Design of a High-Gain Wideband Antenna with Directional Characteristics

Article information

J. Electromagn. Eng. Sci. 2025;25(5):474-482

Publication date (electronic) : 2025 September 30

doi : https://doi.org/10.26866/jees.2025.5.r.319

Lefei He, Youming Miao, Qiangjuan Li, Gui Liu

College of Electrical and Electronic Engineering, Wenzhou University, Zhejiang, China

^*Corresponding Author: Gui Liu (e-mail: iitgliu2@gmail.com)

Received 2024 October 2; Revised 2024 November 18; Accepted 2024 December 19.

Abstract

I. Introduction

With the development of wireless communication technology, wireless local area networks (WLANs) have been widely employed for commercial, medical, and industrial applications. In recent years, various types of WLAN antennas have been developed to meet diverse application requirements, including monopole antennas, band-notched antennas, and lens arrays [1–4]. However, these antennas were primarily designed for mobile terminals, featuring omnidirectional radiation characteristics and relatively low antenna gain (usually below 2 dBi). Certain applications require antennas with high gain and directional radiation characteristics that ensure reduced sensitivity to environmental interference, reliable communication, and high data transmission rates across different frequency bands. A directional antenna can effectively reduce interference from other directions by minimizing the reception of signals from non-target directions.

Microstrip antennas—widely used as directional antennas— are easy to fabricate, highly integrated, and offer significant advantages in terms of weight and volume, making them popular in mobile communications [5, 6]. Notably, various methods have been employed to achieve directional radiation performance in antenna design [7–10]. For instance, a reconfigurable Yagi-Uda antenna was introduced to achieve passive parasitic dipole directional radiation by adjusting the lengths of the parasitic dipoles [9]. Furthermore, a wideband multibeam circular array was proposed for VHF/UHF frequencies, featuring a directional element that combines the figure-eight and omnidirectional radiation patterns of a loop and a monopole, respectively, to achieve a low profile and excellent impedance matching [10]. Additionally, augmenting the original antenna with a metasurface, such as an artificial magnetic conductor (AMC), can significantly enhance its directional performance [11–15]. For instance, a polarization-dependent dual-band AMC surface was integrated with the antenna to improve its radiation performance, reduce dorsal radiation, and improve the directionality of its radiation [11]. The AMC structure allowed in-phase reflection, thereby making the reflected wave in phase with the incident wave. Furthermore, by employing the 3 × 3 Giuseppe Peano AMC reflector, an antenna equipped with AMC achieved a 70% smaller size compared to its counterparts without an AMC, while also exhibiting pronounced unidirectional radiation patterns [14].

High gain performance is also an important consideration for directional antennas. AMC structures can significantly enhance various antenna performance metrics. By compactly integrating with a feed structure or source antenna, it forms an electrically-thin (<0.1λ broadband element [16]. Therefore, when loading AMC structures, a unidirectional radiation mode and low back radiation can be achieved, which can increase the antenna bandwidth [17, 18], front-to-back ratio [19], and gain [20–23] while also improving the specific absorption rate [11, 21, 22]. Along these lines, a frequency-reconfigurable antenna based on AMC was proposed, which achieved a working frequency ranging from 1.51 GHz to 2.12 GHz and a maximum radiation gain of 5.37 dBi at 1.74 GHz—about 3.16 dB higher than an antenna without AMC [24]. In [25], a quad-band coplanar-waveguide antenna was placed over an AMC with four zero-phased reflection coefficients to increase the gain. Compared to the antenna without AMC, the quad-band AMC ground significantly enhanced the gain of the multiband antenna.

This article describes a directional radiating antenna specifically designed for WLAN applications. A defected ground plane with two semi-ellipses is printed on the bottom surface of the upper substrate to meet the antenna’s bandwidth coverage requirements, and AMC cells are employed to improve the antenna’s gain and directionality. The AMC structures are arranged periodically. Various evaluation structures are examined, and optimal parameter analyses are conducted, showing that both gain and directivity are improved by the addition of AMC cells. Overall, the proposed antenna exhibits good directional and high-gain radiation characteristics that are suitable for WLAN applications.

