foreword
The previous article introduced the Russian Internet celebrity Wi-Fi antenna, which can be regarded as a Yagi antenna. For the Yagi antenna, I believe that many friends in the industry are familiar with it, and this type of antenna looks like this. This article introduces the principle of Yagi antenna in detail, and designs and prints Yagi antenna.
Figure 1 Yagi Antenna
How Yagi Antennas Work
How Antennas Work
What is a Yagi antenna? The Yagi antenna is also known as the directional antenna, the wave channel antenna, and it is a kind of end-fired antenna. This kind of antenna was invented, which is called "Yagi-Uda antenna", or "Yagi antenna" for short. It has good directivity and high gain, and is suitable for long-distance communication. The relative bandwidth of traditional Yagi antenna is generally below 20%.
This type of antenna consists of a feed oscillator, a reflector oscillator and several passive parasitic oscillators placed side by side. All the oscillators are arranged side by side in a plane, and all these elements are fixed on the transmission line, that is, the metal rod in the middle above, see the figure below.
Figure 2 physical map
The Yagi antenna is a directional antenna. The driving oscillator and the reflector here are each composed of a pair of oscillators, which can be regarded as a dipole antenna. The length of the reflector oscillator will be slightly longer than the driving oscillator.
The more the number of guiding vibrators, the better the orientation of the antenna, but the more the number of guiding vibrators, the farther the distance from the driving vibrator, the weaker the induced current generated on the guiding vibrator, adding the guiding vibrator to the antenna orientation The sexual enhancement effect also gradually diminishes. The directivity of the Yagi antenna can be enhanced by increasing the number of guiding elements.
Figure 3 Working principle of Yagi antenna
The length of the guide oscillator of the Yagi antenna is slightly shorter than that of the reflector, and the impedance of each guide oscillator is capacitive, while the length of the reflector is slightly longer than that of the drive oscillator, and the impedance is inductive. The phases of the currents on the reflectors and steering elements are determined not only by their size, but also by the spacing of their adjacent cells. For receiving Wi-Fi signals, the guiding vibrator of the antenna is "capacitive" to the induction signal, and the current leads the voltage by 90°; the electromagnetic wave induced by the guiding vibrator will radiate to the driving vibrator, and the radiated signal passes through a quarter of the wavelength. Its lag of 90° just offsets the "advance" caused by the front, and the phases of the two are the same, so the signals are superimposed and strengthened. The reflector is slightly longer than one-half of the wavelength, and it is inductive. The current lags by 90°, and the radiation to the main oscillator lags by 90°. If one direction is strengthened and the other is weakened, there will be strong directionality.
A typical pattern for this type of antenna is shown below:
Figure 4 Yagi antenna radiation characteristics
2.2 Antenna parameters
Relevant theoretical experience, the calculation formula of the length of each vibrator of the Yagi antenna is as follows:
(1) Reflector length:
(2) Length of driving vibrator:
(3) Distance dR between the reflector and the driving oscillator:
(4) Guide vibrator length:
(5) Guide oscillator spacing:
According to the above formula, if the length of the guiding dipole and the distance between the guiding dipoles are equal, it is called a uniform antenna.
Printed Yagi Antenna Design
Basic feed type of PCB Yagi antenna
The printed Yagi antenna is printed on a single-layer dielectric board after planarizing the structure of the Yagi antenna, which reduces the volume while retaining the characteristics of traveling wave radiation, while maintaining excellent front-to-back ratio and cross-polarization value.
1) PCB Yagi antenna with balun structure
The figure below is a PCB Yagi antenna with a balun. Through this structure, the exciting oscillators on the left and right sides can form a phase difference of half a cycle, so that the active oscillator and the microstrip feeder can be designed on the same plane and impedance matched .
Figure 5 Yagi antenna with balun PCB
As can be seen from the figure above, the balun structure in the antenna is larger, which increases the structure of the antenna. At the same time, this structure will also affect the radiation direction of the antenna. At this time, Yagi without balun structure appears. The antenna can simplify the antenna structure.
2) PCB Yagi antenna without balun structure
The structure of a PCB Yagi antenna without a balun structure is as follows. The front microstrip line is connected to the microstrip feeder line on the front of the dielectric substrate, and the back microstrip line is connected to the half floor on the back of the dielectric substrate. This method achieves balanced and unbalanced The conversion can also make the excitation elements on the left and right sides form a phase difference of half a period.
Figure 6 Yagi antenna without balun
By comparing the two antennas with different feeding forms, the Yagi antenna without balun is relatively simple, and the directivity is not affected.
PCB Yagi Antenna Design
1) Antenna structure
Here we build a simple feed-type microstrip quasi-Yagi antenna without a balun structure. The antenna structure is shown in the figure below. The antenna is 100mm long and 50mm wide. As far as the structure of the red Wi-Fi antenna is concerned, the maximum size of the two antennas is similar; from the perspective of assembly complexity, the antenna studied in this study has the characteristics of one-time molding and is easy to process.
Figure 7 Antenna structure
2) Antenna working principle
The antenna structure is different from the two microstrip antennas introduced in the previous chapters: the front microstrip line is not connected to the feeder line, but is fed to the front oscillator through coupling; at the same time, the feeder line is connected to the back floor through the via hole, Excite the antenna element.
Three guide oscillators are placed in front of the excitation oscillator of the antenna, and the excitation oscillator on the reverse side of the PCB is connected to the reflection oscillator.
Figure 8 Working principle
3) Simulation result analysis
After building the model and setting the parameters, the simulation is carried out. The S11 parameter reaches -12.2dB at 2.42GHz, which has good standing wave characteristics.
Figure 9 S11 characteristics
The radiation characteristics of the antenna are shown below. It can be seen from the figure that it has strong directivity and the gain can reach 10dBi.
Figure 10 Radiation diagram
Figure 11 Current distribution of the front vibrator
Figure 12 The current distribution of the anti-oscillator
4) Antenna improvement
As can be seen from the current distribution in the above figure, the current amplitude at the third pilot dipole means that the antenna can further enhance the directivity by introducing the fourth pilot dipole, which will obtain a higher antenna gain than the current 10dBi. The antenna characteristics shown in the figure below are given in the next issue.
Figure 13 The front side of the PCB Yagi antenna after adding four guide oscillators
summary
In this issue, we have studied a microstrip Yagi antenna. By designing a relatively simple feeding method, the complex balun is omitted, the impedance matching of the antenna is realized, and the antenna structure is simplified. At the same time, its working principle is explained. And the parameters that affect the performance of the antenna are analyzed. The antenna has the characteristics of low cost and easy processing. It only needs to weld a SMA connector, and no additional processing is required.
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Original: Simulation and Design of Printed Yagi Antenna - RFASK Radio Frequency Questions
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