GPS antenna
GPS is a terminal that performs positioning or navigation by receiving satellite signals. An antenna must be used to receive the signal. GPS satellite signal is divided into L1 and L2, the frequency is 1575.42MHZ and 1228MHZ respectively, among them L1 is the open civilian signal, the signal is circular polarization. The signal strength is about -166dBW, which is a relatively weak signal. These characteristics determine the need to prepare a special antenna for GPS signal reception.
Classification
⒈In terms of polarization, GPS antennas are divided into vertical polarization and circular polarization.
The effect of vertical polarization is not as good as circular polarization. Therefore, except for special circumstances, GPS antennas will use circular polarization and linear polarization.
⒉In terms of placement, GPS antennas are divided into built-in antennas and external antennas.
The assembly location of the antenna is also very important. Early GPS handsets mostly used flipped antennas. At this time, the antenna is basically isolated from the inside of the whole device, and EMI hardly affects it, and the satellite reception effect is very good. With the trend of miniaturization, GPS antennas are mostly built-in. At this time, the antenna must be above all metal components, and the inside of the shell must be electroplated and well grounded, away from EMI interference sources, such as CPU, SDRAM, SD card, crystal oscillator, and DC/DC.
The application of car GPS will become more and more common. The car shell, especially the car anti-explosive film, has a serious obstacle to GPS signals. An external antenna with a magnet (that can be attached to the roof of the car) is very necessary for car GPS.
3. In terms of power supply, it is divided into active and passive.
External GPS is an active antenna, such as gamma GPS external antenna is basically an active antenna. The passive antenna does not contain the LNA amplifier, but the antenna itself.
The main factors affecting the performance of GPS antennas are as follows:
1. Ceramic chip: The quality of ceramic powder and the sintering process directly affect its performance. The ceramic sheets currently used in the market are mainly 25×25, 18×18, 15×15, and 12×12. The larger the area of the ceramic sheet, the larger the dielectric constant, the higher the resonance frequency, and the better the receiving effect. The ceramic sheets are mostly square in design to ensure that the resonance in the XY direction is basically the same, so as to achieve the effect of uniform star collection.
2. Silver layer: The silver layer on the surface of the ceramic antenna can affect the antenna resonance frequency. The ideal GPS ceramic chip frequency point accurately falls at 1575.42MHz, but the antenna frequency point is very susceptible to the surrounding environment, especially when assembled in the whole machine, the frequency point must be adjusted to maintain the frequency point at 1575.42MHz by adjusting the shape of the silver coating. . Therefore, GPS manufacturers must cooperate with the antenna manufacturers when purchasing antennas and provide complete samples for testing.
3. Feed point: The ceramic antenna collects the resonance signal through the feed point and sends it to the back end. Due to antenna impedance matching, the feed point is generally not in the center of the antenna, but slightly adjusted in the XY direction. Such impedance matching method is simple and does not increase the cost. Only moving in the direction of a single axis is called a single-biased antenna, and moving in both axes is called a double-biased antenna.
4. Amplifying circuit: the shape and area of the PCB carrying the ceramic antenna. Because GPS has the characteristics of ground bounce, when the background is 7cm×7cm uninterrupted ground, the effectiveness of the patch antenna can be exerted to the extreme. Although restricted by factors such as appearance and structure, try to keep a considerable area and uniform shape. The selection of the gain of the amplifying circuit must match the gain of the back-end LNA. Sirf's GSC3F requires that the total gain before signal input should not exceed 29dB, otherwise the signal will be over-saturated and self-excitation will occur.
GPS antenna has four important parameters: gain (Gain), standing wave (VSWR), noise figure (Noise figure), axial ratio (Axial ratio). Among them, the shaft ratio is particularly emphasized, which is an important indicator to measure the difference of the signal gain of the whole machine in different directions. Since the satellites are randomly distributed on the hemispherical sky, it is very important to ensure that the antennas have similar sensitivity in all directions. The axial ratio is affected by the antenna performance, appearance structure, internal circuit of the whole machine and EMI.
Structure
The vast majority of GPS antennas are right-handed polarized ceramic media, and their components are: ceramic antennas, low-noise signal modules, cables, and connectors.
Among them, ceramic antenna is also called passive antenna, dielectric antenna, PATCH, which is the core technology of GPS antenna. The signal receiving ability of a GPS antenna is largely determined by the composition and ingredients of its ceramic part.
The low-noise signal module is also called LNA, which amplifies and filters the signal. The selection of its components is also very important, otherwise it will increase the reflection loss of the GPS signal and cause excessive noise.
The selection of cables should also be based on reducing reflections to ensure impedance matching.
