GPS is a terminal that performs positioning or navigation by receiving satellite signals. The antenna must be used to receive the signal. GPS satellite signals are divided into L1 and L2, and the frequencies are 1575.42 MHz and 1228 MHz, respectively. L1 is an open civil signal and the signal is circular polarization. The signal strength is about 166-DBM, which is a relatively weak signal. These characteristics determine the need to prepare a dedicated antenna for the reception of GPS signals.
The main aspects affecting GPS Antenna performance are the following :
1, ceramics: ceramic powder quality and sintering process directly affect its performance. The ceramic chips 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 greater the dielectric constant, and the higher the resonance frequency, the better the acceptance effect. Most ceramic tiles are designed in a square shape 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 resonant frequency of the antenna. The ideal GPS ceramic chip frequency accurately falls in 1575.42MHz, but the antenna frequency is very vulnerable to the surrounding environment, especially in the whole machine, must adjust the silver surface coating shape, to adjust the frequency point to keep at 1575.42MHz . Therefore, GPS manufacturers must match antenna manufacturers when purchasing antennas to provide complete machine 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 in the XY direction. This impedance matching method is simple and does not increase the cost. Only moving in a single axis direction is called a single bias antenna, and moving in both axes is called a double bias.
4. Amplification circuit: The shape and area of the PCB carrying the ceramic antenna. Because GPS has the characteristics of touchdown rebound, when the background is 7cm × 7cm without interruption, the performance of the
Patch Antenna can be maximized. Although constrained by appearance and other factors, try to maintain a considerable area and uniform shape. The choice of amplifier circuit gain must be matched with the back-end LNA gain. Sirf's GSC3F requires that the total gain before signal input must not exceed 29dB, otherwise the signal oversaturation will generate self-excitation.
The GPS antenna has four important parameters: Gain, VSWR, Noisefigure, and Axial Ratio. Among them, the axial ratio is particularly emphasized. It is an important index to measure the difference in signal gain of the whole machine for different directions. Since the satellites are randomly distributed on the hemisphere sky, it is very important to ensure that the antennas have similar sensitivities in all directions. Axis ratio is affected by antenna performance, appearance structure, internal circuit and EMI.
