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How to choose antenna and design antenna circuit in wireless module
Date:2018-09-15  Source:http://atwzntx.com/en/  Pageviews:309

The communication distance of the wireless module is an important indicator. How to maximize the effective communication distance has always been a question for everyone. This article has made some explanations based on the debugging experience and the choice and use of the antenna.

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 the engineer chooses improperly, it will not only fail to achieve the desired effect, but will waste a lot of time and cost in troubleshooting and debugging. This article will introduce several commonly used antennas and combine the practical experience in engineering to give design suggestions for your reference.

Next, we will introduce the commonly used antenna types:

(1) On-board PCB antenna: It is made of PCB etching, which has low cost, but has 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 antenna, small footprint, high integration, easy to replace, suitable for products with small space requirements, but this type of antenna is slightly more expensive and has a smaller bandwidth.

(3) External rod antenna: good performance, no need for debugging, easy replacement, high gain, suitable for all kinds of terminal equipment.

(4) FPC antenna: connected by feeder, free installation, high gain, usually can be attached to the non-metallic casing of the machine with adhesive, suitable for products with high performance requirements and sufficient space.


 Figure 1 Common antenna

The role of the antenna is to radiate the RF signal into free space. At this time, choosing the right antenna has a great influence on the transmission distance. The antenna is sensitive to the surrounding environment, and in many cases it may not be as expected even if a suitable antenna is selected. Since some customers are unclear about the factors that need to be considered for antenna design, here we give some experience in actual engineering design, so that customers can better design their own circuits and PCBs and increase the chances of success.

Second, the choice of antenna

The primary parameter affecting the communication distance of the wireless module is the transmission power. Under the premise that the transmission power is determined to meet the demand, the antenna selection and the directivity of the antenna are considered.

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 as needed; the standing wave ratio is preferably less than 1.5; the antenna gain also has an effect 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. Pay special attention to the standing wave ratio parameters here. After purchasing the antenna, it is best to test the SWR with a network analyzer.

The comparison table of standing wave ratio, return loss and transmission power is shown in Table 1.

Table 1 Comparison of standing wave ratio and return loss, transmission power

 

As can be seen from the above table, 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 of less than 2. When using an antenna, it is better to be a standing wave ratio. Less than 1.5, a higher transmission power can be obtained.

Followed by the directionality of the antenna:

Antennas are directional, meaning that the antennas have different radiating or receiving capabilities in different directions of space. The direction of the antenna is usually measured using a pattern. Figure 2 shows a pattern with a frequency range from 2400MHz to 2500MHz.

 

Figure 2 Three-dimensional orientation of the antenna

When the antenna is placed vertically, the deepest direction of the red is the direction in which the antenna radiates or receives the strongest force. Therefore, when installing the antenna, install the antenna as far as possible in the direction of red, so as to ensure sufficient signal quality. In addition, the metal plate has a shielding effect on the signal, so there is no metal plane in the direction of transmission and reception.

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

 

Figure 3 Two-dimensional orientation of the antenna

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

When engineers test wireless module communication, they often encounter weak signals, communication distances that do not meet the requirements, or high packet loss rate. In the case of determining that the wireless module itself is ok, it is possible to test the performance of the antenna itself first, and then follow the antenna. Tested in the direction of strong signal radiation, you 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 in the antenna and module RF output pins. The impedance of the antenna is affected by factors such as the grounding of the PCB, the installation of the antenna, and the surrounding metal. This network is reserved to match the antenna to 50 ohms when the antenna deviates significantly from the 50 ohm impedance.

X1, X2, and X3 are all reactive components. If the antenna is a standard 50 ohm impedance, then X2 and X3 may not be soldered, and X1 may be connected to a 220 PF capacitor or a 0 ohm resistor. In the PCB design, these three devices are as close as possible to the RF output pin of the module, and the connected transmission line is short and straight. Do not lay the ground in the 1.5mm area around the matching component to reduce the influence of parasitic parameters on the matching circuit.

 

Figure 1 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 the reflection of energy during transmission, the PCB lead between the RF output pin and the antenna should be a 50 ohm microstrip line. The commonly used sheet is FR4 (dielectric constant 4.2-4.6). According to experience, when the line width is about 2.2 times the distance of the microstrip line from the reference layer, the characteristic impedance of the microstrip line is about 50 ohms. For specific design, it is recommended to use the microstrip line impedance control tool (ADS, txline, etc.) to calculate and complete the microstrip line design through actual debugging. As shown in the figure below, the pavement layer under the microstrip line must be a complete ground, and more grounding vias are needed on both sides of the microstrip line.

 

Figure 2 microstrip line

3, the impact of metal on the antenna

If there is an object with a metal material near the antenna, the metal can reflect the electromagnetic wave, which not only affects the actual use space of the antenna, increases the loss resistance of the antenna, reduces the radiation efficiency, and causes the radiation performance of the antenna to deteriorate. When installing the antenna, be careful:

a: The antenna must be at least 5mm away from the battery;

b: the antenna must be at least 4 mm away from the shield case;

c: When it is necessary to install the outer casing, do not use metal paint or plating on the outer surface of the outer casing.

See you here, about the antenna design skills of wireless products, have you learned?

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