Introduction
GPS is a terminal that receives satellite signals for positioning or navigation. To receive the signal, an antenna must be used. GPS satellite signals are divided into L1 and L2, with frequencies of 1575.42MHZ and 1228MHZ, respectively, of which L1 is an open civilian signal, and the signal is circular polarization. The signal strength is about -166dBW, which is a relatively weak signal. These characteristics determine that special antennas should be prepared for the reception of GPS signals.
Details
SORT
1. GPS antennas are divided into vertical polarization and circular polarization.
Vertical polarization is not as effective as circular polarization. Therefore, except for special cases, the GPS antenna will use circular polarization and linear polarization.
2. GPS antenna is divided into internal antenna and external antenna from the placement method.
The location of the antenna assembly is also very important. The early GPS handheld machine mostly uses the flip-over antenna, at this time the antenna is basically isolated from the inside of the whole machine, EMI almost does not affect it, and the star collection 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 devices, the shell must be plated and well grounded, away from EMI interference sources, such as CPU, SDRAM, SD card, crystal oscillator, DC/DC.
In-car GPS applications will become more and more common. The shell of the car, especially the explosion-proof film of the car, has a serious obstacle to the GPS signal. An external antenna with magnets (which attach to the roof of the car) is necessary for in-car GPS.
3. From the power supply, it is divided into active and passive.
The external GPS is an active antenna, and the Da Gama GPS external antenna is basically an active antenna. Then the passive antenna does not contain the LNA amplifier, just the antenna body.
PROPERTY
The performance of GPS antenna is mainly affected by the following aspects.
1. ceramic sheet: the quality of ceramic powder and sintering process directly affect its performance. The ceramic chips used in the market are mainly 25×25, 18×18, 15×15, 12×12. The larger the area of ceramic sheet, the larger the dielectric constant, the higher the resonance frequency and the better the acceptance effect. Ceramic pieces are mostly square design, in order 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: ceramic antenna surface silver layer can affect the antenna resonance frequency. The ideal GPS ceramic chip frequency accurately falls at 1575.42MHz, but the antenna frequency is very susceptible to the influence of the surrounding environment, especially when assembled in the whole machine, it must be adjusted by adjusting the silver coating shape to maintain the frequency at 1575.42MHz. Therefore, GPS machine manufacturers must cooperate with antenna manufacturers when purchasing antennas, and provide samples of the whole machine for testing.
3. Feed point: The ceramic antenna collects resonance signals through the feed point and sends them to the back end. Due to antenna impedance matching, the feed point is generally not in the exact center of the antenna, but is slightly adjusted in the XY direction. This impedance matching method is simple and does not increase the cost. Moving only in a single axis direction is called a single biased antenna, and moving in both axes is called a double biased antenna.
4. amplification circuit: bearing ceramic antenna PCB shape and area. Due to the characteristics of GPS rebound from touching the ground, the performance of patch antenna can be maximized when the background is 7cm×7cm uninterrupted ground. Although restricted by the appearance of the structure and other factors, but as far as possible to maintain a comparable area and uniform shape. The amplifier circuit gain must be selected to match the back-end LNA gain. The GSC3F of Sirf requires that the total gain before the signal input should not exceed 29dB, otherwise the signal will be susaturated and self-excited.
GPS antennas have four important parameters: Gain, standing wave (VSWR), Noise figure and Axial ratio. In particular, the axial ratio is emphasized, which is an important index to measure the difference of signal gain of the whole machine in different directions. Since the satellites are randomly distributed over the hemispherical sky, it is important to ensure that the antenna has similar sensitivity in all directions. The axial ratio is affected by antenna performance, appearance structure, internal circuit and EMI.
Select
Shenzhen Yusheng Communication Equipment Co., Ltd. can produce its own GPS ceramic antenna.
Most manufacturers purchase ceramic parts from home and abroad to match the designed amplifier circuit.
Super intelligent terminals are becoming more and more the trend of The Times, and the small space puts high requirements on the size of accessories.
The original domestic technology is only mature above 15×15, and 10×10 is basically mature. The minimum can be about 7×7, the yield will be low.
Structure
Most of the GPS antennas are right-handed polarized ceramic media, and its components are: ceramic antenna, low noise signal module, cable, connector.
Among them, ceramic antenna is also called passive antenna, dielectric antenna, PATCH, which is the core technology of GPS antenna. The signal acceptance capacity of a GPS antenna depends largely on the composition of its ceramic part.
The low-noise signal module, also known as the LNA, is the part that 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 reflection as the standard to ensure the matching of impedance.
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