A Miniaturized Broadband and High Gain Planar Vivaldi Antenna for Future Wireless Communication Applications

School of Electronic and Optical Engineering, Nanjing University of Science and Technology (NJUST), Nanjing 210094, China Department of Electronic and Telecommunication Engineering, Mehran University of Engineering and Technology (MUET), Jamshoro, Sindh 76062, Pakistan College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing 211106, China Department of Computer Science, Huanggang Normal University, Huangzhou, Hubei 438000, China

curvatures, and constant width slots [11]. e main concerns for active researchers related to the antenna designing field are mainly the antenna's large electrical and physical size, narrow impedance BW, moderate gain, and low radiation efficiency. Hence, the prime focus of the antenna designers is to refine the main aspects of the antenna structures, such as wide impedance BW, compact size, gain, radiation efficiency performance, and stable far-field radiation features [12].
In the last decade, different structures of the compact planar antennas were investigated to enhance the antenna impedance BW, gain performance, and radiation characteristics [13]. However, very few antenna designs have a compact size with a tapered profile and can attain the antenna's optimal key characteristics. In the literature, various antenna geometries, such as slits, tapered slot radiator, and flared transition with modified cavity stubs [14][15][16][17] have been reported. Also, several designs based on different slot shapes [18][19][20] were suggested to achieve the required gain, acceptable BW, radiation efficiency, and perfect impedance matching. e reported antennas consist of intricate structures and exhibited a satisfactory performance in terms of BW, peak gain, and radiation efficiency. e coplanar waveguide (CPW) fed bowtie slot antenna with large size achieved triple-band response reported in [21]. Awl et al. proposed microstrip antenna for C, X, and Ku band applications with an electrical size of 0.687λ 0 × 0.492λ 0 × 0.1038λ 0 at the 9.8 GHz [22]. e presented antenna design comprised of a semitriangular patch shape with a CPW feedline and attained 107% fractional BW with 5.3 dBi realized gain performance. Besides, the compact size linear tapered slot antenna design with tilt grooves approach is demonstrated in [23]. e designed antenna achieved an impedance BW of 15.5 GHz with broad beam characteristics. A tapered slot antenna (TSA) employing a folded form technique achieved enhanced impedance BW and gain [24]. e proposed antenna design was complex and attained acceptable gain and broad impedance BW with a large electrical size of 4.0λ 0 × 2.4λ 0 × 0.032λ 0 .
Another work on TSA was proposed for imaging applications with the size of 2.09λ 0 × 1.566λ 0 × 0.034λ 0 [25]. e antenna achieved 77% BW and has a larger size. e authors proposed antenna array and obtained ∼55% broad BW, and excellent peak realized gain with antenna size of 1.8λ 0 × 1.38λ 0 × 0.024λ 0 [26]. e substrate integrated waveguide (SIW) based new shaped antenna with broadband impedance BW is reported in [27]. However, the developed antennas reported in the literature were larger in size and employed a high fabrication process and complicated assembly. Another work on high gain bowtie slot antenna achieved the multiple frequency bands [28]. e proposed structure was simple and printed on FR4 substrate. e authors presented wideband loaded linear TSA with broad beam width [29]. Besides, the Chang team designed an antenna on FR4 substrate with a larger dimension of 1.465λ 0 × 0.847λ 0 × 0.02λ 0 and covered 82.2% relative impedance BW. e reported antenna comprises three types of inverted ℿ (omega-shaped) loaded slots and has a complex structure with large size. Krishna et al. [30] developed the antenna for S, C, X, and Ku band applications with a compact size of 2.0λ 0 × 2.5λ 0 × 0.067λ 0 and obtained the impedance BW of 15 GHz with a peak realized gain of 7.0 dBi. However, the work presented in [30] occupied large antenna size with dual exponentially slotted, resulting in computational complexity. A compact wideband CPW-fed taper-shaped monopole antenna with an L-slot has been investigated in [31]. e antenna was realized on FR4 substrate with compact dimensions of 0.395λ 0 × 0.355λ 0 × 0.0315λ 0 and obtained the narrow BW performance ranging from 3.8 to 8 GHz with a modest gain of 3.4 dBi. Another high gain CPW-fed antenna is proposed in [32]. e authors achieved good performance with a large antenna size.
In this paper, a new miniaturized structure of PVA is designed, simulated, and fabricated. e proposed tapered structure is erected with alternate grooves based technique that obtained the wide impedance BW, high gain, excellent radiation efficiency, and stable far-field radiation characteristics. e proposed antenna has the miniaturized size of 0.92λ 0 × 0.64λ 0 printed on a Teflon ™ laminate with a thickness of 0.03λ 0 . A comparison with the recent state-ofthe-art work is provided. e proposed model achieves the relative BW of 117.25% with |S 11 | better than 10 dB, the peak realized gain of 10.9 dBi, the excellent radiation efficiency of 95%, good performance of group delay <2 ns, stable far-field radiation pattern in E-and H-planes, and consistent strong current distribution at multiple resonances.
e main contribution of this paper is as follows: (i) e designed PVA is compact in size, overcomes the simulation and geometrical complexity, and achieves optimal performance (ii) A robust grooves based method is proposed which reduces the design and fabrication complexity of the antenna (iii) e antenna achieves an excellent relative BW of 117.25%, a high realized gain of 10.9 dBi, and excellent radiation efficiency of 95% (iv) e proposed antenna results are validated in the time domain, that is, group delay, which shows the excellent performance <2 ns across the operable frequency span (v) e proposed antenna exhibits the optimal performance features, and their comparison is provided with recently reported literature (comparison presented in Table 1), which shows the clear advantages e rest of the article is organized as follows: Section 2 presents the suggested design material, antenna arrangement, and proposed antenna model strategy. e impact of different variable and sketches of surface current distribution is also provided in this section. Section 3 offers the analysis of the simulated and measured results of the proposed antenna. e peak realized gain, radiation efficiency, group delay, and far-field radiation pattern along the standard planes are analyzed and discussed in this section. Finally, the conclusion of the study is explained in Section 4.

