Antenna design tips for wireless products

The communication distance of the wireless module is an important indicator. How to maximize the effective communication distance has always been a question of doubt. This article gives some explanations based on the debugging experience and the selection and use of the antenna. It is hoped that it will help the engineer to quickly debug the communication distance.

First, the type of antenna

With the advancement of technology, in order to save the development cycle, many manufacturers have introduced a variety of finished antennas. However, if engineers choose improperly, they will not only fail to achieve the desired results, but will waste a lot of time and cost in troubleshooting, which is not worth the candle. This article will introduce several commonly used antennas and combine their practical experience in engineering to give design recommendations for your reference.

Next we introduce the commonly used antenna types:

(1) On-board PCB antenna: It is made of PCB etching and 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 ceramic antennas have small occupied area, high integration, and are easy to replace. They are suitable for products with small space requirements, but this type of antenna is slightly expensive and has a small bandwidth.

(3) External rod antenna: It has good performance, requires no debugging, is easy to replace, and has a high gain. It is suitable for various terminal devices.

(4) FPC antenna: It is connected by feeders, free to install, and has a high gain. It can usually be attached to the non-metallic housing of the machine, and it is suitable for products with high performance requirements and sufficient housing space.

Figure 1 common antenna

The role of the antenna is to radiate radio frequency signals to free space. Selecting the right antenna at this time has a great influence on the transmission distance. The antenna is very sensitive to the surrounding environment. In many cases, even if a suitable antenna is selected, the desired effect cannot be achieved. Since some customers do not understand the factors that need to be considered in the antenna design, here we give some experience in the actual engineering design, so that customers can better design their own circuit and PCB, increase the chance of success of the project.

Second, the choice of antenna

The primary parameter affecting the communication distance of the wireless module is the transmission power. The transmission power of the wireless module and the corresponding ideal transmission distance can be found in the manual. Under the premise that the transmission power is satisfied, the selection of the antenna and the antenna are considered. Directionality.

The first is the choice of antenna:

The main indicators of the antenna include the following: frequency range, standing wave ratio SWR or VSWR, antenna gain, polarization mode, and impedance. The frequency range is selected on demand; the VSWR is preferably less than 1.5; the antenna gain also has influence on the transmission distance; the polarization mode is divided into linear polarization and circular polarization; the impedance needs to match the output impedance of the wireless module, generally 50 ohms. Here we must pay special attention to the VSWR parameters. After purchasing the antenna, it is best to test the SWR with a network analyzer.

The VSWR, return loss and transmission power are shown in Table 1.

Table 1 Comparison of VSWR, return loss, and transmission power

From the above table, the theoretical transmission power is 96% when VSWR=1.5. When VSWR=2, the transmission power is only 88.9%. Some antennas have a VSWR of less than 2, and it is better to use VSWR when selecting an antenna. Less than 1.5 can get higher transmission power.

Followed by the antenna's directionality:

Antennas have directionality, which means that the antenna has different radiation or receiving capabilities in different directions in space. The directional pattern is usually used to measure antenna directivity. Figure 2 shows the pattern of an antenna with a frequency range from 2400MHz to 2500MHz.

Figure 2 Three-dimensional antenna pattern

When the antenna is placed vertically, the deepest direction of red is the direction in which the antenna radiates or receives the strongest, so when installing the antenna, the antenna should be installed in the direction of red as much as possible, so as to ensure good enough signal quality. In addition, the metal plate shields the signal, so there should be no metal plane in the direction of emission and reception.

There are also some antennas. The antenna patterns given in the manual are represented by two-dimensional diagrams and are divided into H-Plane and E-Plane, as shown in Figure 3.

Figure 3 Two-dimensional antenna pattern

When testing wireless modules to communicate with each other, the antenna directionality must be considered. When the communication space is not blocked and the antenna direction corresponds to the strongest radiation direction, the communication distance can reach the maximum. If the antenna is not installed properly, the communication distance will be shortened and even communication will not be possible.

When engineers test wireless module communication, they often encounter weak signals, communication distances do not meet the instructions in the manual, or the packet loss rate is high. In the case of determining that the wireless module itself has no problem, it may be necessary to first test the performance of the antenna itself, and then Then according to the direction of the radiation intensity of the antenna signal to test, will get better test results.

Third, the antenna part of the circuit

1, matching circuit design

In the schematic design, a π-type network needs to be reserved at the antenna and module RF output pins. The impedance of the antenna is affected by factors such as the PCB floor, antenna installation, and surrounding metal. The network is reserved to match the antenna to 50 ohms when the antenna is seriously deviated from the 50 ohm impedance.

X1, X2, and X3 are all reactance components. If the antenna is a standard 50 ohm impedance, then X2, X3 can be soldered, and X1 can be connected to 220 PF or 0 ohm. In PCB design, these three devices have been as close as possible to the RF output pins of the module, and the connected transmission lines are short and straight. Do not pave the area around the matching component within 1.5mm to reduce the effect of parasitic parameters on the matching circuit.

Figure 3 matching circuit

2, microstrip line design

In the PCB design, since most of the antenna and module output impedance is 50 ohms, in order to minimize energy reflection during transmission, the PCB lead between the RF output pin and the antenna should be a 50 ohm microstrip line. The commonly used plate is FR4 (dielectric constant 4.2-4.6). According to experience, when the line width is approximately 2.2 times the distance of the microstrip line from the reference layer, the characteristic impedance of the microstrip line is approximately 50 ohms. In the specific design, it is recommended to use the microstrip line impedance control tool (ADS, txline, etc.) to calculate, and to complete the design of the microstrip line through actual debugging. As shown in the figure below, the ground layer under the microstrip line must be a complete ground, with multiple ground vias on both sides of the microstrip line.

Figure 3 Microstrip line

3, the impact of metal on the antenna

If there is a metallic material near the antenna, the metal can reflect electromagnetic waves, which will not only affect the actual use of the antenna space, increase the loss resistance of the antenna, reduce the radiation efficiency, but also lead to deterioration of the antenna radiation performance. When installing the antenna, pay attention to:

a: At least 5mm from the battery to the antenna;

b: The antenna must be at least 4mm away from the shielding shell;

c: Do not use paint or plating with metal components on the surface of the housing when it is necessary to install the housing.