GPS is a terminal that uses satellite signals for positioning or navigation. And receiving signals requires the use of antennas. GPS satellite signals are divided into L1 and L2, with frequencies of 1575.42 MHz and 1228 MHz respectively. L1 is an open civilian signal with circular polarization. The signal strength is around -166DBM, which is a relatively weak signal. These characteristics determine the need to prepare specialized antennas for GPS signal reception.
　　
construction
　　
The vast majority of GPS antennas are right-handed polarized ceramic media, consisting of ceramic antennas, low-noise signal modules, cables, and connectors.
　　
Ceramic antennas, also known as passive antennas or dielectric antennas PATCH， It is the core technology of GPS antenna. The signal reception capability of a GPS antenna largely depends on the composition and ingredients of its ceramic parts.
　　
The low-noise signal module, also known as LNA, is the part that amplifies and filters signals. The selection of its components is also important, otherwise it will increase the reflection loss of GPS signals and cause excessive noise.
　　
The selection of cables should also be based on reducing reflection to ensure impedance matching.
　　
influenceGPS antenna performanceThe main aspects are as follows
　　
1. Ceramic chips: The quality of ceramic powder and sintering process directly affect its performance. The ceramic tiles currently used in the market are mainly 25 × 25, 18 × 18, 15 × 15, and 12 × 12. The larger the area of the ceramic plate, the higher the dielectric constant, the higher the resonance frequency, and the better the acceptance effect. Most ceramic tiles are designed in a square shape to ensure consistent resonance in the XY direction, thereby achieving uniform star collection.
　　
2. Silver layer: The silver layer on the surface of ceramic antennas can affect the resonant frequency of the antenna. The ideal GPS ceramic chip frequency point accurately falls at 1575.42MHz, but the antenna frequency point is very susceptible to the influence of the surrounding environment, especially when assembled in the whole machine. It is necessary to adjust the silver coating shape to re maintain the frequency point at 1575.42MHz. Therefore, GPS manufacturers must cooperate with antenna manufacturers when purchasing antennas and provide complete machine samples for testing.
　　
3. Feed point: The ceramic antenna collects resonance signals through the feed point and sends them to the backend. Due to impedance matching of the antenna, the feed point is generally not located in the center of the antenna, but is adjusted slightly in the XY direction. This impedance matching method is simple and does not increase costs. Moving only in a single axis direction is called a single offset antenna, while moving in both axes is called a double offset antenna.
　　
4. Amplification circuit: The shape and area of the PCB carrying the ceramic antenna. Due to the ground bounce characteristic of GPS, when the background is a 7cm × 7cm uninterrupted ground, the effectiveness of the patch antenna can be maximized. Although constrained by factors such as appearance and structure, try to maintain a considerable area and uniform shape as much as possible. The selection of amplification circuit gain must be coordinated with the backend LNA gain. Sirf's GSC3F requires that the total gain before signal input should not exceed 29dB, otherwise signal supersaturation will result in self excitation.
　　
The GPS antenna has four important parameters: gain, standing wave ratio (VSWR), noise figure, and axial ratio. Special emphasis is placed on the axial ratio, which is an important indicator for measuring the difference in signal gain of the whole machine in different directions. Due to the random distribution of satellites in the hemisphere sky, it is crucial to ensure that the antenna has similar sensitivity in all directions. The axial ratio is affected by antenna performance, appearance structure, internal circuit of the whole machine, and EMI.