Introduction

The exponential growth of wireless communication technologies has profoundly transformed modern life, enabling real-time connectivity between people, devices, and systems across vast distances. One of the most significant technological trends of the past decade is the emergence and rapid expansion of the Internet of Things (IoT), a paradigm that envisions the interconnection of billions of smart devices capable of sensing, communicating, and processing data autonomously. These devices are expected to operate seamlessly within complex and heterogeneous networks, sharing information efficiently while consuming minimal power and occupying limited physical space1,2

This growing demand for ubiquitous, intelligent, and low-power connectivity has imposed new requirements on the design of wireless communication components, particularly antennas. As the core element responsible for transmitting and receiving electromagnetic signals, the antenna must meet several critical criteria in IoT systems: compactness, frequency flexibility, low energy consumption, and robust electromagnetic performance in dense environments3,4. In portable electronic devices including smartphones, tablets, smartwatches, GPS trackers, and medical wearables, the space allocated for the antenna is increasingly constrained. Nevertheless, the antenna must still maintain high performance across multiple frequency bands and wireless protocols such as Wi-Fi, Bluetooth, Zigbee, and Lora WAN5.

Conventional antenna technologies often fall short when deployed in such demanding scenarios, as they typically require trade-offs between size, bandwidth, gain, and efficiency. In addition, the fixed-frequency operation of many legacy antennas limits their ability to adapt to dynamic wireless environments where multi-standard operation and interference mitigation are essential. IoT systems further exacerbate these challenges by requiring support for diverse device densities, adaptive power control, and coexistence within shared spectrum environments6,7.

In response to these limitations, metamaterials and metasurfaces have emerged as promising solutions in the field of antenna engineering. Metamaterials are artificially structured materials designed to exhibit unconventional electromagnetic properties such as negative permittivity and/or permeability, which do not exist in naturally occurring substances8. These unique characteristics enable unprecedented control over wave propagation, radiation direction, and surface current distribution, opening the door to highly efficient and miniaturized antenna designs suitable for integration into IoT-enabled devices9,10.

Furthermore, the advent of Frequency Reconfigurable Antennas (FRAs) has provided a practical pathway to address the dynamic requirements of modern communication systems. By integrating reconfigurable elements such as PIN diodes, varactor diodes, Micro-Electro-Mechanical Systems) MEMS (switches, or digital tuning components, FRAs allow antennas to switch between frequency bands, alter polarization, or modify radiation patterns in real-time based on network conditions or device requirements11,12. This capability is particularly valuable in IoT networks, where the operating environment may change rapidly, and the need for adaptive performance is paramount.

In this study, we propose an innovative antenna structure that combines the advantages of metamaterials and reconfigurability to meet the stringent demands of next-generation IoT systems. The proposed design incorporates embedded metasurfaces, multi-path configurations, and tunable components to achieve compact size, high radiation efficiency, and dynamic frequency agility. The antenna is optimized to operate in frequency bands relevant to IoT applications, including 2.4 GHz, 5 GHz, and emerging 6 GHz ranges, ensuring compatibility with a wide range of low-power communication protocols13,14.

Compared to conventional antennas, the proposed structure offers several advantages. Its compact form factor makes it suitable for space-constrained environments, while its low power consumption supports battery-powered operation for extended periods. The design’s reconfigurable nature ensures scalability and adaptability across different use cases—ranging from smart home automation, environmental monitoring, and agricultural sensing, to wearable health devices and industrial IoT (IIoT) platforms15,16. By minimizing signal loss, reducing interference, and enhancing spectral efficiency, the antenna design contributes significantly to the realization of reliable and high-performance IoT systems.

