Paper

Multilayer multiband hybrid fractal antenna for public safety and 5G Sub-6 GHz bands

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Published 29 August 2024 © 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.
, , Citation Saurabh Anand and Ashwini Kumar 2024 Eng. Res. Express 6 035346DOI 10.1088/2631-8695/ad710a

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2631-8695/6/3/035346

Abstract

In this work, a multiband hybrid fractal antenna is developed for public safety and 5G Sub-6 GHz bands. Proposed antenna design is designed, analyzed, and optimized using HFSS simulator. Radiating element of the proposed antenna consists of Hybrid Fractal shape which is designed using Koch and Hilbert curve fractal geometry. Hybrid Fractal Antenna (HFA) is designed on a Multilayer FR4 substrate which has an overall size of 0.22 × 0.47 × 0.044 ƛ3. Parametric analysis of the proposed design is done to get the optimum results. Proposed HFA resonates at 2.64 GHz (2.34–2.80 GHz), 3.87, 4.54, 5.02, 5.35 GHz (3.73–5.55 GHz), and 6.42 GHz (5.70–6.72 GHz) with a gain of 4.2, 0.9, 2.9, 3.3, 4.6, and 3.7 dBi respectively. The proposed HFA is useful for 5G bands such as: N7 band, N38 band, N40 band, N41 band, N47 band, N53 band, N79 band, N90 band and N93 band, and also useful for public safety band (4.9 GHz). Prototype of the proposed HFA is fabricated and tested in lab for the verification of simulated results. The results show good performance in terms of reflection coefficient, gain, and bandwidth. Measured results are in good agreement with the simulated results.

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1. Introduction

The advent of 4G has significantly increased the interconnectedness between people, allowing for more efficient, seamless and natural communication. Now, with the emergence of 5G, the goal is to strengthen this foundation and take it to new heights. 5G seeks to not only meet but exceed the demands of better connections between individuals, interconnected devices, and the fusion of people with technology. Imagine a network where users can seamlessly collect the status of everyday gadgets, traffic data, personal health monitoring and home electronics. Such a network would be just a glimpse of the vast range of possibilities within the realm of 5G. While many potential 5G scenarios are unknown, it is clear that upcoming 5G services will be categorized into three main areas: advanced mobile broadband, mission-critical control, and facilitating large-scale Internet of Things (IoT) connectivity [1].

Within the spectrum of antennas available in the market and industry, the microstrip patch antenna stands out as a well-established option for implementing 5G wireless communications. Its appeal lies in several advantages, including its lightweight construction, compact size, affordability, low profile, smaller footprint, and straightforward fabrication process [2, 3]. Despite their compact size, ease of fabrication at low cost, and seamless integration with radio frequency circuits, these antennas are plagued by a significant drawback: their low fractional bandwidth and gain [4]. This limitation shows them antipathetic with modern wireless technologies. It may possible that patch antenna's increment in bandwidth and gain will be the most effectual processes to fulfil the requirement for 5G applications. In recent years, extensive research is only done to reduce the restrictions of these antennas, that will lead and be focused to improve its bandwidth and effective gain [59]. In [5], DGS method is implemented which enables the antenna operation at two distinct frequencies: 4.53 GHz and 4.97 GHz. This was accomplished utilizing the RO5880 substrate, characterized by a relative permittivity of 2.2. The antenna showcases gains of 5 dBi at 4.53 GHz and 4.57 dBi at 4.97 GHz, within an overall dimension of 77 mm × 70.11 mm. However, the bandwidth of the antenna is not reported. In [6], an antenna design was introduced featuring a high-gain, single-band configuration, utilizing Arlon AD300C substrate and resonating at 5.65 GHz. This design boasts an impressive overall gain of 7.15 dBi alongside a bandwidth spanning 135 MHz. An alternative method to enhance antenna gain involves incorporating a reflective layer, as suggested in [7]. This design utilizes four spacers positioned at distinct corners of the patch antenna. Operating at 2.392 GHz, the antenna achieves a total gain of 5.2 dBi and exhibits a narrow bandwidth of 44.7 MHz. Its physical dimensions measure 60 mm × 55 mm × 8 mm. In [8], a broadband antenna is designed on FR4 substrate for 5G n77 and n78 bands. The overall dimensions of the antenna are 28 mm × 20 mm, with a peak gain of 2.5 dBi while offering a bandwidth of 700 MHz. In [10], a dual band antenna for n77, n78, and n96 5G bands is proposed operating at 3.45 and 5.9 GHz with a bandwidth of 160 and 220 MHz. Gain of the antenna is 3.83 and 0.576 dBi.

