Various PCB onboard antennas and PCB design points
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Antennas are important components for all kinds of smart devices, and all devices that need to use wireless need to use them. Now is the wireless era, network routers are all wireless WIFI, computers and mobile phones no longer need network cables to connect to the network, and Bluetooth headsets, Bluetooth mice, Bluetooth keyboards, etc. no longer have wires, so the performance of this antenna is crucial.
There are some factors in choosing an antenna. In addition to performance, cost must also be considered. Therefore, when choosing an antenna, comprehensive considerations are required. Today, Captain Shonway will talk to you about the design and design points of various antennas.
There are generally the following types of antennas:
The first type: PCB onboard antenna
This type of antenna is low cost, but the performance will be slightly worse. There are also several forms of PCB board antennas.
a, Planar Inverted F Antenna, abbreviated as PIFA
The following figure shows an inverted F PCB antenna.
Figure 1
Figure 2
The one below is a variation of the flat inverted-F PCB antenna above. Due to lack of space, it is twisted.
What are the issues that need to be paid attention to in the PCB design of this inverted F antenna? First of all, we need to know the knowledge of radio frequency. Shonway has published an article before. For radio frequency, any copper foil and wire cannot be regarded as a simple wire. It is an equivalent circuit composed of many resistors and capacitors. What you see as a short circuit is not a short circuit for radio frequency. With this idea, let's take a look at the PCB design of this inverted F antenna. As shown in the figure below
Figure 3
There are six points to note here
1. This inverted F antenna is not drawn casually. There is a library of this type of antenna on the Internet. Just take it and put it on as required. If there is not enough space, you can make your own dedicated antenna through simulation. Original Today's Headlines: Wolonghui IT Technology
2. The line impedance from the RF feed point must be 50ohm
3. The grounding feed point must be firmly grounded
4. The ground plane must have more ground vias, as shown in the figure above. If the spacing between the vias is appropriate, we have written an article about it before. You can find it and take a look.
5. All layers of copper foil on the antenna must be clear.
6. The antenna must be placed in the corner of the PCB board. It is best if all three sides are empty. As shown in Figure 2, the top three sides are empty.
The antenna on the mobile phone is called a planar inverted F antenna. In principle, it is composed of a plane connected to a ground plane feed point and an RF feed point, as shown in Figure 4 below.
Figure 4
In Figure 4 above, looking from the arrow at the RF feed point on the lower left, it is an inverted F. It is also an inverted F structure, but the antenna in the mobile phone adopts a planar structure. This inverted F antenna has much better performance than the PCB board antenna, which takes up less space and has low cost, making it the best choice for mobile phone antennas.
The actual antenna is shown in Figure 5 below
Figure 5
In fact, this plane has many shapes for different mobile phones. The principle is a planar inverted F structure. On this plane, one is connected to RF and the other is connected to the ground feed point to form a planar inverted F antenna.
Figure 6
The above picture shows different mobile phone antennas. Their principles are all planar inverted F antennas. Did you learn something new? Remember to like it.
b, Inverted L-shaped PCB onboard antenna
As shown in Figure 7 and Figure 8 below, this is a variation of the inverted L-shaped antenna. It is also twisted to match the frequency due to insufficient space.
Figure 7
Figure 8
The issues that need to be paid attention to for this inverted L-shape are similar to the previous ones and will not be explained again. The inverted L-shaped antenna is not as effective as the inverted F-shaped antenna because the inverted F antenna has a ground feed point that can effectively adjust the frequency.
There are many PCB board antennas on the market, mainly the two types above, and some as shown in the figure below
Fig. 9
Fig.10
Fig.11
Some are made by the manufacturers themselves through simulation.
The second type: patch ceramic antenna
This antenna is made into a patch component, as shown in the figure below
Fig.12
One end of this antenna is connected to RF and the other end is grounded. The principle of ceramic antenna is to convert the high-frequency electric field formed between the antenna and the ground into electromagnetic waves through an electrode called "antenna", so that it can be emitted and transmitted to a distant place.