The next section reviews literature on frequency reconfigurable antennas and metamaterial designs. Ref.17 presents a printed star-triangular fractal microstrip-fed monopole antenna with a semi-elliptical ground plane designed for super-wideband (SWB) applications. Unlike traditional SWB and fractal antennas, this one is compact and achieves a VSWR of less than 2 at 2.1, 2.45, 3.2, and 3.5 GHz. Ref.18 describes an extremely wideband star-star fractal antenna fed by a microstrip line, incorporating a semi-elliptical ground plane with notch loading. This design achieves a bandwidth ratio of 11.31:1 with VSWR values below 2. Ref.19 discusses a metamaterial and slot-based fractal antenna designed for multiband operation at 3.5, 5.01, 3.2, and 5.77 GHz, featuring an L-shaped slot, a Sierpinski triangle (fractal), and a metamaterial circular split ring resonator (SRR) ground plane. Ref.20 proposes a simple, frequency-reconfigurable microstrip patch antenna for multiband applications, with a 1.8 mm gap between two patches on the same FR4 substrate. Three RF pin diodes are used to modify the antenna’s resonance frequencies, allowing operation at 2.4, 4.26, 4.32, and 4.58 GHz with acceptable return loss. Ref.21 introduces a novel reconfigurable sub-6 GHz microstrip patch antenna operating at 3.9 and 4.9 GHz, which includes a metamaterial (MTM) array and a corresponding printed circuit around a strip line. Ref.22 describes a frequency-reconfigurable planar monopole antenna using an FR4 substrate, with operation at 2.4, 3.5, 4.7, and 5.8 GHz and radiation efficiency between 73 and 79%, with a VSWR of less than 1.5. Ref.23 presents a wireless frequency-reconfigurable antenna with a complementary four-split ring resonator (SRR) metamaterial, operating at 3.02, 2.34, 5.06, 6.44, and 4.2 GHz, making it ideal for IoT applications. A major challenge in antenna design is achieving compact size and extended frequency ranges. This research aims to address these issues by leveraging metamaterials, which possess unique electromagnetic properties that enable smaller antenna designs. The research concludes with Sections III and IV, which detail the simulated analysis of the antenna’s performance.

Materials and methods

Design of the metasurface unit cell

Recent advancements in metamaterials have enabled the design of innovative unit cells with exceptional electromagnetic properties. In this study, this work presents a Multi-Path T-Structured Unit Cell with single-negative characteristics, operating at 12.1 GHz. The unit cell is fabricated using an FR-4 substrate (1.4 mm thick) with copper cladding (0.035 mm). At this frequency, it demonstrates a relative permittivity (εr) of − 8 and a relative permeability (μr) of 17.3. The design utilizes multi-path T-structured elements to enhance electromagnetic performance. Figure 1 illustrates the proposed unit cell, and Table 1 details its dimensions. Moreover, Table S6 presents a comprehensive comparison with recently published unit cell designs.

Fig. 1
figure 1

(a) Multi-path T-structured unit cell (b) Material properties (c) Unit cell absorber.

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Table 1 Dimensions of unit cell.
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Proposed antenna design

Everything from the theory and design of the metamaterial-based RA to its fundamental geometry and switching mechanics is addressed here. In order to make the antenna work in two different frequency bands, lumped element switch was installed. PIN diode are used to rebuild the measurement setup circuit. Improved operating efficiency in the distant field is a direct outcome of using a partial ground plane.

Structural geometry

Figure 2 illustrates an antenna incorporating a set of metamaterial unit cells arranged in a 4 × 3 array, totaling 12 cells. These cells form the radiating patch in the upper section of the antenna, carefully arranged to enhance or modify performance across specific frequency ranges. The integrated metamaterials consist of small, repetitive elements with intricate patterns. The primary purpose of incorporating these metamaterials is to improve antenna characteristics, such as frequency control and radiation direction. These cells influence the interaction between electromagnetic signals and the material. The inclusion of 12 metamaterial cells enables the antenna to operate at two distinct frequencies, rather than a single frequency. This effect arises from the unique design of the metamaterial cells, which alter the propagation of electromagnetic waves within the antenna structure, The dimensions of the proposed antenna are listed in Table 2.