Another approach utilized for enhancing the bandwidth and gain of antenna is the use of multilayer substrate. In many of the research in literature this method is successfully used to improve the performance of patch antenna in terms of gain and bandwidth [1117].

In [13], a comparative analysis of the multilayer substrate patch antenna is done, and observations shows that there is a significant increase in the gain as well as bandwidth of the antenna. In [14], a multiband triple layer U-slot patch antenna is proposed which operates at 1.6, 1.9, and at 3.8 GHz. Multilayer structure is successfully used to achieve for enhancing bandwidth of the patch antenna. In [15], a seven layer aperture stacked patch antenna is designed which operates from 5.05 to 10.1 GHz with a gain more than 6dBi. In [16], a miniaturized patch antenna is designed on a multilayer substrate which is consisted of eight layers of Roger RT Duroid 5880 and FR4 epoxy materials. Proposed antenna has an operating bandwidth ranging from 861–873 MHz with a gain of 2.92 dBi, and the overall dimensions of antenna are 100 × 100 × 8 mm3. In [17], a dual band miniaturized stacked patch antenna has been proposed for ISM and WiMAX applications. The antenna has an overall size of 35 × 35 × 5.3 mm3 and resonates at 2.5 and 3.52 GHz, with a operating bandwidth of 150 and 200 MHz respectively. A compact triple band multilayer patch antenna is proposed in [12] which resonates at 2.45, 3.25 and 4.15 GHz with a maximum gain of 6.9 dBi. The proposed antenna has an impedance bandwidth of 180, 350 and 850 MHz at three respective resonating frequencies.

In antenna design, multifunctionality holds significant importance as it enables the integration of multiple applications into a single device. To attain this versatility, antennas must operate across multiple frequency bands, allowing a single antenna to fulfil the role of multiple ones. Researchers have developed various techniques to achieve multiband operation in recent years, with the incorporation of fractal geometry into the radiating element of patch antennas emerging as one of the most effective and widely adopted methods [1827]. In [18], a Minkowski fractal antenna is designed and fabricated on FR4-epoxy substrate for Internet of Things (IoT) applications. Multiband performance and 65% miniaturization are achieved using Minkowski fractal. The proposed antenna resonates at 1.223, 1.58, 2.69, and at 3.54 GHz with impressive impedance bandwidths of 0.11 GHz (1.16–1.27 GHz), 0.23 GHz (1.47–1.70 GHz), and 4.5 GHz (1.99–6.49 GHz). The peak gain for the antenna is 3.4 dBi. In [19], a flexible multiband fractal antenna is proposed which covers frequency bands from 1.37–1.93 GHz, 2.25–2.51 GHz, 3.13–3.81 GHz, and 4.46–5.5 GHz. A wideband with band notched characteristics has been proposed in [20] which resonates at 1.51 GHz (1.19–2.06 GHz), 6.53 GHz and 8.99 GHz (4.44–9.54 GHz) while a band is notched at 10.46 GHz (9.32–11.92 GHz). A patch antenna using hybrid fractal geometry has been proposed in [24] which is designed on jean substrate with a size of 52.3 × 58.7 × 1 mm3. The antenna resonates at 2.44 and 6.31 GHz with an operating bandwidth of 230 and 240 MHz. Similarly in [25, 26], planar antennas are proposed using hybrid fractal geometries and results shows the effectiveness of the hybrid fractal structure implementation. From the literature, it is evident that there remains significant potential for the advancement of multilayer hybrid fractal antennas to attain both multiband and wideband, and high gain capabilities while maintaining satisfactory performance.