The Proposed Antenna Arrangement and Geometry
In this section, we discuss the designed antenna layout and simulation results analysis. e proposed prototype is engraved on Teflon ™ laminate with relative permittivity of 2.1 and loss tangent of 0.001. It consists of a top metal plate etched with tapered lines and the radial cavity stub. Moreover, a strip line feeding network with a fan-made stub is used to connect the 50 Ω SubMiniature version A (SMA) connector. e feedline is placed on the bottom of the substrate. A high-frequency structure simulator (HFSS) V-17.1 is used to design the proposed structure. e compact antenna model aims to produce a broad impedance BW, high gain and radiation efficiency, stable far-field patterns, and strong current distribution at multiple resonances. Figure 1 shows the three-dimensional (3D) view of the designed and simulated antenna model, consisting of a dielectric substrate, metal plate, and strip line feedline framework.
e metal plate is etched with tapered lines, slots, and the radial cavity stub, where the strip line feed network is connected with a balun cavity stub placed on bottom of the substrate. e opening tapered profile depends on aperture H, and its width must be greater than half of the free-space wavelength. A slot line is engraved on the metal ground plate with a radius (r 1 ) and a rectangle slot (L 1SL ) tapered profile depicted in Figure 2. e parallel rectangle slot line segment is used to perform the coupling. Besides, the gradual exponential profile radiates the free space in the steady opening part from the inevitable electromagnetic waves to the tapered aperture (H), which primarily marks the transmission of electromagnetic waves. e gradual exponential profile radiates the free space in the steady opening part from the inevitable electromagnetic wave to the tapered aperture (H).
In the metal plate, two grooves are introduced with 0.3 mm width at the opening profile affecting the lower operating band ranging from 6 to 7 GHz. However, the truncated line is adopted at the 100 Ω transmission line to achieve the load terminal matching equal to the ground terminal. e linear expression of the exponential tapered profile curve is determined by where Two conical points can calculate the linearly exponential tapered profile P 1 (x 1 , y 1 ) and P 2 (x 2 , y 2 ) and R is specified  Figure 1: A 3D view of the proposed antenna structure. Figure 2: e 3D-geometrical arrangement of the proposed planar Vivaldi antenna. by the opening rate of the exponential factor. e parameters c 1 and c 2 are determined by the constants of coordinates of the opening rate. Besides, the slit line transition is indicated by the (W 1SL ) and (W 2GR ) and strip line transition is stated with W 3FL , W 4FL , and W 5FL . Moreover, the taper line T SL is determined as (x 2 − x 1 ) and the height of the aperture section "H" is 2(y 2 − y 1 ) + W 1SL . When the opening rate R is approaching zero, the exponential taper results s 0 � (y 2 − y 1 /x 2 − x 1 ) provide the tapered slope. e taper slope "s" changes continuously from s 1 to s 2 for the exponential taper defined by equation (1), where s 1 and s 2 are the taper slopes at x � x 1 and x � x 2 and s 1 < s < s 2 for R > 0. e angle of the taper flare is characterized by α � tan −1 s that parameters are interrelated to W 1SL , T SL , H, and opening rate R.
e EM waves uncouple from the tapered profile, and the structure radiates into free space from the laminate end. For the TSA to radiate appropriately, it has to behave as a surface-wave antenna. To achieve this, the effective dielectric thickness t eff � ( �� ε r √ − 1) t, where t shows the thickness parameter of actual laminate must meet the following requirements [33]: where λ 0 is the free space wavelength at the center frequency. e thinner the dielectric substrate, the wider the width while the beam width will break if the substrate is thicker. Furthermore, in many wireless applications, alternate fed network performance requires a narrower BW [34]. As a result, different feed networks such as microstrip line, circular cavity stub, and strip line exist. In the proposed design, the strip line feed system is excited through a 50 Ω SMA connector containing the balun stub transformer section, as illustrated in Figure 3. e proposed antenna structure provides a broad impedance BW of 6-23 GHz with good radiation characteristics.
Geometrically optimized values of the proposed antenna are illustrated in Table 2.