Fig. 2
figure 2

Geometry of the suggested antenna.

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Table 2 Dimensions of the planned antenna.
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Theoretically, effective resonant lengths at required frequencies are computed using the transmission line model24. For the selected operating frequency f, the effective permittivity εeff and effective resonant length Lf are determined by Eqs. (1) and (2).

$${\upvarepsilon }_{{{\text{eff}}}} = \frac{{{\upvarepsilon }_{{\text{r}}} + 1}}{2} + \frac{{{\upvarepsilon }_{{\text{r}}} - 1}}{2}\left( {1 + 12\left( {\frac{{\text{h}}}{{\text{w}}}} \right)} \right)^{ - 0.5}$$
(1)
$${\text{Lf}} = \frac{{\text{C}}}{{4{\text{f}}\sqrt {{\upvarepsilon }_{{{\text{eff}}}} } }}$$
(2)

The variables in the equations are defined as follows: The symbol C denotes the velocity of light in a vacuum, whereas \({\upvarepsilon }_{\text{r}}\) indicates the relative permittivity, h denotes the thickness or height of the substrate, whereas w denotes patch width.

Result and discussion

The proposed design has been analyzed using CST Microwave Studio 2022. The radiating structure is excited through a waveguide port. Performance parameters, including S-parameters, gain, and surface current distributions, can be obtained under various conditions within the CST Microwave Studio environment.

Reconfigurability

Figure 3 illustrates the S-parameter characteristics of the proposed antenna across all operational modes. Mode 1 (D1 OFF), The antenna operates in a single-band mode centered at 2.4 GHz, with a bandwidth of 80 MHz (ranging from 2.36 to 2.44 GHz) and an S-parameter value of − 22.2 dB, indicating efficient impedance matching at this frequency. Mode 2 (D1 ON) The antenna switches to a single-band mode at 6.04 GHz, achieving a bandwidth of 300 MHz (ranging from 5.9 to 6.2 GHz) and an impressive S-parameter value of − 38.79 dB, reflecting excellent performance in this mode (Table 3).

Fig. 3
figure 3

Mode 2 (a) Lg = 8.2, (b) L1 = 8.4 mm.

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Table 3 Frequencies and modes of antenna.
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Switching techniques

The SMP1340-079LF PIN diode is widely used in switching applications due to its RF characteristics. It behaves like a variable resistor and operates efficiently from 10 MHz to 10 GHz. Incorporating PIN diodes into antenna systems enables reconfigurability by altering the effective resonant length of the antenna, thereby influencing the operating frequency. Depending on the applied configuration, the PIN diodes can function as either open-circuit or short-circuit elements, with the forward and reverse modes of operation modeled using equivalent circuits, as illustrated in Fig. 4. For this study, the Skyworks SMP1340-079LF PIN diode was selected, featuring an inductance (L) of 0.7 nH, a series resistance (RL) of 0.85 ohms, and a capacitance (C) of 0.21 pF, making it well-suited for the intended application.

Fig. 4
figure 4

A PIN diode’s equivalent circuits and its CST model.

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The antenna demonstrates excellent impedance matching, with a voltage standing wave ratio (VSWR) remaining consistently below 1.18 at all resonant bands. This superior performance ensures minimal reflection and efficient power transfer. Figure 5 highlights this capability, showing that the VSWR stays well below the standard threshold of 2 across all usable frequency bands, confirming the antenna’s effectiveness in maintaining stable and reliable operation (Fig. 6).

Fig. 5
figure 5

(a), (b), VSWR of the suggested antenna in various modes of operation.

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Fig. 6
figure 6

(a), (b), Practical result of the two modes.

