I. Introduction
Benefiting from advances in fifth-generation (5G) and other wireless technologies, Wi-Fi has become increasingly crucial for high-speed connectivity and has significantly enhanced daily life over the past 20 years. Wi-Fi covers the frequency ranges of 2.4–2.484 GHz and 5.15–5.83 GHz [1–3]. Wireless routers, as core components of Wi-Fi systems, provide high-speed, uninterrupted Internet access and are now indispensable in homes, offices, and factories. Notably, research to miniaturize router antennas has paved the way for smaller and more efficient devices, representing a prominent trend in wireless technologies [4–6].
Whip antenna routers are prevalent in numerous contexts. These external antennas frequently comprise conventional dipoles or monopoles featuring vertical polarization. However, with the increasing demand for high integration in today’s electronic devices, the miniaturization of antennas poses a challenge. There are several techniques for antenna miniaturization, such as using high permittivity materials [7, 8], short-circuit loading techniques [9], fractal techniques [10, 11], integration of capacitive and inductive components [12], application of metamaterials and metasurfaces [13], helical and spiral antenna techniques [14], etc. The use of these techniques can effectively reduce antenna size. However, some techniques require complex structures or demand excessive antenna space, which may be unsuitable for practical applications.
Reduced antenna size and high gains are always contradictory. Conventional routers utilize vertical mono-poles or dipoles as external antennas [15], while enhanced high-gain performance requires complex radiation structures that increase overall size. Many researchers have explored gain enhancement for WLAN antennas. In one study [16], the sleeve and ground plane of a high-gain monopole antenna formed a quarter-wavelength cavity and were introduced to enhance antenna gain. In another study [17], an antenna with defected ground structure configuration was realized to improve the antenna gain from 4.93 dBi to 6.27 dBi. A quasi-Yagi antenna was also designed to enhance gain and directivity [18].
In this article, we propose a printed circuit board (PCB) Wi-Fi antenna pair operating in the 2.4 GHz and 5.0 GHz frequency bands, respectively, to realize good input impedance and omnidirectional radiation performance. The antenna is made of FR4 material with a copper coating. The main radiating components of the two antennas are slotted rectangular radiating elements and serpentine microstrips. The metal ground is a U-shaped metal coating with a minigap between the ground and the radiating element for feeding purposes. The antenna pair achieves size reduction by etching a rectangular slot on the radiating structure. The use of serpentine microstrips effectively extends the path lengths of the surface currents, thereby increasing the electrical length of the antenna. Serpentine microstrips also help regulate the surface current distribution and enhance the gain performance of the antenna. The gain improvement of both antennas was achieved by increasing the codirectional currents and enhancing the local currents at different positions.
This paper is organized as follows. Section II provides a brief description of the antenna configuration and design process, with the two characteristics of miniaturization and gain-enhanced performance discussed in detail. The simulation and measurement results are presented in Section III, and conclusions are drawn in Section IV.
II. Design of the Proposed Antenna
In this section, we provide detailed design parameters for the antenna configurations for both the 2.4 GHz and 5.0 GHz frequencies, and we then discuss the evolutionary process and design considerations behind these structures. Fig. 1 shows the top view of the 2.4 GHz and 5.0 GHz frequency band antennas. The detailed dimensions of the antennas for the 2.4 GHz and 5.0 GHz frequency bands are shown in Tables 1 and 2, respectively.
The structure of the 2.4 GHz antenna is illustrated in Fig. 1(a). Component “a” incorporates a U-shaped structure at the end of the antenna to enhance the end current through the end effect of the metal. Components “b” and “d” serve as the primary radiating parts of the antenna, characterized by equally spaced rectangular slots etched onto the radiating element. Following parameter adjustment, the antenna exhibited improved performance with a slot size of 5 mm × 2 mm. Component “c” comprises a serpentine microstrip that does not actively contribute to radiation but effectively increases the electrical length of the antenna. Component “e” corresponds to the metal ground of the antenna, which also contributes to overall radiation. The radiation energy of the 2.4 GHz antenna was generated from the cumulative effect of radiation energies around the x-axis, originating from all the previously mentioned components.
The structure of the 5.0 GHz antenna is illustrated in Fig. 1(b). The main radiating structures of the antenna are represented by components “a,” “c,” and “e,” which utilize metal coatings of varying sizes to generate radiation. Component “a” is narrow, while components “c” and “e” are wider. Following parameter adjustment, the antenna exhibited improved performance with a slot size of 2 mm × 1 mm in component “a.” The antenna includes serpentine microstrips in components “b” and “d,” which not only extend the electrical length but also modify the width to shift the current zero point, thereby enhancing the antenna gain. Component “f” corresponds to a metal ground, while the angled edges of component “e” are specifically designed to minimize radiation interference from nearby metallic elements.
