LoRa Antenna - The Four Key Points of Antenna Development

LoRa is one type of LPWAN communication technology, which is a long-range wireless transmission scheme based on spread spectrum technology promoted and used by the American company Semtech. This scheme changes the traditional trade-off consideration between transmission distance and power consumption, and provides users with a simple system that can achieve long-range, long battery life, and large capacity, thereby expanding the sensing network.

Currently, LoRa mainly operates in free frequency bands worldwide, including 433MHz, 868MHz, and 915 MHz. LoRa technology features long-range, low-power consumption (long battery life), multi-node, and low-cost. The LoRa network mainly consists of four parts: terminals (with built-in LoRa modules), gateways (or base stations), servers, and clouds, and application data can be transmitted bidirectionally.

As LoRa technology continues to gain popularity in the industry, and due to its unique and superior transmission performance, the number of users applying LoRa technology is growing exponentially. However, since many users are new to radio frequency technologies such as LoRa, they may encounter challenging radio frequency circuit design problems during the LoRa antenna product development process. In fact, by mastering a few key points, the best performance of LoRa antenna can be achieved.

1, Matching circuit design

When designing the schematic, a π-type matching circuit should be reserved between the antenna joint and the antenna pin of the module. The impedance of the LoRa antenna is affected by factors such as the grounding of the circuit board, the shell, and the installation angle. Reserving this π-type matching circuit is to correct it to 50 ohms when the antenna deviates significantly from 50 ohms.

By default, the LoRa antenna impedance is relatively close to 50 ohms, and C17 and C18 in the diagram do not need to be soldered. L2 can be 220pF capacitor, 1nH inductor, or 0-ohm resistor, all of which are suitable. In special cases, such as when the antenna is installed inside a mold, the antenna is very small, or the higher-order harmonic suppression needs to be strengthened, these three matching components need to be adjusted.

In theory, the π-type matching circuit can match the antenna to 50 ohms regardless of the impedance value of the antenna. However, in reality, inductors and capacitors have internal resistance, which absorbs energy. If the antenna impedance is too small (a few ohms) or too large (thousands of ohms), matching the circuit to 50 ohms loses its meaning. This is because most of the energy is consumed by the internal resistance of the matching elements.

2, Microstrip line routing rules

The microstrip referred to here refers to the PCB routing between the antenna pin of the LoRa module and the antenna joint. Since the internal impedance of the module and the impedance of the antenna are both designed according to the 50-ohm standard, the best match is achieved when the characteristic impedance of the microstrip is also 50 ohms.

To obtain a microstrip with an impedance of around 50 ohms, on the one hand, impedance processing requirements can be provided to the PCB manufacturer, and capable PCB manufacturers can control the impedance of the trace by adjusting the trace width based on the parameters of the PCB material. On the other hand, after obtaining the PCB material parameters (mainly the dielectric constant) from the PCB manufacturer, the trace width can be calculated using software to control the impedance within the expected range.

Based on experience, if FR4 material is used (dielectric constant between 4.2 and 4.6), the characteristic impedance of the microstrip is closer to 50 ohms when the trace width is 2.2 times the distance between the microstrip and the reference layer. For example, in the case of a double-sided board with a thickness of 0.8 mm, a trace width of 1.7 mm can be used. However, it should be noted that the ground plane below the microstrip must be complete, and the distance between the microstrip and the copper ground plane should be set according to the results calculated by the impedance calculation software. The ground solder pads on both sides of the module's antenna pin must be well grounded.

Since the wavelength of the 470 MHz electromagnetic wave is relatively long, if the length of this microstrip routing is no more than 20 mm, the characteristic impedance of the trace between 25 and 75 ohms has little effect on the performance. In this case, a trace width of 25 mils is recommended.

3, PCB ground plane requirements.

In free space, the wavelength of an electromagnetic wave with a frequency of 470MHz is 63cm. If a standard half-wave dipole antenna is designed, the LoRa antenna should be at least half of the wavelength, which is 31.5cm. However, in practical applications, most products do not have such a large space reserved for LoRa antenna design, so spring antennas are commonly used. When using this type of spring antenna, the performance parameters of the LoRa antenna vary depending on the board it is connected to. This is because the spring is only a part of the entire antenna, and the other part is the ground on the circuit board. Therefore, when designing the ground on the circuit board, there are some principles to follow: first, the antenna should be mounted as vertically as possible to the circuit board, and second, the copper area of the ground should be as large and continuous as possible, and it is feasible to use dense through-holes to make the ground on both sides of the board continuous.

4, Antenna installation specifications

Antenna installation is also a key step. Antenna radiation is directional, and not every direction has equal energy radiation, just like when we speak, some directions have stronger sound while others weaker. When installing an LoRa antenna, it is necessary to align the direction of the strongest radiation of the antenna with the receiving antenna, so that the receiving antenna can receive the strongest signal. To achieve this, you must first know the radiation direction of the antenna.

In the absence of a professional antenna testing laboratory such as a darkroom, how can we test the radiation direction of the antenna? After the product is finalized, we can continuously send data and use a spectrum analyzer to test the signal strength at a certain distance from the product. Then, rotate the tested product and record the signal strength in each direction, which can draw the approximate directional radiation pattern of the product's antenna.

When installing near reflective surfaces such as walls, doors, and metal surfaces, it is necessary to pay attention to the effects of reflection. Both theoretical and test results have proved the following conclusion:

The best and worst positions alternate at a distance of λ/4; RX1 is better than RX2, for example, at 470MHz, the effect at a distance of 16CM from the reflective surface is much better than 32CM.

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