Variation in Slot Line Width (W 1SL ) and Groove Width (W 2GR ).
is section explains the impedance matching of the developed antenna model. e return loss |S 11 | plot with various values is illustrated in Figures 4(a) and 4(b). It is observed that the simulated result resonances obtained inclusive impedance BW performance. e values range from 0.5 mm to 0.8 mm; the width of the slot line (W 1SL ) transition is varied. e best result for impedance matching is obtained at the value of 0.7 mm, as shown in Figure 4(a). e combination of radial cavity stub and tapered profile is used to achieve the antenna parameter accuracy. By applying the variation from 0.1 mm to 0.4 mm of the groove width (W 2GR ), the optimum results have been obtained at the values of 0.3 mm, as illustrated in Figure 4(b). Figure 5 shows the surface current distribution of the proposed antenna. It can be analyzed that the strong current density appears at the higher frequency of the operable BW. Figure 5(a) shows the surface current intensity at the first resonance of 6.6 GHz. e feed line is excited by 50 Ω SMA connector at the bottom of the substrate laminate and transited due to radial cavity and the taper profile as can be seen from Figure 5(a). Moreover, the second resonance point indicated at 12.3 GHz shows the high current intensity to the slot line and the grooves to the taper lines as illustrated in Figure 5(b). us, the current intensity is very high and is concentrated at the high frequency of 20.  11 |. S-parameters, peak realized gain, and radiation characteristics are the critical antenna output indexes. For an optimal antenna design and its high efficiency, these interdependent metrics need to be balanced. S-parameter of the fabricated structure was measured with a calibrated Agilent vector network analyzer N5244A. e simulated and measured results of return loss |S 11 | are illustrated in Figure 6. e simulated results show multiple resonances, indicating that the proposed antenna has achieved comprehensive impedance BW performance. Although with a slight change in frequency, the simulated results (black and red curve) have several resonances observed at 6.6 GHz, 12.3 GHz, 20.2 GHz, and 22.3 GHz.

Return Loss |S
Moreover, a small variation in calculated and simulated outcomes can be seen in Figure 6. ese inaccuracies are primarily due to assembly error and experimental  International Journal of Antennas and Propagation configuration tolerances. Besides, the antenna fabricated model was fed by a practical SMA connector made of metal (causes losses due to incorrect soldering and parasitic losses in terms of capacitance and inductance). erefore, an acceptable difference between the tested and the simulation is observed. One can control this by using efficient substrates, feeding connectors of high precision, and good quality cables to monitor performance accuracy. is slight error in the calculated results often ensures that the fabricated prototype is realized. e fabricated prototype connected with the SMA connector is shown in Figures 7(a) and 7(b) which is measured in the anechoic chamber. Furthermore, the measured and simulated return losses across operable frequency are depicted in Figure 6. It may have been observed to cover the wide BW from 6 to 23 GHz, that is, (∼117.25%) associated with future wireless communication applications.