The best layout and wiring method for PCB is the following method
Fig.13
Place the ceramic patch antenna on the edge of the board, ground it on one side and connect it to the RF signal on the other side. Hollow out all the copper foil layers below (the area shown in the white box). In this way, at least two of the four directions are empty, which is very effective for the antenna. Don't forget to make ground vias on the grounding copper foil, and make more of them.
The third type: rod antenna
This type of antenna is shown in Figure 14 below. This type of antenna has the best effect. It is placed in space and has the best radiation effect, but it is also a bit more expensive and occupies a large space. It can only be exposed outside the casing.
Fig.14
Issues to be aware of when designing PCB for this type of antenna
1. If the RF lead is short, all layers below the RF signal line must be cleared, as shown in Figure 15. If the lead is long, the impedance of this lead must be controlled. For multi-layer boards, the second layer below it needs to be cleared, and the other layers are covered with full copper. Then the interlayer reference ground is used as impedance. (Why is the interlayer reference needed? Please give your opinions in the comment area.) The impedance is controlled at 50ohm. As shown in Figure 16.
2. The nearby grounding copper foil must be firmly grounded, which means more ground holes must be drilled.
Fig.15
Fig.16
Bluetooth antenna design: inverted F-type antenna:
The antenna body of the inverted F-type antenna can be linear or sheet-shaped. When using insulating materials with a higher dielectric constant, the size of the Bluetooth antenna can be reduced. As a type of onboard antenna, the inverted F-type antenna has a low design cost but increases a certain volume. It is the most common type in practical applications. The antenna is generally placed on the top layer of the PCB, and the ground is generally placed on the top layer and near the antenna. However, the ground must not be placed around the antenna, and there should be a clear area around it. As shown in the following figure:
Figure 1: Schematic diagram of inverted-F antenna design
Bluetooth antenna design: meander antenna design:
The length of the meander antenna is difficult to determine. The length is generally slightly longer than a quarter wavelength, and its length is determined by its geometric topological space and the area of application. The meander antenna is generally a PCB package, that is, a board-mounted antenna. Like the inverted F type, the antenna is generally placed on the top layer of the PCB, and the ground is generally placed on the top layer and near the antenna, but the ground must not be placed around the antenna, and there should be a clear area around it. As shown in Figure 2 and Figure 3:
Figure 2: Schematic diagram of meander antenna design 1
Figure 3: Schematic diagram of meander antenna design 2
Note: Antenna length calculation formula:
Antenna length (meters) = (300/f)*0.25*0.96
, where f represents frequency (MHz) and 0.96 represents wavelength shortening rate.
The length of the Bluetooth antenna is approximately 300/2.4G*0.25*0.96, which is approximately 31mm.
Bluetooth antenna design ceramic antenna design:
Ceramic antenna is another type of miniaturized antenna suitable for use in Bluetooth devices. Ceramic antennas are divided into block ceramic antennas and multilayer ceramic antennas. Since the dielectric constant of ceramic itself is higher than that of PCB circuit boards, the use of ceramic antennas can effectively reduce the size of the antenna. In terms of dielectric loss, the ceramic medium is also smaller than the dielectric loss of PCB circuit boards, so it is very suitable for use in Bluetooth modules with low power consumption. When designing the PCB, just leave space around the antenna, and pay special attention to not applying copper. As shown below:
Figure 4: Schematic diagram of ceramic antenna design
Bluetooth antenna design 2.4G rod antenna design:
The 2.4G rod-shaped Bluetooth antenna is large in size, but its transmission distance is stronger than other antennas. When designing the PCB, the area around the antenna should be cleared as in the above three antenna designs. See the figure below:
Figure 5: 2.4G rod antenna design diagram
Other points to note about Bluetooth antenna design:
1) The antenna signal (frequency greater than 400MHz) is susceptible to attenuation, so the distance between the antenna and the nearby ground must be at least three times the line width.
2) For microstrip lines and strip lines, the characteristic impedance is related to the thickness of the board layer, line width, vias, and the dielectric constant of the board.
3) Vias will produce parasitic inductance, which will cause very large attenuation of high-frequency signals, so try not to have vias when running RF lines.
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