Factors Affecting the Performance of GPS Ceramic Antennas

As an important category of antennas, GPS antennas play a crucial role in fields such as agriculture, aviation, environment, maritime transportation, public safety and disaster relief, railways, space, and surveying and mapping. This article will briefly introduce four factors that affect the performance of GPS ceramic antennas.

I. Ceramic Chip

The quality of ceramic powder and the sintering process directly affect its performance. The ceramic chips currently used in the market are mainly 25×25mm, 18×18mm, 15×15mm, and 12×12mm. The larger the area of the ceramic chip, the higher the dielectric constant, the higher its resonance frequency, and the better the reception effect. Most ceramic chips are designed in a square shape to ensure that the resonance in the X and Y directions is basically consistent, thereby achieving uniform satellite reception.

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II. Silver Layer

The silver layer on the surface of the ceramic antenna can affect the antenna's resonance frequency. An ideal GPS ceramic chip has a frequency point accurately falling at 1575.42MHz, but the antenna's frequency point is very susceptible to the surrounding environment. Especially when assembled in the whole machine, it is necessary to adjust the shape of the silver coating to adjust the frequency point to re-maintain it at 1575.42MHz. Therefore, when purchasing antennas, GPS complete machine manufacturers must cooperate with antenna manufacturers and provide complete machine samples for testing.

III. Feed Point

The ceramic antenna collects resonance signals from the feed point and transmits them to the backend. Affected by antenna impedance matching, the feed point is usually not located in the exact center of the antenna but is slightly adjusted along the X and Y directions. This impedance matching method is both simple and does not increase costs. Moving only in a single-axis direction is called a single-offset antenna, and moving in both axes is called a dual-offset antenna.

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IV. Amplifier Circuit

It carries the PCB shape and area of the ceramic antenna. Due to the ground bounce characteristics of GPS, when the background is a 7×7cm uninterrupted ground, the efficiency of the patch antenna can be maximized. Although limited by the appearance structure and other aspects, it is necessary to maintain a considerable area and consistent shape as much as possible. The gain selection of the amplifier circuit must match the gain of the backend LNA. As specified in Sirf's GSC3F, the total gain before signal input should not exceed 29dB; otherwise, signal oversaturation will cause self-oscillation.

GPS antennas have four important parameters: Gain, VSWR (Voltage Standing Wave Ratio), Noise Figure, and Axial Ratio. Among them, Axial Ratio is particularly emphasized, which is an important indicator to measure the difference in signal gain of the whole machine in different directions. Since satellites are randomly distributed in the hemispherical sky, it is very important to ensure that the antenna has similar sensitivity in all di