II. Antenna Structure

The overall geometric shape and detailed parameters of the proposed antenna are presented in Fig. 1. The antenna consists of two FR4 (Flame Retardant 4) substrates, each measuring 80 mm × 50 mm × 1.59 mm, with a relative dielectric constant (ɛ_r) of 4.4 and a loss tangent (tanδ) of 0.02. The distance between the two substrates is d = 25 mm. The antenna’s side view is illustrated in Fig. 1(b). Fig. 1(c) depicts the top surface of the upper substrate, which features a rectangular microstrip line measuring 2 mm × 13 mm. This microstrip line is fed by a coaxial connector through a via in the upper substrate. Both substrates have four holes, each with a diameter of 1.5 mm, positioned at the corners to secure the two substrates together. As depicted in Fig. 1(d), the defected ground plane of the upper substrate consists of two semi-ellipses and a rectangular plane with a rectangular slot. The two semi-ellipses are connected to the rectangular plane printed on the bottom surface of the upper substrate by two rectangular strips. Fig. 1(e) illustrates the top view of the lower substrate, upon which the AMC structures are printed in a 5 × 7 array. The bottom surface of the lower substrate is covered with a layer of metal. Detailed numerical values for the circular cells are provided in Fig. 1(f).

Fig. 1

Geometry of the proposed antenna (unit: mm): (a) geometry of the antenna, (b) side view, (c) top view of the upper substrate, (d) bottom view of the upper substrate, (e) top view of the lower substrate, and (f) detailed dimensions of the AMC cells.

III. Antenna Development

1. Design of the Defected Ground Plane and Radiator

Fig. 2 illustrates the evolution of the defected ground plane for the proposed antenna. As shown in Fig. 3, the rectangular structure depicted in Case 1 resonates at both the lower and higher frequency bands. In Case 2, modifying the rectangular structure to a semi-elliptical shape widens the antenna’s bandwidth to cover a frequency range of 2.46–5.84 GHz. Case 3 further broadens the antenna’s bandwidth by adjusting the width of the rectangular microstrips connecting the semi-ellipses. This adjustment extends the coverage to lower frequency bands, with the range spanning from 2.32 GHz to 5.84 GHz, and enhances resonance at both the lower and higher frequencies. In Case 4, two symmetric C-shaped slots are cut out from the structure in Case 3. Altering the slots on the ground plane changes the distributed parameters of the antenna elements, thereby reducing their distributed capacitance and resistance. Fig. 3 shows that Case 4 shifts the resonance toward higher frequencies, covering the frequency range of 2.35–6.06 GHz, thereby achieving satisfactory performance within the required frequency bands of 2.4–2.48 GHz, 5.15–5.35 GHz, and 5.73–5.85 GHz. To further increase the antenna gain, a periodically arranged AMC board was placed beneath Case 4, as depicted in Fig. 4. This phenomenon is detailed in the next section.

Fig. 2

Design evolution of the defected ground plane for the upper substrate: (a) Case 1, (b) Case 2, (c) Case 3, and (d) Case 4.

Fig. 3

Simulated S₁₁ parameters for the four cases.

Fig. 4

Design evolution of the AMC cells: (a) AMC1, (b) AMC2, and (c) AMC3 (proposed).

2. Design of AMC Cells

Fig. 4 illustrates the design process for the AMC cells. Notably, the primary objective of adding AMC cells was to increase gain and improve the directional ability of the antenna. As shown in Fig. 5, with the distance (d) set to 8 mm, the −10 dB impedance bandwidth is able to cover the desired frequency bands. Fig. 6 depicts the gain of the proposed antenna in response to the various AMC cells presented in Fig. 4. AMC1, featuring the simplest circular patch design, exhibits a higher gain than the antenna without AMC across all operating bands, except for the 5.5–5.7 GHz band. AMC2 introduces grooves to AMC1, dividing the circular patch into nine smaller parts. This modification results in a gain that surpasses that of the antenna with AMC1 at 5.5 GHz. AMC3 further improves the gain in the 5–6 GHz frequency band, surpassing that of the antenna with AMC2. Overall, compared to the antenna without AMC, AMC3 achieved an increased gain in both the lower and higher frequency bands.

Fig. 5

Simulated S₁₁ with and without AMC cells.

Fig. 6

Simulated gain with and without AMC.

