A Triple-Band Microstrip Antenna with a Monopole Impedance Converter for WLAN and 5G Applications

In this study, a triple-band microstrip antenna with compact size and simple structure is proposed for WLAN and 5G applications. The radiating element consists of a circular patch, a Y-shaped patch, and a monopole impedance converter. In order to obtain the desired operating bands, the monopole impedance converter is inserted between the circular patch and Y-shaped patch. The proposed antenna can work in the frequency range of 2.38 ∼ 2.53 GHz, 3.29 ∼ 4.11GHz, and 4.72 ∼ 5.01 GHz, with the corresponding peak gains of 4.09 dBi (2.4GHz), 2.95 dBi (3.5GHz), and 4.01 dBi (4.9GHz), respectively. The measures’ results are in approximate agreement with the simulated values, which shows that the proposed compact antenna can oﬀer omnidirectional radiation, appropriate gains, and suﬃcient bandwidths.


Introduction
In recent years, enhanced vehicle-to-everything, Internet of things, etc., need better data ability with high speed transmission, rapid response, and stable reliability [1]. To meet the above demands, the network technology is being shifted towards the fifth generation (5G), which aims to provide ultra-fast peak data rate, small mobility interruption time, and high transmission reliability [2]. As a terminal of global mobile data traffic, various antennas have been proposed and designed for 5G and other applications. Of all the antenna structures, microstrip antenna has some significant advantages and characteristics such as low profile, easy fabrication, mechanical stability, better compatibility, and flexible scalability.
In order to satisfy the 5G and other applications, multiple-input-multiple-output (MIMO) can highly improve the data rate, capacity, and link reliability [3]. By incorporating multiantenna systems at the transmitter as well as the receiver, m × n MIMO 5G antenna array can achieve high data throughput with good isolation and high efficiency [4][5][6]. In order to satisfy the low isolation degree requirement, a two-port circularly polarized antenna is designed with a rectangle ring ground plane and two T-shaped radiation patches [7]. In [8], a dual-polarization microstrip 5G MIMO antenna array is proposed with eight microstrip antenna elements arranged on the back cover of a mobile phone. Feeding networks and shorted patches' structure are proposed to realize the broadband dual-polarized filtering antenna [9]. Metamaterial structures are also used to realize the circularly polarized antenna with miniaturized size and high radiation efficiency [10]. A beam-switchable antenna is proposed with a crossover including substrate integrated waveguide and dielectric slab [11]. In addition, the fiberreinforced plastic materials [12], the solar cells [13], airy beam at microwave frequency [14], and high-order mode in circular patch [15] are also introduced to improve the performance of the antennas.
Due to the complexity of the application environment, the pure 5G antenna cannot satisfy the demands of different users. us, it is required to integrate 5G frequency band with other spectrum such as WLAN, LTE, WiMAX, and GPS application. e multiband slotted antenna is formed by a T-shaped feed patch and a rectangular slot on top side of the substrate, which covers the GPS/WiMAX/WLAN bands [16]. A triple-band antenna consisting of a Franklin monopole strip and a rectangular patch is proposed with the operating bands at WLAN, WiMAX, and 5G, respectively [17]. In [18], a dual-band LTE-R and 5G antenna is realized by stacked configuration consisting of a double dielectric substrate, a circular patch, and five elliptical patches. By integrating a stepped patch and a ground plane [19], a lowprofile multislot patch antenna is presented for LTE and 5G applications. Usually, the above multiband slot antennas have a larger size to ensure adequate current path on the radiating plane or ground plane. In order to facilitate the integration into the back cover of the handheld devices, a multiband antenna with more compact size and simple structure is desirable for WLAN and 5G application.
In this study, a compact triple-band microstrip antenna is proposed and designed for WLAN and 5G applications. e proposed antenna consists of a circular patch, a Y-shaped patch, and a monopole impedance converter. e realization principle is based on the half-wavelength (quarter-wavelength) resonance in the circular patch and the Y-shaped patch structure. e proposed antenna can generate the operating bands of 2.38∼2.53 GHz, 3.29∼4.11 GHz, and 4.72∼5.01 GHz, respectively. A prototype of the antenna is manufactured and measured, and the simulation and measurement results confirm that the triple-band antenna has enough bandwidth to cover the desired WLAN and 5G applications.