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Biasing circuit

On the same FR-4 substrate, a biasing circuit for the proposed antenna’s PIN diode switch was designed and implemented. Figure 7 illustrates the control mechanism, where a bias line connects to a bias pad to activate the PIN diode switch as a proof of concept. The switch is connected to the negative terminal of a 3-V DC battery, while the positive terminal is directly linked to the bias circuit. This setup enables the switch to control the flow of voltage, toggling the diodes ON and OFF. The direct connection of the DC terminal, as shown, makes this design adaptable for future applications, such as cognitive radio systems. To protect the PIN diode, specific design gaps were incorporated: a 1 mm gap between the bias pad and the ground to accommodate inductors (H), and an additional 1 mm gap between two pad lines to place a 1 kΩ resistor (R) for added protection. The DC circuit is isolated from the antenna’s feeding line using RF chokes, which block RF signals while permitting DC current flow. Key components include a 1 kΩ resistor to regulate and protect the voltage and 47 nH inductors for additional stability. Figure 6 presents the measurement results of the reconfigurable monopole antenna’s reflection coefficient (S11) using an Agilent Technologies E5071C network analyzer (30 kHz–20 GHz) at Al-Nahrain University, Baghdad, Iraq. The antenna operates with a bias current of approximately 2.2 mA (calculated using Ohm’s law with a 1 kΩ resistor) and a bias voltage of 3 V, powered by two 1.5-V DURA Cell alkaline batteries.

Fig. 7
figure 7

Biasing circuit and antenna proposed.

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For the inductors (47 nH), the inductive reactance (\(X_{L}\)) is calculated as:

$$X_{L} = 2\pi fL$$
(3)

where \(f\) at 1 GHz:

$$X_{L} = 2\pi \times 10^{9} \times 47 \times 10^{ - 9} = 295\,\Omega$$

At DC (frequency = 0 Hz), the reactance is 0, and the inductor acts as a short circuit, allowing DC to pass freely. This can be calculated as:

$$X_{L} = 2\pi \times 0 \times 47 \times 10^{ - 9} = 0\,\Omega$$

To determine the current passing through the PIN diode, we use Ohm’s law:

$$I_{R} = \frac{{V_{cc} - V_{diode} }}{R}$$
(4)

where \(Vcc = 3\,{\text{Volts}}\) (power supply voltage) and \(R = 1\,{\text{k}}\Omega\). For example:

$$I_{R} = \frac{3 - 0.8}{{1000}} = 2.2\,{\text{mA}}$$

This ensures that the current passing through the PIN diode remains within the safe operating range to ensure its proper functioning.

As for the power consumption in the ON state (D1 ON), the consumed power is calculated using the equation:

$${\text{P}} = {\text{V}} \times {\text{I}}$$
(5)
$$3\,{\text{V}} \times 2.2\,{\text{mA}} = 6.6\,{\text{mW}}$$

This value represents the power consumed when the PIN diode is activated to switch to the higher frequency (6.04 GHz). Regarding the total energy cost, assuming the antenna operates in the ON state (D1 ON) for 50% of the time, the average power consumption would be:

$${\text{P}}_{{{\text{avg}}}} = \frac{{6.6\,{\text{mW}}}}{2} = 3\;{\text{mW}}$$
(6)

This is a very low value, making the antenna highly suitable for battery-powered applications such as Internet of Things (IoT) devices.

Physical interpretation

The operation of the antenna at the frequencies 2.4 and 6.04 GHz depends on the relationship between the antenna’s effective length and the electromagnetic properties resulting from its design, including the addition of metamaterials. Resonance in the antenna occurs when its effective length is equal to half the wavelength of the signal at the resonant frequency. The effective length (Leff) is mathematically determined by the following equation24:

$${\text{Lf}} = \frac{{\text{C}}}{{2{\text{f}}}}$$
(7)

The effective length at 2.4 GHz is 62.5 mm and at 6.04 GHz is 24.8 mm.

