AMC-Loaded Low-Profile Broadband Printed 2×2 Array With Gain and Isolation Enhancement Using Equivalent Circuit Model for Wireless Systems

A <inline-formula> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> array of printed antenna loaded by planar Giuseppe Peano artificial magnetic conductor (AMC) is presented for vehicular wireless systems. The four-element of the proposed array by utilizing diverse polarized directions of the printed dipoles with a <inline-formula> <tex-math notation="LaTeX">$5\times 5$ </tex-math></inline-formula> AMC reflector achieves a wideband antenna with enhanced properties. It shows the measured S-parameters of the wide bandwidths from 3.30 to 6.02 GHz with increased stable gains and efficiencies (more than 90%) for all elements. Moreover, the appropriate isolation of greater than 30 dB between the elements is achieved. The proposed design of the printed dipoles includes T-shaped microstrip dipoles and a feedline system as a T-shape to broaden the bandwidth in 4.7-6.02 GHz (<inline-formula> <tex-math notation="LaTeX">$\text{S}_{11}\le $ </tex-math></inline-formula> -10 dB). By utilizing the <inline-formula> <tex-math notation="LaTeX">$3\times 3$ </tex-math></inline-formula> Giuseppe Peano AMC reflector into the one element of the printed dipole, enhanced radiation efficiency and −10 dB measured bandwidth from 3.26 to 6.02 GHz (60%) is achieved for vehicular wireless systems. The proposed design with AMC with respect to the antenna without AMC indicates a reduced size of 70%, increased gain up to 8.4 dBi, and uni-directional radiation patterns. The new AMC unit cell is configured by the Giuseppe Peano design to resonate at 5.36 GHz in 4.27-6.34 GHz (39%). Finally, the suggested equivalent transmission line model of the <inline-formula> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> array with AMC surface is introduced with a suitable agreement between the results of reflection coefficients.


I. INTRODUCTION
By developing various daily issues, such as contamination, traffic jams, and accidents, vehicular communication has signified the extension of transport systems. It covers the general technology to provide vehicle-to-vehicle and vehicle-toinfrastructure systems. Wideband antennas have been taken into account for vehicular wireless networks by progressing the wireless communication systems [1]. Microstrip antennas utilize as a good selection to prepare the proper quality signals for vehicular wireless communications. These antennas show prominent features such as compactness, low-profile, and simple production [2], [3]. In such studies, miniaturized The associate editor coordinating the review of this manuscript and approving it for publication was Sandra Costanzo . broadband microstrip antennas can be used for intelligent transport systems (ITS) and wireless devices. In the last decades, ITSs can be affected various applications for traffic security and proficiency. Vehicular communication is capable to realize these applications as it activates the straight signal replacement among vehicles. Also, many studies have been done about the protocols, channel models, and applications of vehicular communications [4], [5], [6].
Due to high data rate transmission, confined area, and envelopment multipath surroundings, low-profile broadband microstrip antennas are chosen for vehicular mobile systems [7], [8], [9], [10], [11], [12]. In these studies, different methods are presented to broaden the bandwidth of microstrip antennas. Many works have used electromagnetic band gap (EBG) surfaces in wireless systems due to their considerable VOLUME 11, 2023 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ features [13]. In this regard, multiple low-profile antennas are used AMCs as various surfaces to improve the radiation properties [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. It should be noted that broadband microwave devices and printed antennas with AMCs suffer from a narrow AMC bandwidth [24]. Recently, low-profile microstrip antennas with broadband AMC surfaces are utilized with improved properties by developing a variety of printed circuit technologies [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. A reported antenna with AMC introduces a measured bandwidth of 2.2-2.72 GHz (18%) with 67.5 × 67.5×4.5 mm 3 and a high gain of 5 dBi [34]. This study presents the new 4-element array of printed antenna applying broadband 5 × 5 planar AMC for vehicular wireless systems. Firstly, a broadband printed antenna with microstrip T-shaped dipoles is presented. Two T-shaped dipoles with the T-shaped feedline widen the frequency band. In the next step, a broadband Giuseppe Peano AMC is presented to resonate at 5.36 GHz (4.27-6.34 GHz) for C-band applications. Then, the 3 × 3 Giuseppe Peano AMC reflector array is loaded into the printed antenna to achieve the -10 dB measured bandwidth in 3.26-6.05 GHz for WiMAX, WLAN and 5G vehicular base stations. The excellent compactness with increased gain and good impedance matching over the band is achieved. The four-element array with diverse polarized directions of dipoles is designed to attain high isolations between the elements from 30 dB to 67 dB. Finally, the proposed equivalent transmission line model is presented to forecast the reflection coefficient of the 2×2 array with AMC.