1. Design of the 2.4 GHz Frequency Band Antenna
The simulation results for a 2.4 GHz PCB dipole are shown in Fig. 2. At 2.4 GHz, the antenna exhibited a gain of 1.9 dBi. To enhance this gain, the electrical length was increased by enlarging its physical dimensions. After introducing serpentine microstrips and rectangular radiating elements, the gain rose to approximately 3.9 dBi. This led to the development of the original structure for the 2.4 GHz antenna, as illustrated in Fig. 3(a) for antenna (Ant.1).
Fig. 3 depicts the optimization process for the 2.4 GHz antenna. The initial structure of the antenna is shown in Fig. 3(a), consisting of a simple symmetric rectangular radiating patch with a serpentine microstrip and a metal ground. When the serpentine line was adjusted, the antenna operated at 2.4 GHz with a simulated gain of 3.9 dBi. Directly reducing the dimensions resulted in a decrease in gain to 3 dBi. As shown in Fig. 3(b), to further optimize antenna performance, slots of the same size were etched equidistantly on the main radiating structure using the end effect of the metal. The dimensions were adjusted until the antenna operated at 2.4 GHz, with a simulated gain of 3.6 dBi. As shown in Fig. 3(c), the final structure was determined by placing slots of the same size at equal intervals across the main radiating elements. This increased the simulated gain to 3.9 dBi.
The optimized simulation results for S11 and the gain curves for the three structures are shown in Fig. 4. In summary, the antenna size was reduced from the initial 124 mm to 104 mm, while the gain and bandwidth remained almost unchanged. This reduction was achieved by etching slots on the main radiating body, which effectively increased the current path length as the surface current flowed along the edges of the slots. Additionally, the radiation structure narrowed at the slots, enhancing the local current and thereby improving antenna gain.
2. Design of the 5.0 GHz Frequency Band Antenna
Fig. 5 describes the optimization process for the 5.0 GHz antenna. We based the initial structure of the 5.0 GHz antenna on the design principles of the initial 2.4 GHz antenna structure, as illustrated in Fig. 5(a). It comprised three radiating elements with varying sizes and shapes, multiple serpentine microstrips, and a metal ground. Adjusting the dimensions of the serpentine microstrips and radiating elements in the antenna design led to a gain of over 5.3 dBi in the 5.1–5.8 GHz frequency range. However, reducing the antenna size, as shown in Fig. 5(b), resulted in a significant decline in gain. To optimize performance, slots were introduced in component “a” of the radiating element, enhancing the current on the narrow radiation element and improving the overall gain.
The simulation results in Fig. 6 show that incorporating slots and adjusting the microstrip width of the antenna led to increased electrical size and improved gain. The final antenna design, with a size of 80 mm, achieved a gain of over 4.9 dBi in the 5.1–5.8 GHz frequency range, meeting the requirements of the 5 GHz Wi-Fi frequency band (5.125–5.825 GHz) while maintaining miniaturization and gain enhancement characteristics.
3. Study on Surface Current Distribution and Dual-Element Antennas in Routers
The descriptions provided in the last two sections prove to some extent that both miniaturization and gain enhancement can be achieved by manipulating the surface current distribution along the antenna-radiating elements. Hence, in this section, we explain the study of current distribution across the antenna to elucidate the reason for performance enhancement from a theoretical perspective. Taking a simple dipole antenna as an example, provided that the amplitude of the surface current along the dipole follows a sinusoidal pattern, this current distribution can be expressed as shown in Eq. (1), while if the surface current distribution along the dipole exhibits a triangular pattern, it can be expressed as shown in Eq. (2). By integrating the results from Eqs. (1) and (2) into Eq. (3), respectively, we could derive the radiation field of the dipole from either type of current distribution, thus allowing us to understand the influence of the current distribution on the radiation characteristics of the antenna.
Simulation analysis of a simple dipole antenna showed that with a triangular current amplitude distribution, the radiation pattern exhibited a narrower main lobe and suppressed side lobes, enhancing both directionality and gain. In contrast, a sinusoidal distribution produced a wider main lobe accompanied by slightly elevated side lobes. Thus, it was likely that antennas with steeper declines in current amplitude distribution would achieve higher gains.
We inferred that the current distribution across Ant.3 should be steeper than that across Ant.2. The surface current distribution of the proposed 2.4 GHz planar antenna, simulated and depicted in Fig. 7, aggregated the overall gain along the x-axis. Comparing the current amplitude distributions of the proposed 2.4 GHz antennas revealed that Ant.3 exhibited a steeper decline in current amplitude across its main radiation areas than Ant.2.
As indicated by the blue dashed line in Fig. 7, the current amplitude distribution across the main radiation areas closely resembles a triangle with a sharp gradient, whereas Ant.2 had a smoother distribution. Consequently, Ant. 3, with its slotting technique, altered the current distribution, providing gain enhancement.