This work introduces a novel hybrid fractal antenna designed for both public safety and 5G sub-6 GHz applications. The design innovation lies in the amalgamation of Koch and Hilbert fractal geometries to create a unique radiating element. Implemented within a multilayer structure, this hybrid approach enhances antenna performance across key metrics such as bandwidth, gain, and multi-frequency operation. The overall dimension of the proposed antenna is 0.22 × 0.47 × 0.044 ƛ3, here ƛ is the wavelength as per lowest operating frequency. The antenna configuration comprises five layers of FR4 epoxy substrate housing three radiating elements, including two parasitic ones. The proposed antenna resonates at 2.64 GHz (2.34–2.80 GHz), 3.87, 4.54, 5.02, 5.35 GHz (3.73–5.55 GHz), and 6.42 GHz (5.70–6.72 GHz) with a maximum gain of 4.2 dBi. A 50-ohm transmission line is used to excite the radiating element. Proposed antenna design can be used for 5G bands such as: N7 band, N38 band, N40 band, N41 band, N47 band, N53 band, N79 band, N90 band and N93 band, and also useful for public safety band (4.9 GHz).

2. Antenna design

The process of designing hybrid fractal geometry using Koch and Hilbert curve fractal geometry is shown in figure 1. Design is initiated with a straight strip as shown in figure 1(a) which is transformed in hybrid fractal structure as shown in figure 1(d) using the fractal geometry shown in figures 1(b) and (c). Benefits of using the hybrid fractal structure in antenna design is the lengthening of antenna structure in the same space which helps in miniaturization of the antenna design. The 1st stage of the antenna is shown in figure 2 with its dimension details which has been designed using the hybrid fractal which is shown in figure 1(d). Figure 3 shows the reflection coefficient (S11) parameters of the 1st stage of design with the variation of ground height (gh). The variation in ground plane height is done from 10 to 23 mm. The best results in terms of S11 is observed at ground height equals to 17 mm. Further, stages have been developed with the ground height equals to 17 mm.

Figure 1. Refer to the following caption and surrounding text.

Figure 1. Transformation of strip line to hybrid structure using Koch and Hilbert curve.

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Figure 2. Refer to the following caption and surrounding text.

Figure 2. Hybrid fractal antenna.

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Figure 3. Refer to the following caption and surrounding text.

Figure 3. Effect of ground height (gh) on Reflection Coefficient (S11) of HFA at first stage.

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Figure 4 illustrates the effect of strip width (sw) on the reflection coefficient of the antenna design at first stage. The strip width has been varied from 0.5 mm to 2.0 mm and it is noticed from the figure 4 that the antenna at stage 1 has best response in terms of reflection coefficient when the width is equals to 1.5 mm. Figure 5 illustrates the progression of layers within the proposed HFA, spanning from the 1st stage to the 5th stage. Initially, in the 1st stage, a single layer of FR4 substrate measuring 28 × 60 × 1.6 mm3 is utilized. Moving to the 2nd stage, an additional layer of dimensions (28 × 50 × 1 mm3) is stacked atop the first layer, incorporating a downscaled (2/3) replica of the HFA employed in the 1st stage as a parasitic patch. Transitioning to the 3rd stage, a similar-sized FR4 substrate layer (28 × 50 × 1 mm3) is employed, albeit with a downscaled (1/3) version of the HFA, utilized as a parasitic patch, as depicted in figure 5. Subsequently, in the 4th and 5th stages, two layers of FR4 substrate measuring 28 × 50 × 1 mm3 each are added on top of the 3rd layer.

Figure 4. Refer to the following caption and surrounding text.

Figure 4. Effect of strip width (sw) on Reflection Coefficient (S11) of HFA at first stage.

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Figure 5. Refer to the following caption and surrounding text.

Figure 5. All layers of the proposed HFA at different stages.

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Figure 6 illustrates the S11, while figure 7 depicts the gain of the proposed antenna across all stages. It can be noticed from the figure 6 that there is a significant improvement in matching of the antenna as well as in the bandwidth of the antenna, and at the same time the gain of the antenna also shows a significant improvement at resonating bands. The enhancement in both S11 and gain parameters is evident from these visual representations. Notably, at stage 5, the antenna demonstrates optimal matching and operating bandwidth compared to all other stages.

Figure 6. Refer to the following caption and surrounding text.

Figure 6. S11 of Hybrid fractal antenna at all stages.

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Figure 7. Refer to the following caption and surrounding text.

Figure 7. Gain of Hybrid fractal antenna at all stages.

