In the context of modern agriculture, the integration of advanced technologies to enhance productivity and operational efficiency has become an inevitable trend. Agricultural robots, equipped with intelligent automation and control systems, play a crucial role in reducing labor requirements, optimizing farming processes, and improving accuracy in tasks such as planting, harvesting, and crop monitoring. To achieve high precision and efficiency in outdoor environments, these robots must be equipped with a robust and stable real-time kinematic global positioning system (GPS-RTK) [1]. The GPS-RTK system not only provides precise location information but also enables robots to maintain their trajectories, avoid obstacles, and perform tasks autonomously. Particularly in agricultural settings, where large-scale fields and complex terrain conditions are common, the demand for an accurate positioning system becomes even more critical.

A key factor influencing the performance of the GPS system is the antenna. Dual-band GPS antennas (1.227 GHz for L2 and 1.575 GHz for L1) [2] have the capability to receive signals from a greater number of satellites, thereby enhancing the accuracy and stability of the positioning system [3]. In areas with weak or obstructed signals, dual-band antennas effectively mitigate errors and improve connectivity reliability. The design and development of a dedicated dual-band GPS antenna for agricultural robots not only fulfills the requirement for high accuracy but also ensures compatibility and efficiency within specific working environments.

Currently, numerous studies have been conducted on dualband GPS antennas with various structural configurations [4–16]. In [4], researchers used two stacked square patches with truncated corners placed on a double-layer fractal electromagnetic band gap (EBG) substrate. This design, utilizing low-cost FR4 material, successfully provided full GPS bandwidth coverage. Despite its features, it has a large footprint of 130 mm × 130 mm. Another study [5] describes an antenna built on a multi-layer low-temperature co-fired ceramic (LTCC) substrate. This configuration features two stacked patches: the bottom patch handles lower frequencies and acts as the ground for the top patch, which operates at higher frequencies and connects directly to the probe. However, this study overlooks the size of the ground plane. The antenna achieves dual-band functionality by utilizing a patch mode for the L2 band and a slot mode for the L1 band [7]. It also incorporates additional slot stubs, allowing for independent tuning of the L1 frequency. The use of high-dielectric, low-loss substrates and meander slot designs are employed to enhance the antenna’s performance and manufacturability. A miniature coupled double loop (CDL) antenna is introduced for dual-band GPS applications [8]. This antenna’s design incorporates lumped capacitors and inductive pins on a Rogers TMM10i substrate with moderately high permittivity. This configuration aims to concurrently achieve a smaller size, high radiation efficiency, and appropriate spacing between the L2 and L1 band resonance frequencies.

Meanwhile, the researcher in [9] proposed a compact quadband inverted F/L antenna (QIFLA) with right-hand circular polarization (RHCP), comprising four helical dual-band antennas. Traditional multi-band four-port CP antennas tend to be large, making frequency tuning and impedance matching more challenging. The inverted F and L antennas incorporate inductive, capacitive, and resistive elements to achieve resonance. The author in [10] designed a compact dual-coupled short-circuit probe-loaded (DCSP) patch antenna with dimensions of λ₀/8.8 × λ₀/8.8 × λ₀/19. This dual short-circuit probe structure functions as two parallel LC loads integrated into the patch antenna.

Studies [11–13] examined antennas with various slot configurations. The work in [11] introduced an antenna based on the reduced surface wave (RSW) mechanism, minimizing susceptibility to low-angle multipath interference compared to conventional high-precision GPS antennas. The author in [12] developed a circularly polarized, single-fed slotted patch antenna featuring an S-shaped slot at the center of the square patch radiator for dual-band operation. A single microstrip feed line is positioned beneath a coupled aperture ground plane, allowing frequency ratio control by adjusting slot arm length.

Additionally, research on antenna isolation has been conducted. Study [14] introduced a tag structure with a reflecting plane and an air gap to enhance directivity. Study [15] explored transmitter-receiver isolation, utilizing a metamaterial cavity composed of metal reflectors and a two-layer complementary metasurface to improve isolation by up to 10 dB over the 2.5–2.7 GHz range. Lastly, the author in [16] examined an ultra-high frequency (UHF) folded tag antenna with a U-shaped impedance matching structure designed for metal surfaces, achieving a maximum theoretical reading distance of 3.6 m. The aforementioned studies have designed dual-band GPS antennas. However, certain limitations persist, such as large size [12, 17], narrow bandwidth [8, 9], and the use of high permittivity substrates [8]. Additionally, some results are only based on simulations and have not yet undergone fabrication and measurement [18].

