Performance of GPS Antenna
The performance of a GPS antenna is primarily influenced by the following factors:
1. Ceramic Sheet: The quality of the ceramic powder and the sintering process directly impact its performance. Commonly available ceramic chips in the market include sizes such as 25x25, 18x18, 15x15, and 12x12 mm. A larger ceramic sheet area results in a higher dielectric constant, resonance frequency, and improved reception. Ceramic chips are predominantly designed in a square format to ensure that resonance in the XY directions is nearly identical, thereby achieving uniform satellite signal acquisition.
2. Silver Coating: The silver coating on the ceramic antenna surface can influence the antenna's resonance frequency. Ideally, the frequency of a GPS ceramic chip should precisely align with 1575.42 MHz; however, this frequency is highly susceptible to environmental factors, particularly when integrated into a device. To maintain the frequency at 1575.42 MHz, adjustments to the silver coating shape may be necessary. Consequently, GPS device manufacturers should collaborate closely with antenna suppliers and provide complete device samples for testing purposes.
3. Feed Point: The ceramic antenna receives resonant signals through the feed point and transmits them to the backend. Due to the positive reactance matching requirement of the antenna, the feed point is typically not centered but slightly offset in the XY directions. This impedance matching technique is straightforward and cost-effective. An antenna with a single-axis offset is referred to as a single-biased antenna, while one with offsets in both axes is known as a double-biased antenna.
4. Amplification Circuit: The shape and area of the PCB supporting the ceramic antenna are crucial. Given the characteristics of GPS signals reflecting off the ground, a 7 cm x 7 cm uninterrupted ground plane optimizes the performance of a patch antenna. Despite constraints imposed by structural aesthetics and other factors, maintaining a substantial and uniformly shaped ground plane is advisable. The gain of the amplifier circuit must be compatible with the gain of the backend Low Noise Amplifier (LNA). For instance, the GSC3F chipset from SiRF stipulates that the total gain prior to signal input should not exceed 29 dB to prevent signal saturation and self-excitation.
Four key parameters define GPS antennas: Gain, Voltage Standing Wave Ratio (VSWR), Noise Figure, and Axial Ratio. Notably, the axial ratio is a critical metric for assessing the directional consistency of signal gain across the entire device. As satellites are randomly distributed across the hemispherical sky, ensuring the antenna exhibits comparable sensitivity in all directions is essential. The axial ratio is influenced by the antenna’s performance, physical structure, internal circuitry, and Electromagnetic Interference (EMI).
