The quality of the ceramic material and the sintering process plays a critical role in determining the performance of a GPS antenna. In the current market, commonly used ceramic patch sizes include 25×25 mm, 18×18 mm, 15×15 mm, and 12×12 mm. Generally, a larger ceramic patch area provides a higher dielectric constant, resulting in a lower resonant frequency and improved GPS signal reception performance.
The silver electrode layer on the surface of a ceramic GPS antenna also has a direct impact on antenna resonance. The ideal operating frequency for a GPS ceramic patch antenna is 1575.42 MHz. However, the resonant frequency is highly sensitive to the surrounding environment, particularly when the antenna is integrated into a complete device. For this reason, the thickness and coverage of the silver coating often need to be adjusted during system integration to ensure the antenna remains tuned to 1575.42 MHz. As a result, GPS device manufacturers are advised to work closely with the antenna manufacturer and provide complete device samples for tuning and verification during antenna selection.
Feed Point Design and GPS Antenna Performance
The feed point location significantly affects the performance of a GPS ceramic antenna. The antenna receives the resonant signal through the feed point and transmits it to the RF backend. Due to impedance matching requirements, the feed point is typically offset from the center of the ceramic patch and adjusted along the X and/or Y axis.
Adjustment along one axis is commonly referred to as a single-offset (single-biased) antenna.
Adjustment along both axes is known as a dual-offset (double-biased) antenna.
This impedance matching approach is widely used because it is effective and does not increase manufacturing cost.
Amplifier Circuit and System-Level Performance
The PCB size and ground plane area supporting the ceramic antenna also influence GPS antenna performance. Due to GPS signal reflection characteristics, optimal performance is achieved when the antenna operates over an uninterrupted ground plane of approximately 7 cm × 7 cm. Although industrial design constraints may limit PCB dimensions, maintaining a uniform ground shape and area helps improve antenna efficiency.
The gain of the amplifier circuit must be carefully matched with the gain of the backend LNA. For example, systems using SiRF GSC 3F require the total gain before signal input to remain below 29 dB, otherwise the GPS signal may become oversaturated, leading to instability or self-oscillation.
Key Performance Parameters of GPS Antennas
A GPS antenna is typically evaluated using four critical parameters:
Gain
Voltage Standing Wave Ratio (VSWR)
Noise Figure
Axial Ratio
Among these, the axial ratio is particularly important, as it reflects the antenna’s ability to receive signals evenly from different directions. Since GPS satellites are distributed randomly across the hemispherical sky, maintaining consistent sensitivity in all directions is essential. The axial ratio is influenced not only by the antenna itself, but also by the mechanical structure, internal circuitry, and EMI conditions of the complete device.