In this paper, a compact, dual-band antenna utilizing a low-cost substrate for GPS applications in agricultural robots is designed and fabricated. The technique employs stubs combined with a slotted square DGS to generate two resonant bands while simultaneously miniaturizing the antenna size. A cavity is used to significantly reduce backward radiation, improve forward radiation, and simultaneously isolate the antenna’s influence from other components within the robot. Section II outlines the design considerations, while Section III presents the experimental results. The paper concludes with a summary of findings.

Fig. 1 illustrates the geometric structure of the proposed antenna. Its two main components are a square radiating element and a square cavity, with dimensions of g and g₂, respectively. The radiating element is formed by four stubs along with a microstrip line mounted on one side and a DGS structure consisting of two square slots mounted on the opposite side. This combination not only creates a dual-band antenna but also helps reduce its size. However, it introduces the limitation of significant back radiation. Therefore, a cavity is attached to the rear of the antenna to overcome this drawback.

The simulated surface current distribution results (using CST Studio Suite 2021 software) at phase = 0° in Fig. 2 show that the dual-band characteristic is primarily determined by two square slots. At lower frequencies, the surface current distribution is more concentrated in the larger slot, while at higher frequencies, it’s concentrated in the smaller slot. Notably, the surface current distribution at both frequencies is mainly focused on the edges of each slot. This, combined with the relatively large size of the DGS structure, means that the dimensions of the slots have minimal impact on the antenna’s input impedance and resonant frequency. Therefore, during the parameter study, the central and middle metal squares were partially cut with isosceles right triangles, whose square sides were x and y, respectively. The results of the S₁₁ parameter study with respect to x are shown in Fig. 3(e). With x = 5 mm (equivalent to a = 5√2 mm), the antenna’s S₁₁ achieves the best results at two frequency bands. The same phenomenon occurs when y = 10 mm (b = 10√2 mm).

The front section of the antenna’s radiating element comprises the feed structure and four stubs (stub 1, stub 2, stub 3, and stub 4). Both their dimensions and placement impact the input impedance, which in turn influences the antenna’s resonant frequency. Fig. 3 illustrates the S₁₁ investigation results, detailing how stub size and position affect the antenna’s S₁₁.

The dimensions of stub 1 and stub 2 significantly affect the resonant frequency at 1.227 GHz, whereas the resonant frequency at 1.575 GHz is primarily determined by the position of stub 3, in conjunction with stub 4 and the slots. The electromagnetic interaction between stub 3 and stub 4 enables fine-tuning of the 1.575 GHz resonant frequency without impacting the resonance at 1.227 GHz. A parametric study was conducted on key variables, including ws₁, ws₂, ws₃, ws₄, and d₃, to evaluate their effects on the antenna’s performance as shown in Fig. 3.

Each stub contributes differently to the antenna’s overall performance. Stub 1 and stub 4 (the lower section of stub 1) influence both resonant frequencies, whereas stub 2 and stub 3 primarily affect only one of the two resonant frequencies when appropriately positioned. When ws1 increases (i.e., when the upper part of stub 1 is widened), the lower resonant frequency tends to decrease, while the higher frequency splits into two distinct resonance points. Conversely, when ws₅ increases (i.e., when the lower part of stub 1 is extended), both the lower and higher resonant frequencies increase, with S₁₁ deepening, indicating improved impedance matching.

It is evident that as stub 3 is positioned farther from the feed point, the high resonant frequency decreases, whereas the low resonant frequency remains unaffected. Since d₃ is appropriately placed, it influences only the higher frequency, making it a key parameter for high-frequency tuning. To enhance this tuning capability, stub 4 is introduced at a position farther from the feed point than stub 3, ensuring optimal placement. The interaction between stub 3 and stub 4 allows for precise fine-tuning of the high resonant frequency.

The impact of the reflector cavity dimensions on the antenna’s performance is illustrated in Fig. 4. The impact of the reflector cavity dimensions on the antenna’s performance is illustrated in Fig. 4. As both the cavity’s width and thickness increase, the antenna’s resonant frequency decreases. However, the cavity’s thickness affects the antenna’s resonant level more significantly than its width. The proposed antenna dimensions are shown in Table 1.

In this paper, a dual-band antenna with an F-shaped feed and a reflector cavity attached to the back is proposed for use on a robot. The proposed antenna operates at the central frequencies of the L1 and L2 bands, with a high gain of over 2.5 dBi, a wide beam, and an upward direction. The high gain, along with the wide beam and upward direction, is very advantageous for receiving GPS signals. Experimental results of the GPS using the proposed antenna mounted on the robot show that the proposed antenna performs well and provides high accuracy, comparable to the U-blox antenna. With its low production cost, the proposed antenna is a promising candidate for use in agricultural robotics applications.