To further enhance the antenna’s gain and improve its directionality, a metal layer was attached below the lower substrate. To ensure that the −10 dB impedance bandwidth still covered the desired frequency bands, the distance (d) was optimized to 25 mm. Fig. 7 compares the S₁₁ parameters with and without AMC cells, showing that the −10 dB impedance bandwidth continues to cover the 2.4 GHz and 5.5 GHz frequency bands. Fig. 8 compares the antenna gains achieved with and without AMC cells. The results show that adding the AMC cells significantly improved the gain. More specifically, compared to the antenna without AMC cells, the gain at 2.4 GHz increases from 3.67 dBi to 6.78 dBi, and the gain at 5.5 GHz rises from 5.91 dBi to 7.56 dBi.

Fig. 7

Simulated S₁₁ parameters with and without AMC cells after adding the metal layer.

Fig. 8

Simulated gain with and without AMC cells after adding the metal layer.

Fig. 9 shows the 3D radiation patterns of the antenna with and without AMC cells. The incorporation of AMC cells led to an enhancement in the radiation performance of the proposed antenna at both 2.4 GHz and 5.5 GHz frequencies, with the reflections from the AMC cells being more concentrated in the middle of the antenna’s yoz axis. Consequently, the addition of AMC cells significantly bolstered the antenna’s ability to resist interference, ensuring directional and focused radiation.

Fig. 9

Simulated 3D radiation patterns: (a) without AMC cells at 2.4 GHz, (b) without AMC cells at 5.5 GHz, (c) with AMC cells at 2.4 GHz, and (d) with AMC cells at 5.5 GHz.

IV. Parametric Analysis

Fig. 10 illustrates the simulated surface current density distributions of the two substrates at frequencies of 2.4 GHz and 5.5 GHz. In Fig. 10(a), the maximum current values for the upper substrate of the antenna appear mainly on the rectangular microstrip line and the rectangular slot on the defected ground plane. Therefore, it may be inferred that the sizes of the two symmetrical semi-ellipses have significant effects on the 2.4 GHz frequency band. In Fig. 10(b), the current is mainly concentrated at the rectangular slot on the defected ground plane and the two symmetrical semi-ellipses. Meanwhile, Fig. 10(c) indicates that the AMC cells have a relatively small impact on the lower frequency bands, while Fig. 10(d) shows that the current density of the AMC cells increases significantly at 5.5 GHz. This comparison suggests that the AMC cells primarily affected the higher frequency range, exhibiting a more pronounced impact on the 5.5 GHz band than on the lower frequency bands.

Fig. 10

Simulated surface current distribution: (a) upper substrate at 2.4 GHz, (a) upper substrate at 5.5 GHz, (c) lower substrate at 2.4 GHz, and (d) lower substrate at 5.5 GHz.

Based on the current distribution results, the proposed antenna was optimized by adjusting the size of the semi-ellipses (L₁) and the distance between the two FR4 substrate (d). During the optimization process, significant changes in the reflection coefficient were observed when varying the length parameter while keeping the other parameters constant. Fig. 11 illustrates the S₁₁ parameters obtained for different values of L₁ and d, confirming the results of the surface current distribution analysis. As shown in Fig. 11(a), variations in L₁ lead to corresponding changes in the antenna’s S₁₁ parameters. Notably, when L₁ is set to 20 mm, the antenna achieves optimal performance within the desired frequency band. Fig. 11(b) presents the S₁₁ parameters for different values of d, highlighting that the antenna’s frequency coverage expands as the value of d increases. When d = 22 mm, the S₁₁ parameter in the 2.4 GHz band is just below −10 dB. To account for potential measurement errors and ensure that the physical antenna can cover the entire frequency band, the value of d had to be increased further. At d = 28 mm, the S₁₁ parameters met the required performance, although the antenna’s overall size had to be increased. Therefore, d = 25 mm was chosen as the optimal value.

Fig. 11

Simulated S₁₁ for different values: (a) different L₁ values and (b) different d values.