Antenna Design Process and Parametric Analysis
Figures 1(a)∼1(c) illustrate the schematic views of the proposed antenna including the front view, back view, and the radiating element. e radiating element is printed on top of the substrate, consisting of a circular patch, a Y-shapelike strip with two fan-shaped patches and a monopole impedance converter. e feed line with 50-ohm characteristics impedance is etched on the FR4 substrate, and the rectangle ground plane is printed on the backside of the dielectric substrate. In addition, the relative permittivity of the FR4 substrate is set as ε r � 4.4 with the dielectric loss tangent of δ � 0.02. e proposed antenna is analyzed and optimized using the HFSS Microwave Studio simulator. e detailed parameters of the proposed antenna are listed in Table 1. Figure 2 displays the design evolution of the proposed antenna, and the corresponding simulated reflection coefficients (S 11 ) are shown in Figure 3. From Figure 2(a), it can be seen that antenna-1 is a simple monopole antenna composed of a circular ring and a rectangular ground plane. As demonstrated in Figure 3, there is only one operating band between 2.40∼2.50 GHz centered at 2.45 GHz. In our design evaluation section from antenna-1 to antenna-2, we repeatedly test different kinds of radiating element to obtain another two resonant frequencies. After consulting a lot of relevant references, the Y-shape-like strip with two fan-shaped patch is selected inside the circular patch with a more compact scale. By introducing a Y-shape-like strip with two fan-shaped patches into the circular ring (antenna-2), the antenna can excite the other two operating bands, which can be used in 5G scenes. In order to obtain more compact structure, the Y-shaped monopole patch can be easily bested within the circular monopole patch and the impedance converter, which can be set as the unit cell of the MIMO antennas and easy to integrate in the back cover of the handheld devices. In order to optimize the first and third operating bands to better satisfy the requirements working frequencies, a monopole impedance converter is inserted between the Y-shaped patch and the circumscribed circle patch. It is obviously observed from Figure 3 that the operating bands of antenna-3 are 2.38∼2.53 GHz, 3.29∼4.11 GHz, and 4.72∼5.01 GHz with the center frequencies of 2.4 GHz, 3.5 GHz, and 4.9 GHz, respectively, which satisfy the bandwidth requirements of WLAN and 5G applications.
In our proposed antenna, the realization principle is based on the half-wavelength (quarter-wavelength) resonance in the monopole radiating element with a circular patch or a Y-shaped patch structure. From Figure 3, it can be seen that the antenna-1 is a simple monopole antenna composed of a circular patch, which is fed by a 50 Ω microstrip line. e length of half of the circular patch    International Journal of Antennas and Propagation (π * r − w 2 ) is optimized to satisfy half-wavelength of the first resonance frequency at 2.45 GHz. e corresponding resonant frequency of antenna-1 is given by where ε eff ≈ ε r + 0.5 and c is the speed of light. And then, antenna-2 is obtained by introducing a Y-shape-like strip with two fan-shaped patch into the circular ring. By carefully optimizing the structure parameters of the Y-shape-like strip, Antenna-2 can generate another two resonant frequencies working in 5G scenes without increasing the size of the antenna. From Figure 3, it can be found that antenna-2 has two operating bands with the resonant frequencies in the vicinity of 3.5 and 4.9 GHz. e effective length of Y-shape-like strip with two fanshaped patch should be half (a quarter) of the guided wavelength, which can be approximately calculated by (2) In order to better analyze the operating principle of the proposed antenna, the variations of different dimensional parameters are investigated. Figure 4(a) demonstrates the variation of the simulated reflection coefficients with L g It can be observed from the plot that the main operating frequency band of 5G is strongly dependent on the value of L g . As the value of L g decreases, the third resonant mode moves towards the second mode, and the bandwidth of the first resonant bandwidth becomes narrower than before. Figure 4(b) presents the simulated reflection coefficients for different values of R 2 . As the figure describes, the first and second resonant mode are almost unchanged, and the third frequency demonstrates redshift obviously. From the above analysis, we can see that the desired third band for 4.9 GHz can be tuned by adjusting R 2 . As described in Figure 4(c), by increasing the value of ht, the first and second operating bands have an obvious redshift while the third band changed slightly. us, the ranges of these two operating bands can be effectively adjusted by changing the value of ht. Figure 4(d) displays the variation of the reflection coefficients for different values of R 6 . It can be observed that, with the increase of R 6 , the first and third bands show redshift while the middle band keeps unchanged.  Figure 5(c) shows its actual environment for the far-field measurements. e overall size of the anechoic chamber is 8 m (length) × 6 m (width) × 3.5 m (height). During the measurement process, the distance between the auxiliary horn antenna and the antenna under test is set as 5 m, which can completely satisfy the requirements of far-field measurement. e measured reflection coefficients together with the simulated values are shown in Figure 6, from which we can see that three operating frequency bands of the antenna (S 11 ≤ 10 dB) are 2.38∼2.53 GHz, 3.29∼4.11 GHz, and 4.72∼5.01 GHz, respectively. As shown in the plot, the simulated and measured results have an approximate agreement in the working frequency band of the antenna.    International Journal of Antennas and Propagation e discrepancies between the simulated and measured results may be attributed to the fabrication inaccuracy, the abrasion of the patch, and the welding errors of the connector.

