A Dual-Band Flexible Antenna With an Adaptable Design Technique

This paper presents a dual-band flexible antenna operating at 2.45 GHz and 5.8 GHz, utilizing dual resonant slots. A design technique is developed through extensive simulations, optimizing the slots’ separation and enhancing performance while simplifying the design process. The antenna achieves an impedance bandwidth of 23% (2.25-2.85 GHz) and 9% (5.5-6.1 GHz), with peak gains of 4.4 dBi and 5.7 dBi, respectively. The measured results align well with the simulations, confirming the antenna’s superior performance. Additionally, the antenna demonstrates improved performance and standard design technique compared to existing works in the literature. This research contributes valuable insights and establishes a foundation for further advancements in flexible dual-band polymide antenna development.


I. INTRODUCTION
Traditional antennas are known to be rigid and inflexible, which makes them not a popular choice for wearable applications [1], [2].On the contrary, wearable antennas are required to be flexible as they need to follow the body curves and be bent for the comfort of the users.In addition, wearable antennas must be small and lightweight as they are designed to be incorporated into flexible substrates [3], as the choice of these materials affects the antenna parameters, such as radiation efficiency, gain, and bandwidth.There are a number of materials like paper, textile, polymer, and liquid crystal polymer are all flexible substrates that can be used to make flexible antennas [4].Polyimide is one of the most commonly used substrates for flexible antennas because of its high thermal stability, mechanical strength, chemical resistance, and low dielectric loss [5].It is suitable for flexible antenna fabrication because it can be bent or folded without affecting its performance.In conclusion, Polymide is one of the ideal substrates which can be used for fabricating flexible devices for conformal and wearable applications, such as displays, and sensors for biomedical, telemedicine, etc.These devices demand antennas that are low-power and short range [6].To meet this requirement technologies like Bluetooth and ZigBee are commonly employed [6].However, for commercial wireless connectivity, a wireless local area network (WLAN) is considered the most cost-effective and reliable solution.Notably, ongoing WLAN advancements demand the integration of all IEEE 802.11a/b/g/n standards operating in the 2.4 GHz (2400-2484 MHz) and 5.8 GHz (5725-5875 MHz) bands into a single antenna unit [7]- [9].
There are a number of dual-band antennas proposed, where various techniques are adopted.Generally, techniques used for obtaining dual band resonances by making various complex geometries of slots [10] or employing the artificial composite material [11], or the meander line technique [12].These previous works make it abundantly clear that the design used is sophisticated, and that no standard design technique is reported.Therefore this work aims to solve these issues by following a simple antenna design, with a design technique that can be altered to generate other bands, as required based on the specifics of the application involved.The technique followed in this work to generate a dual-band frequency of operation is by introducing a slot on the radiating patch so as to alter the current path, which governs the formation of each resonant frequency [13], [14], and its performance evaluation is carried out.
The paper is organized as follows.The geometrical structures of the proposed dual-band antenna and the systematic design flow, and optimizing of the separation of slots for enhancing the performances are covered in Section II.Section III describes the prototype of a dual-band flexible antenna and covers a discussion and comparison of simulated and measured results, and performance comparison finally, conclusions are drawn.

II. PROPOSED DUAL-BAND ANTENNA DESIGN
The antenna shown in Figure 1 comprises a rectangular patch with two slots, and a ground plane, and is operated at 2.45 GHz and 5.8 GHz.The slot sizes are given in Table 1 and calculated using the technique available [15].The patch and the ground plane are made of copper, and the substrate is made of polyimide, which is a flexible and low-loss material.
The antenna is fed by a micro-strip line that connects to the patch through the cut slots in the patch.

A. Design Optimization
To optimize the performance of the proposed flexible antenna after two independent slots are created, a parametric analysis is performed focusing on the seperation between the slots of lower and higher resonance x j is varied, and its impact on various antenna performances, namely the reflection coefficient S 11 , radiation efficiency, and realized peak gain is investigated ((See Figure 2(a)-(c)).The x j is varied from -4.0 cm to 2.0 cm in the step of 0.2 cm steps with slots lengths (l 1 , and l 2 )and widths (w 1 , and w 2 ) kept fixed.The conclusion noted from this analysis is that x j is critical variable which influences the performance in terms of reflection coefficient S 11 , radiation efficiency, and realized peak gain.As illustrated in Figures 2(a)-(c), optimum performance is reached at x j ∼ = −0.8.As a result, in this work x j is set -0.8, similar analysis was also carried out in the case of other dual band antennas, covering other bands, however the results for redundancy are not provided.These findings were then used to develop a design technique for designing dual band flexible antennas.

B. Dual Band Design Technique
Based on extensive observations and comprehensive analysis of various cases, a design methodology has been devised for dual band flexible slot antenna development.The proposed technique entails initially establishing a lower resonance slot antenna and rigorously validating its operational characteristics at lower frequencies.Subsequently, a higher resonance slot is introduced, followed by a meticulous parametric investigation to optimize the x j parameter.The goal is to identify the specific value of x j that yields superior performance metrics, including S 11 , radiation efficiency, and realized peak gain.Through rigorous experimentation and comprehensive testing, multiple scenarios were examined, resulting in successful outcomes.While the detailed results are not presented here to avoid redundancy.

C. Design Finalization
Once the value of x j was determined to be -0.8 through careful evaluation, extensive simulations were conducted to analyze the performance of the dual band flexible slot antenna design.These simulations encompassed impedance matching, radiation pattern characterization, and current distribution analysis.The obtained results demonstrated good performance, thus providing the authors with confidence to proceed with the measurement phase of the proposed dual-band antenna.To validate the design, a prototype (See Figure 5) is fabricated on polyimide substrate.The parameters used in the prototype are as given in Table 1, and its various performances are measured.The loss of the coaxial adapter is carefully calibrated by following the OPEN, SHORT and THROUGH standards, and its operation at 2.45 GHz and 5.8 GHz are experimentally verified.Comparision of simulated and measured performances S 11 , and gain as shown in Figure 5 and 6, respectively.As observed from Figures 5 and 6 a good agreement is observed.The antenna performances are usually separately compared in terms of gain.Overall, the simulations and subsequent measurements validated the efficacy of the proposed antenna design, showcasing its suitability for dualband operation as well as its ease of design.The measured results were then compared with the previously reported work, performance improvement was observed, mainly in terms of gain and radiation efficiency, as well as the addition of standard dual-band design technique.

CONCLUSION
In conclusion, this paper introduces a flexible slot antenna that operates at dual resonances, with slots positioned at higher and lower frequencies.The detailed parametric analysis reveals that the separation between these slots plays a crucial role in determining the antenna's performance.By conducting additional investigations, a design technique is proposed, streamlining and expediting the overall design process.This technique offers significant benefits in terms of ease of implementation and efficiency as well as an increase in antenna gain.The presented antenna design holds promise for various wireless applications, showcasing improved performance and offering design flexibility.Further research and development in this area are warranted to explore the full potential of this novel antenna design and design technique.