II. SUGGESTED PRINTED 2 × 2 ARRAY WITH AMC REFLECTOR
The three-dimensional (3D) view of the 2 × 2 array of the printed dipole antenna by utilizing an AMC surface is plotted in Fig. 1. It consists of the printed dipoles with diverse polarized directions to adjust the effect of the mutual coupling of the elements. The proposed design includes a 2 × 2 array with a 5 × 5 AMC surface (60 mm×60 mm). The separation length between the printed elements is selected d 12 = 14 mm,    Table 1. The rectangular microstrip patch with the primary width (W) and length (L) is presented with the following equations (1)-(4) [3]: where, ε eff , h, and L are effective permittivity coefficient, thickness, and additional length, respectively. For this purpose, the primary width and length of the printed dipole are chosen on basis of the aforementioned formulas (1)-(4) at the C-frequency band. In the present study, dielectric constant substrate, ε r = 2.55 is selected. Also, for the rectangular simple patch at 5.36 GHz, the basic width and the length are selected for height h 1 = 0.8 mm. To obtain the optimum sizes and arms' length, the parametric simulations are utilized in the high-frequency simulator for broadband response in C-band. The suggested design of printed dipoles is exhibited based on the T-shaped arms with various arms. As shown in Fig. 1 (c), when two T-shaped slots insert into the patch, an additional series inductance L and an additional capacitance C between the arms of the T-shaped patch are produced. Thus, the reactive couplings in the printed design lead to a broad bandwidth with two resonances. Fig. 2 illustrates the structure of a proposed Giuseppe Peano fractal AMC unit cell. This unit cell is composed of the fractal patch with the first iteration and inserted slot [17]. The optimized size of its line sections in various iterations is realized in the simulator. The relative permittivity and thickness of ε r = 4.4 and h = 3 mm, respectively, are chosen for an FR4 substrate. The dimension of the ground plane is selected as 12 × 12 mm 2 . The AMC frequency band is optimized by tuning the optimum electrical characteristic as h and ε r . The size of self-similar structures marked by D is determined by the below relation [17]: where h n is the ratio of the sub-portion length and the starter length and k n is the number of sub-portions with the length ratio h n . The physical and electrical characteristics of the AMC take into account for acquiring the bandwidth enhancement. The physical characteristic of the wide bandwidth is concluded from the reactive couplings of slots, arms, and parasitic elements. The final AMC bandwidth is achieved by considering the proper electrical characteristic as h and ε r . Finally, the little variation of reflection properties can be regarded with acceptable stability for a broadband response. On the other hand, various sections and embedded slots into the planar AMC conclude to a further bandwidth.
The EBG surface like a mushroom-type is configured by a via-loaded patch and it is modelled by an equivalent parallel LC resonator with a resonance of f r = 1/(2π √ LC) [18]. The current route between the patch surface and ground plane concludes the inductance of L. Moreover, the gap distance between two near patches results in the capacitance of C. The values of L and C parameters and the frequency band according to L, C, and intrinsic impedance are given by the following relations [18]: where, ε 0 , and µ 0 are the permittivity and permeability of free space, respectively. Also, η is equal to 120π.
The proposed AMC is simulated by applying the finite element method (FEM) for periodic surfaces to obtain the AMC reflection properties. In Fig. 2 (b), a periodic boundary condition as an infinite model is shown. This model results in infinite conditions to achieve the reflection properties for various incident waves.
As can be seen in Fig. 3, the diagram of the design procedure is illustrated. It presents the detailed model to achieve an optimum proposed printed antenna design with the improved features. The new broadband AMC design in 4.27-6.34 GHz (39%) for C-band operation is proposed, in the first step. In the next step, broadband printed dipoles with the proposed feeding system in C-band (4.7-6.02 GHz) for WLAN/WiMAX applications are designed. Finally, a lowprofile antenna with T-shaped diploes by using the AMC surface for wideband wireless systems is realized with enhanced properties.
