The communication distance of wireless modules is an important indicator, and how to maximize the effective communication distance has always been a question of confusion. This article provides some explanations on antenna selection and usage based on debugging experience, hoping to help engineers quickly adjust communication distance.
 
1、 Types of antennas
With the advancement of technology, many manufacturers have launched various finished antennas to save research and development cycles. However, if the engineer chooses improperly, not only will it not achieve the desired effect, but it will also waste a lot of time and cost on troubleshooting and debugging, which is not worth the loss. This article will introduce several commonly used antennas and provide design suggestions based on practical experience in engineering, for your reference.
Next, we will introduce the commonly used antenna types:
(1) Board mounted PCB antenna: Made of PCB etching, it has low cost but limited performance and good adjustability. It can be used in large quantities for Bluetooth and WiFi wireless communication modules.
(2) SMT patch type: Commonly used are ceramic antennas, which occupy a small area, have high integration, are easy to replace, and are suitable for products with small space requirements. However, this type of antenna is slightly expensive and has a small bandwidth.
(3) External rod antenna: Good performance, no need for debugging, easy replacement, high gain, suitable for various terminal devices.
(4) FPC antenna: Connected through a feeder, it can be installed freely with high gain. It can usually be attached to the non-metallic casing of the machine with adhesive backing, suitable for products with high performance requirements and sufficient housing space.
The function of an antenna is to radiate RF signals into free space, and selecting the appropriate antenna has a significant impact on the transmission distance. Antennas are very sensitive to the surrounding environment, and in many cases, even if a suitable antenna is chosen, the expected effect may not be achieved. Due to some clients not being clear about the factors that need to be considered in antenna design, we provide some experience in practical engineering design to help clients better design their own circuits and PCBs and increase the chances of project success.
2、 Antenna selection
The primary parameter that affects the communication distance of wireless modules is the transmission power. The transmission power of wireless modules and the corresponding ideal transmission distance can be found in the manual. After determining that the transmission power meets the requirements, the selection of antennas and their directionality should be considered.
Firstly, the selection of antennas:
The main indicators of an antenna include the following: frequency range, standing wave ratio SWR or VSWR, antenna gain, polarization mode, and impedance. Frequency range can be selected as needed; The standing wave ratio should preferably be less than 1.5; Antenna gain also has an impact on transmission distance; Polarization methods are divided into linear polarization and circular polarization; The impedance needs to match the output impedance of the wireless module, usually 50 ohms. Special attention should be paid to the standing wave ratio parameter here, and it is best to test the SWR using a network analyzer after purchasing the antenna.
The comparison table between standing wave ratio, echo loss, and transmission power is shown in Table 1.
Table 1 Comparison Table of Standing Wave Ratio, Return Loss, and Transmission Power
As shown in the table above, when VSWR=1.5, the theoretical transmission power is 96%. When VSWR=2, the transmission power is only 88.9%. Some antennas have a standing wave ratio index less than 2. When selecting an antenna, it is best to have a standing wave ratio less than 1.5 to obtain a higher transmission power.
Secondly, the directionality of the antenna:
Antennas have directionality, which refers to the fact that antennas have different radiation or reception capabilities in different directions of space. The directional pattern is commonly used to measure antenna directionality. Figure 2 shows the directional pattern of an antenna in the frequency range of 2400MHz to 2500MHz.
When the antenna is placed vertically, the direction with the deepest red color is the direction with the strongest radiation or reception ability of the antenna. Therefore, when installing the antenna, try to install it in the direction pointed by the red color as much as possible to ensure sufficient signal quality. In addition, metal plates have a shielding effect on signals, so there should be no metal planes in the direction of transmission and reception.
Some antenna manuals also provide antenna directional patterns represented in two-dimensional diagrams, divided into H-Plane and E-Plane, as shown in Figure 3.
When testing wireless module communication with each other, the directionality of the antenna must be considered. When there is no obstruction in the communication space and the direction of the antenna corresponds to the strongest radiation direction, the communication distance can be maximized. If the antenna is installed improperly, it can lead to a shorter communication distance or even inability to communicate.
When testing wireless module communication, engineers often encounter weak signals, communication distances that do not meet the instructions in the manual, or high packet loss rates. If it is confirmed that the wireless module itself is not a problem, it is recommended to first test the performance of the antenna itself, and then test according to the direction of antenna signal radiation strength to obtain better test results.
3、 Antenna circuit
1. Matching circuit design
When designing the schematic, it is necessary to reserve a π - network between the antenna and module RF output pins. The impedance of the antenna is affected by factors such as PCB flooring, antenna installation, and surrounding metal. This network is reserved to match the antenna to 50 ohms when it deviates significantly from the 50 ohm impedance.
X1, X2, and X3 are all reactance components. If the antenna has a standard 50 ohm impedance, X2 and X3 can be soldered, and X1 can be connected to a 220PF capacitor or a 0 ohm resistor. When designing the PCB, these three devices have been placed as close as possible to the RF output pins of the module, and the connected transmission lines are short and straight. Do not lay the ground within a 1.5mm area around the matching components to reduce the impact of parasitic parameters on the matching circuit.
2. Microstrip Line Design
In PCB design, due to the output impedance of most antennas and modules being 50 ohms, in order to minimize energy reflection during transmission, the PCB lead between the RF output pins and the antenna should be a 50 ohm microstrip line. The commonly used board is FR4 (dielectric constant 4.2-4.6). Based on experience, when the line width is about 2.2 times the distance between the microstrip line and the reference layer, the characteristic impedance of the microstrip line is about 50 ohms. When designing, it is recommended to use microstrip line impedance control tools (ADS, txline, etc.) to calculate and complete the microstrip line design through actual debugging. As shown in the figure below, the ground layer below the microstrip line must be completely grounded, and more grounding holes need to be drilled on both sides of the microstrip line.
3. The influence of metal on antennas
If there are metal objects near the antenna, the metal can reflect electromagnetic waves, which not only affects the actual usage space of the antenna, increases the loss resistance of the antenna, reduces radiation efficiency, but also leads to the deterioration of antenna radiation performance. When installing the antenna, pay attention to:
a: The antenna should be at least 5mm away from the battery;
b: The antenna should be at least 4mm away from the shielding shell;
c: Do not use paint or coatings with metallic components on the surface of the casing when it is necessary to install it.
Shenzhen Xunman Technology Co., Ltd. is a development and production oriented technology limited company that integrates research and development and manufacturing, and is composed of professional wireless communication design and production technology personnel. The company focuses on the field of short distance wireless communication and multimedia players. Developed various WIFI and Bluetooth industry application solutions, achieving supporting functions for various products. The products include: WIFI module, wireless network card, wireless router, wireless AP, switch, Bluetooth module, Bluetooth speaker, Bluetooth GPS, etc. The company adheres to the corporate spirit of "professionalism, integrity, and innovation", has established a leading position in China, and has won the recognition of domestic and foreign brand merchants.
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