V. Experimental Results

To validate the simulation results, a prototype of the proposed antenna was fabricated, and subsequent measurements were conducted. Photographs of the designed antenna are shown in Fig. 12. The S-parameters were measured using a Keysight vector network analyzer (VNA) with model number N5223A. Furthermore, for environmental testing, 2D radiation patterns were obtained in a microwave anechoic chamber. Fig. 13 presents a comparison of the measured and simulated S₁₁ parameters of the proposed antenna. The measured −10 dB impedance bandwidths were observed to be 0.88 GHz and 1 GHz, covering frequency bands ranging from 2.32 GHz to 3.2 GHz and from 4.9 GHz to 5.9 GHz, respectively. Notably, these bands effectively cover the required frequency bands of 2.4–2.4835 GHz, 5.15–5.35 GHz, and 5.725–5.85 GHz.

Fig. 12

Photograph of the fabricated antenna: (a) prototype, (b) VNA setup, and (c) microwave anechoic chamber.

Fig. 13

Simulated and measured S₁₁ parameters.

Fig. 14 illustrates the measured radiation patterns at 2.4 GHz and 5.5 GHz, with the black and red lines representing copolarization and cross-polarization, respectively. It is evident that the proposed antenna exhibits unidirectional radiation performance, with beam splitting observed along the main radiation direction at the desired frequencies. The cross-polarization levels in the xy- and yz-planes are less than −10 dB, indicating low cross-polarization across both frequency bands. Furthermore, the measured results demonstrate the desired directionality of the radiation patterns at 2.4 GHz and 5.5 GHz.

Fig. 14

Simulated and measured normalized radiation patterns: (a) xy-plane at 2.4 GHz, (b) yz-plane at 2.4 GHz, (c) xy-plane at 5.5 GHz, and (d) yz-plane at 5.5 GHz.

The simulated and measured gains and efficiencies are depicted in Fig. 15. The peak gains measured at 2.4 GHz and 5.5 GHz are 4.37 dBi and 7.67 dBi, respectively, with the measured efficiencies being 67.9% at 2.4 GHz and 85.2% at 5.5 GHz. Moreover, it is worth noting that the proposed antenna exhibits gains exceeding 4.37 dBi and efficiencies exceeding 67.9% across the entire desired operating frequency range.

Fig. 15

Simulated and measured gain and total efficiency.

Fig. 16 compares the measured gain attained with and without using the AMC cells. It can be observed that, after adding the AMC cells, the proposed antenna exhibits increased gain across the entire desired operating frequency range. Specifically, the gain increases from 1.92 dBi to 4.37 dBi at 2.4 GHz, while it increases from 6.22 dBi to 7.67 dBi at 5.5 GHz. These results validate the expectation that adding AMC cells can significantly increase gain.

Fig. 16

Measured gain with and without AMC cells.

Table 1 compares the performance of the proposed directional high-gain antenna with that of other similar antennas. Notably, the main highlights of the antenna proposed in our work include excellent high gain and a wider frequency band. The experimental results indicate that the antenna can be considered a strong competitor in the field of WLAN communication.

Table 1

Performance comparison of the proposed antenna and other reported designs

VI. Reflection and Future Research Directions

This section discusses the limitations of the proposed antenna design and explores potential areas for further research. Initially, it is important to note that the design proposed in this study has certain limitations that require attention. First, with the advent of new wireless standards, including those for the 6 GHz band, there is a pressing need for antennas that offer expansive frequency coverage to accommodate this new band. Second, the antenna’s relatively high profile may not be suitable for applications that require a compact form factor.

Looking ahead, future research on WLAN directional antennas should aim not only to optimize broadband performance, directivity, and gain but also to accomplish greater miniaturization and low-profile designs. As technology continues to evolve, future WLAN applications are likely to demand more agile control over frequency and directionality. Consequently, the development of intelligent reconfigurable antennas that can dynamically adapt their radiation directions and frequencies in response to network requirements will become a pivotal area of future research.

VII. Conclusion

This paper presents a directional antenna for operation in the WLAN frequency bands (2.4–2.4835 GHz, 5.15–5.35 GHz, and 5.725–5.85 GHz). After analyzing the evolution of the antenna design, dimensional optimization was performed. In this paper, it is proposed that an antenna’s directional radiation capabilities can be enhanced by adding a substrate composed of periodically arranged AMC cells. The proposed antenna exhibited stable directional radiation, with measured gains exceeding 4.37 dBi across the desired frequency bands. Overall, the proposed dual-band directional antenna can be considered a promising candidate for WLAN station antennas that offer directional radiation and high gain.

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