The suggested equivalent circuit model for the 2 × 2 array with AMC reflector is presented in Fig. 4. The resonant circuit L 1 C 1 for conventional microstrip patch antenna is considered [12], [14]: where y 0 , L e , f r , h, and ε e are the interval of feed location from the edge, the effective length of the patch, the operating, and substrate specifications, respectively. The printed dipole antenna as the element 1 with folded T-shaped dipoles is modelled by an L 1 C 1 resonator. The proposed model for the symmetric design of the printed antenna is considered by using the impedance model for radiating elements of broadband antennas, according to Fig. 4. The other elements 2, 3, and 4 of 2 × 2 array as L 2 C 2 , L 3 C 3 , and L 4 C 4 resonators are coupled with C c to the element 1 (see Fig. 4). The air gap height between the array and AMC is modelled by the transmission line The real section is calculated to obtain simply the values of RLC parameters: The simulated results of the 2 × 2 array by HFSS are considered into the function of input impedance attained from the equivalent model which includes several parameters in the formula (17). A least-square optimization and curve-fitting technique are utilized to optimize all of the components R k , L k , and C k .

III. EXPERIMENTAL AND SIMULATION RESULTS WITH COMPREHENSIVE STUDY
The Floquet theory on basis of FEM is used in Ansoft HFSS to attain the simulation results of the AMC design. Fig. 5 illustrates the reflection phase of the AMC by the normal TE/TM incident waves (θ = 0 • ). The simulation result for TE/TM waves includes 4.27-6.34 GHz (39%) with a resonance of 5.36 GHz. Also, it shows the phases of the reflection response of the AMC for oblique incident waves from θ = 0 • to 60 • in two polarization angles of ϕ = 0 • and 90 • . Fig. 6 demonstrates the effect of a square slot on the AMC patch for a normal incident wave (θ = 0 • ). Fig. 6 compares the proposed AMC with and without the first iteration of the suggested fractal unit cell. It shows that the proposed AMC without the first iteration of Giuseppe Peano fractal and square slots cover 4.47-6.31 GHz and 5.04-6.50 GHz, respectively.  The measurement and simulation reflection coefficient S 11 of the printed design with AMC and without the reflector are shown in Fig. 7. The antenna without VOLUME 11, 2023 AMC covers the calculated range of 4.7-6.02 GHz (24.6%) in C-band for S 11 < -10 dB. According to Fig. 7, the printed antenna with the Giuseppe Peano AMC achieves the measured bandwidth (S 11 ≤ -10 dB) of 60% in 3.26-6.05 GHz for WLAN/WiMAX, and 5G communications. The simulated reflection coefficient S 11 of the printed design with AMC covers 3.29-6.02 GHz. It shows a similar impedance bandwidth between the measurement and simulation results.
Moreover, the proposed antenna without the first iteration of fractal AMC (simple square AMC reflector with a unit cell size of 11 × 11 mm 2 ) covers 4.27-5.85 GHz, according to Fig. 7. It is comprehended that using the Giuseppe Peano AMC reflector leads to a broader bandwidth with the further compact size.
The influence of the height of the air distance, h 2 , between the array and the AMC surface is illustrated in Fig. 8. The optimum S 11 parameter is obtained with a wide bandwidth and proper impedance matching for h 2 = 3 mm. By increasing in the h 2 , the bandwidth enlarges until h 2 closes to the optimum value.
The overall sizes (width× length× height) of the printed antenna without AMC are 0.56λ L , 0.56λ L , and 0.012λ L , respectively. However, the total sizes of the printed antenna with the AMC are 0.39λ L , 0.39λ L , and 0.073λ L , respectively. Therefore, a reduced size of 69.36% is obtained with respect to the antenna without AMC.
The comparative performance of the antenna design is illustrated in Table 2. It indicates the considerable outputs including significantly reduced size, extended bandwidth, and improved gain. In this regard, the design with respect to the other reports such as [8], [9], [13], [24], [28], [29], [30], [32], [33], [34], [35], [36], [37], [38], and [39] demonstrates a larger bandwidth of 60% and an improved gain of 8.4 dBi with good impedance matching until -50 dB. For example, a suggested array in [24] with the size of 75 × 75 × 12.7 mm 3 covers a wide frequency band of 3-4.1 GHz and a high gain of 7.1 dBi. Also, a presented array with   EBG in [9] operates in 8-9.25 GHz (14.5%) with 96 × 96 × 1.6 mm 3 and a gain of 7 dBi. In comparison with the reported works of [29], [30], and [39], the present study shows a new design of printed T-shaped dipoles and planar AMC for lower frequencies of C-band which can extensively be used for wideband vehicular communications and 5G vehicular base stations.