Results and Discussion
To better understand the working mechanism of the proposed antenna, the simulated current and current vector distributions at the resonant frequencies of 2.4 GHz, 3.5 GHz, and 4.9 GHz are shown in Figure 7. From  Figures 7(a) and 7(b), it can be seen that the surface current is mainly distributed along the lower part of the circular ring and monopole impedance converter. It is illustrated that the resonant frequency of 2.4 GHz is generated by the combination between the circular ring and the monopole impedance converter. Due to the introduction of the monopole      International Journal of Antennas and Propagation impedance converter, the distribution of the surface current is not symmetrical along the circular ring. Despite the emergence of the middle band is attributed to the introduction of the Y-shape-like strip with two fan-shaped patches, the current is mainly distributed in the left side of the circular ring, as shown in Figures 7(c) and 7(d). is is because that the currents on the monopole impedance converter and the right side of the circular ring have opposite directions, which causes the radiation energy to be cancelled. At 4.9 GHz, it is found in Figures 7(e) and 7(f ) that the current is mainly focused on the circular ring and the Y-shape-like strip. e simulated and measured 2D co-polar and cross-polar radiation patterns at the frequencies of 2.4 GHz, 3.5 GHz, and 4.9 GHz are shown in Figure 8. It can be observed from the plot that the proposed antenna has a stable omnidirectional radiation pattern. Moreover, the cross-polarized radiation distributions are significantly less than the co-polarized counterparts.
e difference between the simulated and measured 2D radiation pattern in E-plane may be caused by fabrication tolerance, chamber scattering, measurement error, etc. us, the designed antenna exhibits stable radiation patterns over the operating band, which satisfies the requirement for 5G and other wireless applications.
As shown in Figure 9, the simulated peak gain of the proposed antenna is 3.56∼4.16 dBi, 2.45∼3.6 dBi, and 3.07∼4.01 dBi in 2.38∼2.53 GHz, 3.29∼4.11 GHz, and 4.72∼5.01 GHz, respectively, while the measured values are 3.60 dBi, 2.95 dBi, and 3.96 dBi at 2.4 GHz, 3.5 GHz, and 4.9 GHz, respectively. From Figure 9, it can be found that the gains of the middle band are lower than that of the upper and lower bands. Due to the opposite current vector direction on the monopole impedance converter and the right side of the circular ring at 3.5 GHz, the peak gain is relatively weaker than that at 2.4 GHz or 4.9 GHz.
e gain values mainly depend on the current intensity on the radiating element in   Figure 7, we can see that the current intensity in Figure 7(d) is weaker than any other distribution in Figures 7(b) or 7(f ). Table 2 displays the comparison of the performance between proposed and other previously represented antennas. rough the comparison with the references, it can be observed that our proposed antenna can provide enough bandwidths, appropriate gains, and omnidirectional radiation characteristics with more compact size.

Conclusions
A triple-band compact microstrip antenna is proposed for WLAN and 5G applications. e radiation element of the proposed antenna consists of a circular ring, a Y-shape-like patch, and a monopole impedance converter. With the help of the monopole impedance converter, the proposed antenna can be optimized in the operating bands of 2.38∼2.53 GHz, 3.29∼4.11 GHz, and 4.72∼5.01 GHz, respectively. From the experimental and measured results, we can see that the antenna has stable omnidirectional patterns, appropriate gains, and enough bandwidth. By comparing the performance with the previously presented antenna, the proposed antenna is a suitable choice to be applied for the 5G antenna design and other wireless applications.
Data Availability e underlying data supporting the results of your study can be found within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.