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The impact of utilizing parasitic HFA radiating elements in the 2nd and 3rd stages is evident in the S11 and gain plots, as well as in table 1. In the 2nd stage, enhancements in S11 and antenna bandwidth are observed. In the initial stage, the maximum bandwidth reaches 900 MHz, while in the 2nd stage, it expands to 1780 MHz, albeit with a reduction in the number of operating frequencies from four to three. Similarly, comparing the antenna's performance between the 2nd and 3rd stages reveals an increase in the number of operating frequencies from three to five. The maximum bandwidth achieved is 2940 MHz in the 3rd stage, compared to 1780 MHz in the 2nd stage. However, in the 3rd stage, the gain is negative at the first resonating frequency (2.65 GHz). Furthermore, upon comparing the 4th and 5th stages, noticeable improvements are observed across all parameters, including S11, bandwidth, and antenna gain.

Table 1. Performance parameter at different stages.

Stagefr (GHz)−S11(dB)Gain (dBi)Bandwidth (MHz)
1st2.8713.06−1.75260 (2.74–3.0)
 3.8827.692270 (3.76–4.03)
 5.1122.682.85280 (4.96–5.24)
 6.1521.072.7900 (5.61–6.51)
2nd2.9821.50.11370 (2.78–3.15)
 4.0216.22.58180 (3.95–4.13)
 5.3734.752.521780 (4.83–6.61)
3rd2.6514.37−0.5420 (2.55–2.97)
 4.4215.412940 (3.95–6.89)
 5.2828.13.3 
 5.8417.14.75 
 6.64214.25 
4th2.531.90.7460 (2.38–2.84)
 2.6931.2−0.33 
 3.9225.81.163030 (3.78–6.81)
 4.2228.30.95 
 5.0939.53.25 
 5.8515.63 
 6.4938.123.88 
5th 2.46 22.96 4.15 460 (2.34–2.8)
  3.87 25.34 0.9 1820 (3.73–5.55)
  4.54 23.35 2.9  
  5.02 42.44 3.32  
  5.35 23.94 4.97  
  6.42 31.88 3.73 1020 (5.7–6.72)

Specifically, at stage 5, the proposed antenna resonates at frequencies of 2.64, 3.87, 4.54, 5.02, 5.35, and 6.42 GHz, with corresponding operating bandwidths of 460, 1820, and 1020 MHz, respectively. Analysis of figure 7 reveals that the gain at the lower resonating frequency, as well as across all other frequencies, surpasses that of all other stages. Gain of the proposed design is highest at 5.35 GHz which is 4.97 dBi, the second highest gain is available at 2.46 GHz which is 4.15 dBi, similarly the proposed design has a good gain at all resonating frequency except at 3.87 GHz where it has a gain of only 0.9 dBi. Figure 8 Shows the radiation efficiency plot of the antenna. Radiation efficiency of the proposed antenna is lies between 65 to 83%. Figure 9 shows the XZ-Plane and YZ-Plane of 3D Polar plot of gain at all resonating frequencies. Furthermore, table 1 provides a comprehensive tabulation of results for all stages, further elucidating the improvement in performance of the antenna.

Figure 8. Refer to the following caption and surrounding text.

Figure 8. Radiation efficiency (%) of proposed HFA.

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Figure 9. Refer to the following caption and surrounding text.

Figure 9. 3D Polar plot of gain.

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3. Experimental results and discussion

Figure 10 shows the fabricated prototype of the proposed HFA antenna which is tested to validate the simulated results. The fabricated antenna is fed using a 50-ohm SMA female connector. Experimental validation of the simulated reflection coefficient has been done using Vector Network Analyzer and radiation characteristics are characterized in anechoic chamber. Figure 11 shows the simulated and measured S11 values for the proposed multilayer antenna design.

Figure 10. Refer to the following caption and surrounding text.

Figure 10. Fabricated hybrid fractal antenna.

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Figure 11. Refer to the following caption and surrounding text.

Figure 11. Simulated and measured S11 hybrid fractal antenna.

