The Tire Pressure Monitoring System (TPMS) is a critical 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 safer driving conditions for both the driver and passengers [1]. The TPMS transmitting antenna operates at 433.92 MHz, with a short-range signal transmission capability of less than 10 meters. This antenna is integrated within the tire pressure sensor module, which is mounted inside the tire. To ensure reliable data transmission during operation, the antenna must be omnidirectional. Additionally, due to space constraints and power limitations from a single lithium battery, the antenna needs to be compact and highly efficient in signal emission. As TPMS technology advances, the development of miniaturized antennas has become increasingly important without compromising performance.
Currently, common TPMS antenna designs include inverted-F spiral antennas [2-3] and small-loop antennas [4]. While the inverted-F spiral offers better performance, it occupies more space, whereas the small-loop antenna is compact but suffers from lower emission efficiency. In this study, a PCB-based spiral antenna was developed to meet the specific requirements of TPMS applications. The antenna was fabricated on a Teflon substrate measuring just 20 mm × 16.7 mm, significantly smaller than traditional helical antennas while maintaining similar performance. The use of printed metal wires on the PCB allows for precise control over dimensions, making the antenna not only compact but also easier to manufacture. Experimental results confirmed that the antenna functions effectively at 433.92 MHz and exhibits good omnidirectionality, meeting the necessary performance criteria for TPMS transmitters.
The structure of the PCB spiral antenna is illustrated in Figure 1. The antenna consists of 11 turns of spiral, with metal traces printed on both sides of a rectangular dielectric substrate. The metal wires have the same width, and through-holes are placed at both ends, with copper plating on the inner walls to connect the two layers. A feed point is connected to the largest via hole located in the upper right corner of the design, while the other vias maintain the same diameter. To ensure proper electrical connection, pads are added around each via hole in the layout.
Simulation was conducted using CST Microwave Studio, with a dielectric constant of 2.5 and a thickness of 1.6 mm. To achieve miniaturization, the minimum PCB fabrication process was selected, and certain parameters were fixed during simulation. The resonant frequency was adjusted by varying the length (L) and pitch (S) of the spiral. Simulation results showed that increasing L decreases the resonant frequency, while increasing S raises it. After optimization, the final antenna parameters were determined as shown in Table 2.
Due to the low input impedance of the antenna (~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, resulting in an S11 curve that shows a working bandwidth of 432.6–435.2 MHz (S11 < -10 dB). Although the bandwidth is narrow, the S11 at the operating frequency of 433.92 MHz reaches -40 dB, which is sufficient for reliable signal transmission.
The antenna was manufactured using a polytetrafluoroethylene (PTFE) substrate known for its stability and low loss. The physical dimensions of the antenna are 20 mm × 16.7 mm × 10 mm. The measured S11 curve closely matches the simulation results, confirming the feasibility of the design. However, due to the use of a homemade inductor, the measured S11 value is slightly higher than expected. The effective bandwidth remains within 432.2–435.3 MHz (S11 < -10 dB), and the S11 at 433.92 MHz is below -15 dB, making it suitable for use in TPMS modules. For practical applications, high-quality chip components can be used to improve matching performance.
The TPMS transmitter operates at a relatively low frequency of 433.92 MHz and is installed inside the tire, where space is extremely limited. This presents significant challenges in antenna design. To address these challenges, a compact PCB spiral antenna was designed and tested. The results show that the antenna provides good omnidirectional radiation, is lightweight, and meets the miniaturization requirements of TPMS. Additionally, the design offers advantages such as simple manufacturing, low cost, and easy integration with electronic circuits. However, due to its limited bandwidth, the antenna is best suited for fixed-frequency transmission. A separate receiving antenna would still be needed for full TPMS functionality.
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