion loss and is well-matched to the antenna’s operational frequency bands. The configuration of the rectifier circuit is illustrated in Fig. 11. Integrating this rectifier within the system enables a direct evaluation of the antenna’s real time energy harvesting performance under realistic environmental conditions. A Power Management Module (PMM) was later integrated to regulate and store the DC output harvested from the rectifier. Additionally, the PMM maintains voltage stability and facilitates continuous power delivery to connected IoT sensor nodes. Furthermore, the system incorporates a precise method for monitoring the extracted power, enabling a detailed evaluation of its energy harvesting efficiency. To characterize the ambient RF environment, a 6 GHz Aim spectrum analyzer (TTiPSA6005) was employed, facilitating real time observation of the incident spectral components. The DC output voltage generated by the system was concurrently measured using a DMM. Under typical ambient environmental conditions, the measured voltage was approximately 0.437 V. This result confirms the effective integration and performance of the antenna and rectifier system for ambient RFEH.

Fig. 14
Fig. 14
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Simulated and measured cross polarization patterns of the antenna for E and H planes: (a) Sim/Meas: 0.9 GHz E Plane; (b) Sim/Meas: 0.9 GHz H Plane; (c) Sim/Meas: 1.6 GHz E Plane; (d) Sim/Meas: 1.6 GHz H Plane.

Fig. 15
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Harvested DC power, PCE vs input power of the Rectenna.

Table 1 Performance evaluation of the proposed antenna compared to literature works.
Full size table

The antenna measurements and rectenna validation procedures were carried out under unbiased operating conditions, with no DC bias applied to the RF front end. The rectifier used employs a passive topology based on a Schottky diode and was characterized without the application of any external bias. Consequently, no biasing network was incorporated at the antenna input port. In systems where an active device is biased at the input, the associated biasing network may alter the input impedance matching, slightly affect radiation efficiency due to load variations, and necessitate the use of appropriate RF chokes and decoupling components to suppress unwanted coupling effects. Consequently, the results presented in this work characterize the antenna and rectenna performance under zero-bias operating conditions.

Figure 12 presents a comparison between the simulated and measured radiation efficiencies of the proposed antenna over the entire operating frequency range. The results indicate that the radiation efficiency remains largely consistent, with the measured values exhibiting close agreement with the simulated trend. A maximum efficiency of 86% is simulated and 84.5% measured at 2 GHz. Although small deviations are observed, particularly at higher frequency ranges, the measured differences remain within acceptable limits. These variations are primarily attributed to fabrication tolerances, connector related losses, and minor alignment inaccuracies introduced during the assembly process. The efficiency characteristics demonstrate that the antenna exhibits stable performance across the entire wideband operating region with, a feature that is essential for ensuring dependable RF energy harvesting. The simulated surface current distributions at 0.9 GHz, 1.47 GHz and 1.6 GHz, are shown in Fig. 13, providing insight into the antenna’s current behavior across the operating band. At 0.9 GHz, the surface current is primarily distributed along the edges of the rectangular slot and the inverted T-shaped stub, which signifies effective EM coupling between the feeding structure and the radiating components. Also, at 1.6 GHz, the surface current distribution is predominantly concentrated on the E-shaped stubs, indicating that these elements play a significant role in exciting higher order resonant modes. The observed redistribution of surface currents among the various structural elements confirms the effectiveness of the proposed design methodology, in which multiple stubs are deliberately incorporated to realize wideband performance through efficient modal interaction and merging. Figure 14 presents the measured and simulated cross-polarization radiation patterns for both the E-plane and H-plane at 0.9 GHz and 1.6 GHz. At both operating frequencies, the antenna exhibits low cross-polarization levels, indicating that polarization purity is effectively maintained throughout the intended BW. This characteristic is critical for reducing signal degradation while enabling efficient energy capture under practical deployment conditions43. Figure 15 shows the variation of RF-to-DC PCE with respect to \(P_{in}\) for the proposed wideband antenna integrated with the rectifier. To emulate practical operating scenarios of low-power IoT devices, the input power level was evaluated over a range extending from -30 dBm to 5 dBm. The results indicate a peak conversion efficiency of approximately 78.8%, achieved at an \(P_{in}\) of 4 dBm. The decrease in efficiency observed at lower input power levels is mainly attributed to the intrinsic threshold voltage characteristics of the diode.

A fabrication tolerance sweep of \(\pm 0.1\,\textrm{mm}\) on \(L_1\), \(L_3\), \(L_5\) and \(\pm 0.05\,\textrm{mm}\) on \(W_f\) yields a frequency shift of \(\le \pm 20\) - \(30\,\textrm{MHz}\) for the lower resonance and a negligible impact on the upper band, while preserving an FBW of approximately \(77\%\) and the main beam profile, consistent with the measured and simulated.

Table 1 presents a comprehensive comparison between the proposed antenna and existing designs, considering dielectric substrate, impedance BW, peak gain, and normalized electrical dimensions. The proposed antenna demonstrates a measured impedance BW of 1.05 GHz, ranging from 0.84 GHz to 1.89 GHz, and achieves a peak realized gain of 4.86 dBi. The design maintains a compact form factor of 0.59\(\lambda _g\) \(\times\) 0.44\(\lambda _g\). While29 and34 exhibit commendable performance, they typically demand larger physical dimensions 2.7\(\lambda _g\) \(\times\) 1.7\(\lambda _g\) and 0.44\(\lambda _g\) \(\times\) 0.38\(\lambda _g\), designed on RO6002 and RO4003C respectively. In contrast, the proposed design shows significant reduced size. Moreover, the antenna presented in30 provides an extended BW of 3.51 GHz, but posses relatively larger dimensions 1.12\(\lambda _g\) \(\times\) 1.66\(\lambda _g\). While designs in31 and32 offer compact footprints, they are constrained by limited BW, thereby limiting their applicability in high efficiency RFEH systems. In contrast to existing designs, the proposed antenna achieves a more favorable balance by maintaining compact dimensions while providing wideband operation and consistent gain performance.

Conclusion and future work

A novel wideband slot antenna is presented in this study, specifically designed to support RFEH for IoT sensor nodes. The integration of an inverted T-shaped stub along with dual E-shaped stubs within a rectangular slot configuration significantly improves the antenna’s impedance BW and gain characteristics, while maintaining a compact physical profile. Extensive parametric optimization was conducted, and the performance was validated through both numerical simulations and experimental measurements. The measured results demonstrate a wide impedance BW of 1.05 GHz and a peak gain of 4.86 dBi, exhibiting strong agreement with the simulated data. The integration of the antenna with a rectifier circuit resulted in effective power conversion efficiency in ambient RF environments, demonstrating its potential for practical deployment in ambient energy harvesting applications. The proposed antenna demonstrates an enhanced balance among physical size, operational BW, and radiation efficiency when compared to prior designs, thereby positioning it as a viable solution for sustainable energy delivery in IoT sensor nodes. Future research will focus on achieving further miniaturization and integrating adaptive power management systems to improve deployment flexibility and operational efficiency.