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From figure 11 it can be noticed that the measured value of S11 is in good agreement with the simulated. Figure 12 shows the 2D- plot of the radiation pattern in XZ-Plane and YZ-Plane at 2.64, 5.02, and 6.42 GHz. Radiation characteristics of the antenna have been measured in anechoic chamber in receiving mode, and obtained patterns are quite similar to simulated patterns which show the practicality of the proposed design. In XZ-Plane antenna radiates nearly equal energy in all directions at all frequencies. while in the YZ-Plane the radiation pattern at 2.64 GHz, and at 5.02 GHz are bidirectional, and at 6.42 GHz patterns are broadside. The following possible causes are identified for little disagreement in the experimental results: Due to soldering bumps, copper loss, misalignment of the SMA connector with the feed-line, fabrication tolerances (Like the adhesive used to glue all layers of fabricated prototype), limitations of the measurement facility, particularly regarding the antenna mounting and positioning system employed in the laboratory. and scattering from surrounding area are cause of frequency detuning in experimental results.

Figure 12. Refer to the following caption and surrounding text.

Figure 12. Simulated and measured radiation pattern of hybrid fractal antenna.

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In additions to the above-discussed performance parameters, the proposed multilayer multiband fractal antenna is compared with the existing antennas in table 2. The proposed antenna has a multiband performance, compact size with larger operating BW, fractional bandwidth, and good gain as compared to antenna proposed in literature. From table 2, it is very clear that the proposed antenna has compact size compared to [7, 14, 16], higher gain than [8, 10, 13, 16], and the proposed antenna provides highest bandwidth and fractional bandwidth compared to other antenna designs present in the table.

Table 2. Performance comparison of the proposed HFA with existing literature.

Referencesfr (GHz)Dimension in mm3 Gain (dBi)Bandwidth (MHz)/Fractional BandwidthSubstrate
[5]4.5377 × 70.11 × 1.6 = 8,637.5525Single Layer RO5880
 4.97 4.57 
[6]5.6552.92 × 55.56 × 1.2 = 3,528.282247.15135/2.4%Single Layer Arlon AD300C
[7]2.39260 × 55 × 8.3 = 27,3905.244.7/1.9%Two Layer FR4 separated with airgap
[8]3.6528 × 20 × 1.6 = 8962.5700/19.18%Single Layer FR4
[10]3.4536 × 35 × 1.6 = 20163.83160/4.64%Single Layer FR4
 5.9 0.576220/3.73% 
[12]2.4527.3 × 27.3 × 2.4 = 1788.7Max. 6.9180/7.35%Two Layer FR4
 3.25  350/10.77% 
 4.15  850/20.48% 
[13]2.4538 × 45 × 4.8 = 82083.81102/4.15%Three Layer FR4
[14]1.652 × 71 × 11.7 = 43,196.46.5Max. 600/15.79%Two Layers FR4 separated with airgap
 1.9    
 3.8    
[16]0.865100 × 100 × 8 = 800002.9213/1.5%Eight Layers FR4 and Roger RT Duroid 5880
[17]2.535 × 35 × 5.3 = 6,492.52.93 dBic150/6%Three Layers Rogers R04003 and FR4
 3.52 6.26 dBic200/5.68% 
Proposed 2.46 28 × 60 × 5.6 = 9408 4.15 460 (2.34–2.8)/18.7% Five Layer FR4
  3.87   0.9 1820 (3.73–5.55)/36.25%  
  4.54   2.9   
  5.02   3.32   
  5.35   4.97   
  6.42   3.73 1020 (5.7–6.72)/15.89%  

4. Conclusion

In this work, multiband multilayer fractal patch antenna is designed. Initial dimension and boundary are given according to the physical model. Proposed HFA resonates at 2.64 GHz (2.34–2.80 GHz), 3.87, 4.54, 5.02, 5.35 GHz (3.73–5.55 GHz), and 6.42 GHz (5.70–6.72 GHz) with a gain of 4.2, 0.9, 2.9, 3.3, 4.6, and 3.7 dBi respectively. The proposed HFA is useful for 5G bands such as: N7 band, N38 band, N40 band, N41 band, N47 band, N53 band, N79 band, N90 band and N93 band, and also useful for public safety band (4.9 GHz). Simulated and measured results of the design prototypes demonstrate the effectiveness of use of multilayer substrate and parasitic fractal patch in enhancement of the gain and bandwidth of the antenna.

Acknowledgments

The authors ate thankful to Poornima university, Jaipur and SLIET, Longowal for their support in this research.

Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files). The simulation and test data used to support the findings of this study are included within the manuscript.

Conflicts of interest

The authors declare that there are no conflicts of interest. There is no conflict of interest with the coauthors.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

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