The S-parameters of the 2 × 2 array with the AMC are illustrated in Fig. 9 for simulation and measurement results. The measured result of S 11 for element 1 in 3.41-6.02 GHz is achieved to cover WLAN (5.2/5.8 GHz), WiMAX (5.5 GHz), and 5G systems. Moreover, the reflection coefficients of S 22 , VOLUME 11, 2023     In Fig. 12, the measured gains of four elements of 1, 2, 3, and 4 are shown. The maximum gains of the elements of 1, 2, 3, and 4 are more than 8.7 dBi over the operating frequency band. The maximum gain of the antenna without AMC structure over the frequency band is almost 6 dBi, according to Fig. 13. Therefore, the average gain of the antenna is improved over the operational band up to greater than 4 dBi. As seen in Fig. 14 (a), similar results of four elements for the measured radiation efficiencies are achieved. Also, Fig. 14 (b) shows the radiation efficiencies of four elements of the proposed array without AMC. Hence, it is observed that by loading the AMC structure into the printed array the efficiency of the proposed antenna is improved over the operating frequency band. The measurement results are very similar to the corresponding simulation results. The simulation efficiencies of whole the frequency band are 91%-95% and the corresponding measurement results are 89%-94%.
The measured and simulated radiation patterns in the XZand YZ-planes for the 2 × 2 array with AMC are plotted in Fig. 15 at 3.8, 5.2, and 5.8 GHz. It is seen that proper accordance with the simulation and measurement results is achieved. It is comprehended that the radiation patterns for the four elements illustrate a little difference owing to polarized directions.
The simulation radiation patterns in the XZ-and YZ-planes for the antenna without AMC (element 1) are shown in Fig. 16 at 5.2 GHz. It is obvious that by adding the AMC reflector to the printed antenna the backside radiation is reduced and the directive radiation patterns achieve. The photos of the created antenna by loading AMC are drawn in Fig. 17.
The measurement setup for far-field monitoring and the measurement of the radiation properties of the suggested  antenna in the anechoic chamber is demonstrated in Fig. 18. A standard horn antenna with tunable calibration is utilized VOLUME 11, 2023 as a transmit (TX) antenna and the given case is measured as a receiving (RX) antenna. This horn antenna is located at the far-field distance 2D 2 /λ (D is the total dimension) from the antenna (RX). The amplifiers are applied to provide stable power reception. To test and measure the radiation intensity in diverse directions, the antenna is rotated by a mechanical retainer. The network analyzer of Agilent 8720C is utilized to measure the S-parameters. Furthermore, the gain of the antenna can be measured according to dBi. For this purpose, the gain of the reference horn antenna (G ref ) is tested. In continuation, the gain of the suggested antenna (G Relative ) with respect to the reference antenna is measured. Finally, the sum of G ref and G Relative for all frequency steps is calculated to achieve the gain of the antenna.
The simulated and measured results such as S-parameters, radiation patterns, gains, and efficiencies have a good agreement with little difference. This difference can be concluded from the fabrication and measurement tolerances due to the used equipment.

IV. CONCLUSION
The 4-element printed array is introduced by utilizing the 5 × 5 Giuseppe Peano AMC reflector to include the band from 3.3 to 6.02 GHz. The various polarized directions of four elements are considered to obtain larger isolations (more than 30 dB) between the array elements and high stable gains of more than 8.7 dBi and good efficiencies (more than 90%) for WLAN and WiMAX base stations. The proposed design of the Giuseppe Peano fractal AMC unit cell is presented for providing a wideband response of 4.27-6.34 GHz (39%) in C-band. This AMC design introduces the proper stability over the AMC frequency band. The proposed antenna with T-shaped arms by using the 3 × 3 Giuseppe Peano AMC reflector exhibits the compact broadband antenna in 3.26-6.05 GHz (60%) for wireless communications systems. Also, good impedance matching until −50 dB, acceptable size reduction, and wider bandwidth is achieved. Moreover, the uni-directional radiation patterns and a high gain of up to 8.4 dBi are achieved. Finally, the proposed equivalent transmission line model is presented to forecast the reflection coefficient of the 2 × 2 array with AMC surface.