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As the most important receiving device in navigation and positioning equipment, the antenna plays a role like a human's "ear"; it converts the electromagnetic wave energy sent by the satellite into an electric current that can be analyzed by electronic devices. Therefore, the performance of the antenna will directly affect the performance of the entire GPS product.
At present, the open civilian positioning systems of GNSS systems are mainly the US GPS L1 band, with a central frequency of 1575.42MHz; the Russian GLONASS L1 band, with a central frequency of 1602.5625MHz; the Chinese Beidou B1 band, 1561.098MHz, etc. When debugging GNSS antennas, small-sized (very small-sized) ceramic antennas can generally only be compatible with 2 frequency bands, while larger ones can be compatible with 3 frequency bands. This requires us to confirm customer needs during debugging; confirm whether to use single GPS or Beidou; or use a combination of GPS+Beidou, GPS+GLONASS, etc. In this way, there is a focus during debugging, and the performance can be optimized.
Types of GNSS antennas: Based on the installation method, they are divided into external antennas and internal antennas; based on the power supply method, they are divided into active antennas and passive antennas; based on the polarization method, they are divided into vertical polarization and circular polarization.
The following is a brief introduction to the most widely used ceramic patch antennas on the market:
1. Ceramic PATCH antenna structure diagram
1. Ceramic sheet:
The quality of ceramic powder and sintering process directly affect its performance. The ceramic sheets currently used in the market are mainly 2525, 1818, 1515, 1212, and 9*9. The larger the area of the ceramic sheet, the greater the dielectric constant, the higher the resonant frequency, and the better the reception effect; the thickness of the ceramic sheet will also have a significant impact on the performance of the antenna. If space allows, it is recommended to choose a thickness of 4mm.
2. Silver layer:
The silver layer on the surface of the ceramic antenna can affect the antenna resonant frequency. The ideal GNSS ceramic chip frequency is exactly 1575.42MHz, but the antenna frequency is very easily affected by the surrounding environment, especially when assembled in the whole machine. The silver coating shape must be adjusted to adjust the frequency to maintain it at 1575.42MHz. Therefore, when purchasing antennas, GPS whole machine manufacturers must cooperate with antenna manufacturers and provide whole machine samples for testing.
3. Feed point:
The ceramic antenna collects the resonant signal through the feed point and sends it to the back end. Due to the impedance matching of the antenna, the feed point is generally not in the center of the antenna, and most of them are single-biased or double-biased.
2. Recommendations for using passive antennas on our products
1. Our GNSS modules all have built-in LNA with 18dBm gain. The passive antenna with ceramic dielectric can be directly soldered to the GNSS_ANT PIN of the module. When laying out the product, the GNSS ceramic antenna should be placed upwards; the module can be placed on the other side of the PCB. In this way, the trace from the GNSS_ANT PIN to the antenna pad can be as short as possible.
2. Matching circuit: If the antenna pad is very close to the GNSS_ANT PIN of the module, then the matching circuit can be omitted. If the GNSS antenna is far away from the module GNSS_ANT PIN due to other reasons such as structure, it is recommended to reserve a pi-type matching circuit. The trace from the module GNSS_ANT PIN to the GNSS antenna pad must be controlled to have a 50 ohm characteristic impedance; if it is a multi-layer board, it is recommended that the impedance line be routed on the L1 layer, and the L2 layer be hollowed out to refer to the ground of L3. The trace width of the 2-layer board can refer to the trace width of the GSM antenna part.
3. Do not run wires under the antenna and make copper leak treatment to serve as the reflective surface of the antenna; see the figure below:
4. There should be no interference sources around the antenna, especially DCDC and other devices; in addition, there should be no metal devices higher than the GNSS antenna around it: as shown below:
3. Active Antenna
1. The structure and physical object of the active antenna are shown in the figure below:
The components of the GNSS active antenna in the red frame are: ceramic antenna, SAW filter, low-noise amplifier circuit, RF cable, and RF connector. The low-noise amplifier circuit is the part that amplifies and filters the signal.
2. The impact of PCB size on antenna performance:
The shape and area of the PCB that carries the ceramic antenna. Due to the ground bounce characteristic of GNSS, the performance of the patch antenna can be maximized when the PCB is 7cm×7cm without interruption. Although it is restricted by factors such as the appearance structure, try to maintain a considerable area and uniform shape. In addition, the selection of the amplifier circuit gain must be coordinated with the back-end LNA gain; it is generally not recommended that the active antenna gain exceed 29dBm, otherwise the signal oversaturation may cause self-excitation.
3. Internal and external antenna compatibility and power supply processing;
The reference circuit is the development board M4 of the Air800 module. R5 and R6 are designed to be compatible with ceramic PATCH antennas and active antennas. L6 and C38 are the power supply filter circuits for active antennas. The schematic diagram is shown below.
4. Power supply processing when the GNSS module uses an external antenna. PCB part; as shown below:
In the above figure, R5 and R6 are the active antenna designs compatible with the PAD. R5 is connected to the passive ceramic antenna pad; R6 is connected to the RF Connector of the active antenna. L6 and C38 are the filter circuits for powering the active antenna.
4. GPS Antenna Selection Recommendations
1. If the terminal structure space allows and the GNSS antenna can be uniformly installed and used with the face upward, and there are no large metal objects blocking the surrounding area, it is recommended to use a GNSS ceramic antenna. If space allows, try to choose a large-sized ceramic antenna.
2. When the terminal usage status cannot be guaranteed and the space is limited: such as mobile phones, badges with positioning functions; it is recommended to use FPC antenna
3. When the terminal installation environment is harsh and the GNSS performance is required to be high, it is recommended to use a GPS active antenna.
4. If the product cannot be installed and used properly, but the space is not limited, you can also choose an external rod antenna similar to GSM.
5. Requirements for antenna manufacturers when looking for antenna manufacturers to debug ceramic patch antennas
1. VSWR: The voltage standing wave ratio of the GPS antenna is generally required to be adjusted to about 1.5
2. Efficiency: Efficiency is generally required to be around 40%
3. Average Gain: The average gain is required to be -0.5dB
4. OTA: Most antenna manufacturers do not have an OTA test environment for GNSS antennas. After the antenna is debugged, it can be measured against actual test data. Generally, our GNSS measurement requirements are: the number of satellites that can be used for positioning is greater than 6, the strongest signal is around 45dB/Hz, and 3 satellite signals must be greater than 40 dB/Hz.
6. GPS Antenna Test Report (Reference)
1. Antenna Return loss and Smith chart:
2. Actual verification:
Most antenna manufacturers do not have an environment for GNSS antenna OTA performance testing, and most of them conduct actual network tests; if conditions permit, more road tests can be conducted.
Test environment: company rooftop; antenna facing upwards
Test direction: East-West
Test direction: North-South
Test direction: West to East
Test direction: North-South
7. GPS Antenna Manufacturer Selection
Domestic manufacturers that can produce GPS ceramic antennas by themselves mainly include manufacturers in Jiangsu, Jiaxing, Zhejiang, Shenzhen, Taiwan, etc. However, most antenna factories can complete the debugging of GPS antennas, but they will outsource the production to other manufacturers with independent ceramic powder formula and firing process.
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