The current distribution for the 5-GHz antenna, shown in Fig. 8, exhibited a similarly steep variation along the x-axis. In line with the principles observed for the 2.4 GHz antenna, by altering the current distribution via slotting and increasing the electrical length, the proposed structure delivered a gain that surpassed that of Ant.2.
The surface current distributions of the 2.4 GHz and 5.0 GHz antennas with different structures are illustrated in Figs. 9 and 10. The strategy of using microstrips to extend the electrical length, coupled with the slotting technique that further lengthened the current path, facilitated a significant reduction in the physical dimensions of the antenna. In the area of the slot, the narrow radiating body resulted in an increase in local current intensity.
The proposed PCB Wi-Fi antennas for installation within the router enclosure are shown in Fig. 11. According to the dimensions and configurations of general router models from China Mobile Group Device Co. Ltd., Shenzhen, China, we have arranged the two 2.4 GHz antennas and the two 5.0 GHz antennas at varying intervals. Ant.1 and Ant.3, and Ant.2 and Ant.4, had 10-cm spacing, while Ant.1 and Ant.2 had 5-cm spacing. With this configuration, we performed simulation studies for the 2.4 GHz and 5.0 GHz antennas.
The simulation results, depicted in Fig. 12, showed that the S21 parameter near the 2.4 GHz frequency point was less than - 16 dB, with an envelope correlation coefficient (ECC) lower than 0.05 for a 10-cm separation between the two 2.4 GHz antennas. Additionally, a simulated dual-element gain of 5 dBi was achieved at 2.4 GHz. With a 10-cm separation between the two 5.0 GHz antennas, the simulation results showed that the S21 parameter was less than −18 dB across the 5.0–5.9 GHz frequency range, with an ECC of less than 0.1. Moreover, the dual-element array gain was greater than 7.5 dBi in the 5.0 GHz frequency band.
III. Measurement and Discussion
An antenna prototype was fabricated as shown in Fig. 13 and measured using a Ceyear 3671D vector network analyzer (Ceyear Technologies Co. Ltd., Qingdao, China). Fig. 14 compares the simulation and measurement results for the S11 and gain curves for the 2.4 GHz Wi-Fi antenna. The measurement results indicated that the 2.4 GHz Wi-Fi antenna exhibited an S11 of less than −10 dB in the 2.25–2.5 GHz frequency band, effectively encompassing the bandwidth for 2.4 GHz Wi-Fi. The peak gain around 2.4 GHz was 3.85 dBi. However, due to factors such as transmission line losses and discrepancies in soldering during testing, the match quality declined slightly, leading to a modest extension of the measured bandwidth of around 2.4 GHz relative to the simulation. Moreover, the gain curve showed that the measured gain was approximately 0.1 dBi below the simulated value.
As with the 2.4 GHz antenna, measurement errors led to reduced matching for the 5.0 GHz Wi-Fi antenna. Fig. 15 compares the simulation and measurement gain curves. The measured S11 values did not fall below −10 dB at the beginning (around 5.125 GHz) and the end (around 5.825 GHz) of the 5.0 GHz band, indicating suboptimal antenna performance at these frequencies. Nonetheless, the gain consistently surpassed 4.6 dBi across the operational frequency, closely aligning with the simulations.
The bandwidth requirement for Wi-Fi applications was nearly satisfied, and the antenna pair demonstrated good matching performance across both the lower (2.4–2.485 GHz) and upper (5.125–5.825 GHz) Wi-Fi frequency bands.
The 2D far-field patterns for the proposed antenna pair are depicted in Fig. 16. Fig. 16(a) shows the pattern of the 2.4 GHz antenna at its operational frequency band, with simulation and measurement results for elevation and azimuth demonstrating reasonable consistency. Fig. 16(b) shows the pattern of the 5.0 GHz antenna at a central frequency of 5.5 GHz, with measurement and simulation results for elevation and azimuth demonstrating good radiation characteristics. Evaluation errors and precision issues resulted in some discrepancies between the measurement and simulation results in both the azimuth and elevation patterns. In summary, the antenna pair demonstrated commendable performance, with both measurement and simulation results aligning with the expected radiation properties.
IV. Conclusion
The proposed PCB-based Wi-Fi antenna exhibited optimal resonance in both the 2.4 GHz and 5.0 GHz frequency bands. Surface slotting and the strategic use of microstrip lines enhanced the surface current intensity of the antenna, achieving miniaturization and high gain. The measurement results coincided well with the simulation results, validating the effective performance of the antennas within the specified frequency bands. Compared with traditional router antennas, the proposed antennas are compact and have enhanced gain, which is crucial for external router antennas.
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