The snake-shaped on-board antenna is one of the most widely used types of antennas for wireless communication modules. It is used in fields such as Bluetooth, WiFi, and ZigBee, which have low performance requirements but high space requirements.
When designing the onboard serpentine antenna, grasp the basic principle, and then meet the design requirements according to the actual clearance of the clearance area provided by the board, combined with the surrounding environment of the antenna such as metal, large capacitor, inductor, shield.
1、Current distribution of a serpentine antenna
As shown below:
Figure 1 Analysis of current direction of serpentine antenna
It can be seen from the figure that the currents on the two adjacent bends of the serpentine trace are equal in magnitude and opposite in direction; from the principle of electromagnetic field generation, if the serpentine trace is adjacent to the two bends infinitely close, the electromagnetic radiation is completely Offset, no external radiation energy, the gain is very poor. Therefore, when designing the trace, it is necessary to balance the antenna area and miniaturization requirements with a given antenna "clear area". It is not possible to withdraw without the principle, in order to sacrifice the gain of the antenna in exchange for the beauty of the product.
2、The current common serpentine antennas mainly have the following types, as shown in the figure:
Figures① and ② show a conventional monopole serpentine antenna.
Figure ③ shows a serpentine trace with parasitic elements that increase bandwidth.
Figure ④ is a unipolar serpentine deformation-inverted F antenna
Example Design Demonstration
Now we take the B-type structure as an example to simply design a 2.45GHz Class B antenna structure model. The bending and segmentation of each segment of the antenna is as follows:
Figure 3 Antenna initial size settingHFSS
It should be noted that since the model is designed as a monopole antenna, the influence of the ground plane on the antenna should be fully considered in the design. The ground plane needs to have a large enough area to enable the antenna to obtain a good image and achieve f-shot. The model is as follows:
Figure 4 HFSS model
Return loss S11 simulation:
Figure 5 S11 simulation results
It can be seen from the simulation diagram that the simulation structure of S11 is relatively good, and it can fully meet the working frequency band and bandwidth requirements of 2.45 GHz.
Some friends may have doubts, because they are beginners of the antenna or have insufficient experience. They may not have enough experience when setting the initial size, which leads to poor simulation structure of the initial size, such as the deviation of the working frequency from the expected one, S11 Too big, etc., these situations are all there. Now let's analyze how we should solve this kind of situation:
1、Working frequency adjustment
The resonant frequency band of the antenna is determined by the effective current path length of the antenna. Therefore, to adjust the working frequency band, it is necessary to consider starting from the physical length of the antenna.
Generally, in our design, we need to reserve a branch with variable expression at the end of the serpentine antenna. As shown in the figure below, the branch with the length L indicated at the far right end. When optimizing, you only need to change the length of the segment simply. For example, I am now making an example on the model just created, so that L is equal to 1.5mm, 2mm, 2.5mm and 3mm, respectively, to solve the corresponding working frequency band, the solution results are as follows:
Figure 6 The effect of the length of L on the resonant frequency
It can be seen from the figure that when L changes, the resonant frequency of the antenna also changes very significantly. As L decreases, the resonant frequency of the antenna decreases.
2、 S11 improvement
The decisive factor of S11 is the input impedance of the antenna. Generally, the default input impedance of the monopole antenna is 50 ohms. When the designed antenna input impedance is infinitely close to 50 ohms, S11 will approach infinitely small. Otherwise, when the input impedance deviates from 50 In the case of ohms, S11 will be deteriorated. In other words, the larger the input impedance deviates from 50 ohms, the worse S11 is. For the antenna structure designed in this paper, the L2 short-circuit branch as shown in the figure below can adjust the input impedance of the antenna in the 2.45 GHz band by adjusting the length of L2, and then adjust the S11 parameter. I now make an example on the model, let L2 be equal to 4mm, 4.5mm, 5mm, 5.5mm and 6mm respectively, to see the corresponding value of S11, the simulation results are as follows:
Figure 7 Effect of short-circuit branch L2 on S11
It can be seen from the figure that when the length of L2 changes, the resonant frequency of the antenna remains almost the same, but S11 has a very significant change. As the length of L2 increases, S11 gradually becomes better.
Therefore, in the actual design, the S11 parameter can be improved by adjusting the short-circuit branch.