Peak Realized Gain, Radiation Efficiency, and Group Delay.
is section covers the simulated and measured result analysis of the radiation efficiency, peak realized gain, and group delay. e antenna gain measures how well an antenna radiates energy in a specific direction. In contrast, radiation efficiency measures how well an antenna uses received power to emit energy in all directions. It can be observed from Figure 8 that the proposed antenna achieved the peak realized gain at its maximum value. However, due to conductor losses and reduced size due to putting a half-wave of wire into a smaller volume, the gain goes below zero for that prescribed frequency range. e interfering radiation caused by the matching network at these particular frequencies could be the cause of this gain reduction. Figure 8 shows the simulated and tested comparison of the gain and radiation efficiency of the proposed antenna. e radiation efficiency of the antenna is defined as the ratio of accepted power and radiated power. e following equation is used to calculate the measured efficiency: where η meas and G meas signify the fabricated prototype measured efficiency and gain, respectively, and D sim denotes the designed prototype simulated directivity. e excellent simulated efficiency of 95% and tested 87% is realized at the higher frequency at the operable BW. Moreover, the proposed antenna shows a gain of 3.41 dBi at the first resonance of 6.6 GHz, the noted gain of 3.7 dBi at 12.3 GHz, the high gain of 9.35 dBi at 20.2 GHz of third resonance, and the final resonance has been obtained at 22.3 GHz with an excellent gain of 10.9 dBi. us, the proposed antenna received the realized gain at its peak value at multiple resonances for future wireless communication applications. Figure 9 depicts the group delay (ns) performance parameter in the context of a time-domain analysis. It is analyzed that the proposed antenna exhibited a constant group delay of <2 ns across the operable frequency range, confirming that the antenna has a broadband capability.

Radiation Characteristics (E and H) Plane.
e twodimensional (2D) pattern in the E-and H-planes of the proposed antenna was determined inside the anechoic    International Journal of Antennas and Propagation Table 1 provides a comparison of the antenna main features with the existing state-of-the-art work. e excellent BW and moderate gain antennas for C, X, and Ku band applications were studied in [22,30] that employed large space. e SIW based tapered slot antenna was investigated in [27], which occupied a large size. e presented antenna provided ample dimensions with a prolonged fabrication process and complicated assembling. Furthermore, a tapered slot array antenna for underwater communications was investigated in [26]. e presented antenna covered the microwave band of 4.0 GHz-8.0 GHz with a large physical size. In [29], a wideband loaded tapered antenna with a broad beamwidth and improved FBR was investigated. e technique used a wideband loaded three types of ℿ slots technique with plenty of space. e reported works were time-consuming and complicated. In this research, the groove technique is used, and broad BW is achieved. Due to the trade-off between the antenna key features presented and results published in the literature, it is determined that the performance analysis results would not be better than the obtained results of the antenna presented. It is concluded that the antenna's approach can produce wide BW, high gain, stable far-field patterns, and strong current flow across the tapered profile.

Conclusion
In this paper, miniaturized broadband and high gain PVA is presented.
e proposed structure comprises Teflon ™ dielectric substrate, taper section, and strip line feedline with cavity stub. e feedline section is soldered with a standard 50 Ω SMA connector. e single tilt groove was engraved on the top of the taper profile, which improved the proposed structure impedance BW performance. e wideband antenna possessed the compact dimension of 0.92λ 0 × 0.64λ 0 × 0.03λ 0 . e proposed antenna has been fabricated and tested in a real-time environment. e antenna exhibited a good relative impedance BW of 117.25%, and the peak realized gain of ∼10.9 dBi at higher resonances. Moreover, the antenna structure exhibited excellent radiation efficiency of 95% and better performance of group delay (<2 ns) in the time domain. Stable radiation pattern performance and vigorous current intensity of the antenna were realized at multiple resonances. Finally, the proposed antenna key characteristics have  International Journal of Antennas and Propagation been compared with the recent state-of-the-art work. e designed antenna offers versatile advantages and exhibited optimal performance features making it a suitable candidate for future wireless communication applications. Besides, the proposed PVA may be further extended to wideband and high gain antenna array topology with an efficient Wilkinson power divider approach.
Data Availability e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this manuscript.