The Tire Pressure Monitoring System (TPMS) is an essential safety feature in modern vehicles, designed to continuously monitor tire air pressure while the vehicle is in motion. It alerts drivers to potential issues such as leaks, low or high pressure, ensuring the safety of both the driver and passengers [1]. The TPMS transmitting antenna operates at a frequency of 433.92 MHz and has a limited transmission range of less than 10 meters. It is integrated within the tire pressure sensor module, located inside the tire. To ensure reliable data transmission during vehicle operation, the antenna must be omnidirectional. Additionally, due to space constraints and the fact that the entire module is powered by a single lithium battery, the antenna must be compact and highly efficient in signal emission. As TPMS technology advances, the development of miniaturized antennas with optimal performance has become increasingly critical.
Currently, common TPMS antenna designs include the inverted-F spiral antenna [2-3] and the small-loop antenna [4]. While the inverted-F spiral offers better performance, it occupies significant space. On the other hand, the small-loop antenna is more compact but suffers from lower emission efficiency. In this study, a small PCB spiral antenna was developed to meet the specific requirements of TPMS. The antenna was fabricated on a Teflon substrate measuring just 20 mm × 16.7 mm. Compared to traditional helical antennas, the PCB-based design achieves a significant reduction in size while maintaining similar total length. The metal wires are printed on both sides of the PCB, allowing for precise control over dimensions like length, width, and spacing, making the antenna easy to manufacture. Experimental results confirm that the antenna operates effectively at 433.92 MHz and exhibits good omnidirectionality, meeting the necessary performance standards for TPMS applications.
The structure of the PCB spiral antenna is illustrated in Figure 1. The antenna consists of 11 turns of spiral, with metal conductors printed on both sides of a rectangular dielectric substrate. Each turn has the same width, and through-holes are placed at both ends, with copper plating on the inner walls to connect the two layers. The feed point is connected to the largest via hole in the upper right corner of the structure, while the remaining vias have the same diameter. To ensure proper electrical connection, pads are added around each via hole in the design.
For the antenna design and simulation, software such as CST Microwave Studio was used. A dielectric substrate with a relative permittivity of 2.5 and a thickness of 1.6 mm was selected. To achieve miniaturization, the smallest available PCB manufacturing process was chosen. During simulation, some parameters were fixed based on practical fabrication conditions, and the desired frequencies were achieved by adjusting the wire length (L) and pitch (S). The relationship between these parameters and the resonant frequency was analyzed, showing that increasing L decreases the resonant frequency, while increasing S increases it.
After optimization, the final antenna parameters were determined, as shown in Table 2. Due to the low antenna impedance (approximately 3.58 Ω), an external matching circuit was required to match the 50 Ω input impedance. A T-type matching network was simulated using ADS software, and the resulting S11 curve is presented in Figure 3. The antenna shows an effective operating bandwidth of 432.6–435.2 MHz (S11 < -10 dB), with a very low S11 value of about -40 dB at 433.92 MHz, which meets the requirements for signal transmission.
The antenna was manufactured using a polytetrafluoroethylene (PTFE) dielectric board, known for its stability and low loss. The dimensions of the antenna are 20 mm × 16.7 mm × 10 mm. The measured S11 curve (Figure 4) closely matches the simulation results, confirming the feasibility of the design. However, due to the use of a custom inductor in the matching circuit, the measured S11 value is slightly lower than the simulated one. The effective bandwidth of the antenna is 432.2–435.3 MHz (S11 < -10 dB), and at 433.92 MHz, the S11 is below -15 dB, making it suitable for use in the TPMS transmitter module. In practice, higher-quality chip components can be used to improve performance.
The TPMS transmitting antenna operates at a relatively low frequency of 433.92 MHz and is mounted inside the tire along with other components. Its extremely small footprint presents significant challenges in antenna design. This paper addresses these challenges by designing and fabricating a compact PCB spiral antenna. The experimental results demonstrate that the antenna is not only small and lightweight but also highly directional, fulfilling the needs of TPMS for miniaturization. Moreover, the design allows for simple manufacturing, cost-effectiveness, and seamless integration with electronic circuits. However, due to its narrow bandwidth, the antenna is suitable only as a fixed-frequency transmitter. A separate receiving antenna is needed for the TPMS system.
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