Metamaterials perform a crucial role in reducing the effective length of an antenna, allowing it to operate at lower frequencies while maintaining smaller physical dimensions. The effective wavelength is modified according to the relationship24:

$$\lambda_{{{\text{eff}}}} = \frac{\lambda }{{\sqrt {\varepsilon_{{{\text{eff}}}} { }\mu_{{{\text{eff}}}} } }}$$
(8)

Metamaterials with negative values, such as \({\upvarepsilon }_{\text{eff}}\) (-8) and μeff (17.3), reduce the effective wavelength, allowing the antenna to resonate at desired frequencies with smaller dimensions. When the switch is OFF, the ground plane is small, causing surface currents to extend more over the radiating patch and along the edges. This increases the effective length, resulting in a lower resonant frequency of 2.4 GHz. Conversely, when the switch is ON, the ground plane area increases. The larger ground plane reduces the spread of surface currents across the antenna. Instead of extending to the edges, the currents are concentrated within a shorter path. This reduction in the effective length leads to a higher resonant frequency of 6.04 GHz.

Far field radiation pattern

In Mode 1, the simulation results show a gain of 3.7 dBi at 2.4 GHz with a radiation efficiency of 95.6%. In Mode 2, the antenna achieved a gain of 4.82 dBi and a radiation efficiency of 92% at 6.04 GHz. Figure 8 presents the simulated radiation patterns of the antenna in both the H-plane and E-plane across the respective frequency bands and Fig. 9 illustrates the gain and radiation intensity for both modes.

Fig. 8
figure 8

The Radiation pattern in the E-plane and H-plane. (a) 2.4 GHz, (b) 6.04 GHz.

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Fig. 9
figure 9

Gain and efficiency pattern at frequencies (a), MODE 1(2.4 GHz), (b) MODE 2 (6.04 GHz).

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Surface currents

Figure 10 illustrates the surface current distribution on the antenna’s radiating structure at various frequencies, demonstrating how the distribution changes across different operating modes. At 2.4 GHz, the primary radiating element shows a surface current density that plays a significant role in radiation. As the frequency increases to 6.04 GHz, the surface current distribution undergoes noticeable alterations due to the shorter wavelength. At this higher frequency, the antenna excites higher-order modes, resulting in more complex and rapid variations in the surface currents. These variations create multiple regions of concentrated current density, which influence the radiation pattern and overall performance of the antenna at the higher frequency.

Fig. 10
figure 10

Distribution of current (a) at 2.4 GHz (b) at 6.04 GHz.

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Finally, A comparison between the theoretical and experimental results is presented in Table 4, a summary of the design as shown in Table 5, and a comparison of the proposed antenna to previously published studies in Table 7 (Table 6).

Table 4 Comparison between simulated and experimental results.
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Table 5 Summary of the design.
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Table 6 Comparison of the proposed work with recently published metamaterial.
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Table 7 Proposed antenna is contrasted with previously published research.
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Conclusion and future work

There is increasing interest in designing compact antennas without compromising performance, particularly at lower frequencies crucial for IoT applications. The proposed antenna, with dimensions of 20 × 15 × 1.4 mm3, operates at 2.4 GHz and 6.04 GHz, achieving reconfigurability through strategically placed PIN diodes while maintaining a reflection coefficient below − 10 dB. This compact design addresses the need for efficient, space-saving antennas in IoT systems. Despite the good agreement between simulation results and theoretical predictions, minor discrepancies were observed. These variations can be attributed to manufacturing inaccuracies, slight misalignments of components, and environmental variations such as temperature or humidity changes during testing. Addressing these challenges through improved fabrication techniques and controlled testing conditions could further enhance performance consistency. The integration of metamaterials into antenna design enhances performance for a wide range of applications, including communications and remote sensing. For IoT, the antenna demonstrates high efficiency and adaptability in constrained spaces. Future advancements, such as expanding frequency ranges and integrating AI technologies, could enable autonomous operation, minimize electromagnetic interference, and further miniaturize the design to suit wearable devices and embedded systems. Future work will focus on improving manufacturing precision, addressing environmental challenges, and exploring new applications in healthcare, environmental monitoring, and industrial automation, these improvements will enable innovative solutions for wireless communication and IoT systems.