As an electromagnetic transducer, the position of the antenna in the entire radio communication system is very important. The quality of the antenna directly affects the distance and communication effect of the transmission and reception. It can be said that without the antenna, there will be no radio communication.
As a classic directional antenna, the Yagi antenna is widely used in HF, VHF and UHF bands. The Yagi antenna is an end-fire antenna composed of an active element (usually a folded element), a passive reflector and several passive directors arranged in parallel. In the 1920s, this antenna was invented by Hideji Yagi and Tasaki Uda of Tohoku University in Japan. It was called "Yagi Uda Antenna", or "Yagi Antenna" for short. This article first introduces the principle of the Yagi antenna, and then explains the process of making a Yagi antenna. Let’s follow the editor to learn more about it.
The principle of Yagi antenna
The principle of the directional operation of the Yagi antenna can be deduced in detail based on electromagnetic theory, but it is more complicated and difficult for ordinary readers to understand. Here is only a simple qualitative analysis: we know that the wavelength λ is closely related to the electrical indicators of the antenna. , The wire length slightly longer than an integer multiple of λ/4 is inductive, and the wire length slightly shorter than an integer multiple of λ/4 is capacitive.
Since the main vibrator L adopts a half-wave symmetrical vibrator or a half-wave folded vibrator with a length of about λ/2, it is in a resonance state when working at the center frequency point, and the impedance appears as a pure resistance, while the reflector A is slightly longer than the main vibrator and is inductive Assuming that the distance a between the two is λ/4, taking the receiving state as an example, the electromagnetic wave coming from a certain point in front of the antenna will reach the main oscillator first, and generate induced electromotive force ε1 and induced current I1, and then after a distance of λ/4, the electromagnetic wave will reach the main oscillator. Reaching the reflector, induced electromotive force ε2 and induced current I2 are generated. Due to the distance of λ/4 in space, ε2 lags behind ε1 by 90°, and because the reflector is inductive I2 lags behind ε2 by 90°, so I2 lags behind ε1 by 180° °, the magnetic field H2 formed by the reflector induced current I2 to reach the main oscillator lags behind I2 by 90°. According to the law of electromagnetic induction, the induced electromotive force ε1' on the main oscillator lags behind H2 by 90°, that is, ε1 lags behind ε1. 360°, that is, the induced electromotive force ε1' generated by the reflector in the main oscillator and the induced electromotive force ε1 directly generated by the electromagnetic signal source are in phase, and the antenna output voltage is the sum of the two.
In the same way, it can be deduced that for the signal from a certain point behind the antenna, the induced electromotive force generated by the reflector in the main oscillator and the induced electromotive force directly generated by the signal are in antiphase, which has the effect of offsetting the output. The directors B, C, D, etc. are all slightly shorter than the main vibrator, and the impedance is capacitive. Assuming that the distance between the vibrators b, c, and d is also equal to λ/4, the signal from the director to the front can also be deduced according to the above method. Plays the role of enhancing the antenna output. In summary, the reflector can effectively eliminate the backlobe of the antenna pattern, and work with the director to enhance the sensitivity of the antenna to the forward signal, so that the antenna has strong directivity, and the antenna gain is improved. For the launch state, the derivation process is also the same. In the actual production process, through careful design and proper adjustment of the length and spacing of each vibrator, it is possible to obtain Yagi antennas that work at different center frequencies, have a certain bandwidth, a certain impedance value, and a better end-fire pattern.
Yagi antenna production process
The structure of the Yagi antenna is shown in Figure 1. It consists of an active vibrator, a reflector and several directors. Among them, the reflector slightly longer than the active vibrator plays the role of reflecting energy, and the director that is slightly shorter than the active vibrator plays the role of guiding energy. The reflectors and directors on both sides of the active oscillator make the original two-way radiation into one-way radiation to improve the gain of the antenna. Yagi antennas are simple in structure, convenient to feed, and have high gains, and are widely used in VHF/UHF frequency bands.
1. Antenna size
The number, length and spacing of the Yagi antenna elements have a great impact on the antenna gain, front-to-rear radiation ratio and bandwidth. The theoretical calculation of the size of the Yagi antenna is more complicated. In most cases, some approximate formulas and empirical data are used for preliminary selection, or modified on the basis of a finished antenna, and then the relevant data is determined after repeated adjustments through experiments.
The size of the Yagi antenna needs to be compromised from the various performance indicators of the antenna. The length of the antenna reflector is 35 cm (0.5λ, wavelength λ=70cm), the lengths of the three directors are equal, all of which are 31cm (0.44λ), the length of the active oscillator is temporarily set to 34cm (0.486λ), the actual length It must be determined in the antenna adjustment.
There are two options for the pitch of the director: variable pitch and equal pitch. The unit spacing can be selected from 0.1λ to 0.34μm. When the distance between the directors is large, the antenna gain is high; when the distance is small, the frequency band characteristics of the antenna are good. The distance of the antenna director is 0.2λ. It should be noted that the distance between the first director and the active oscillator should be smaller, generally 0.14λ. The distance between the reflector and the active oscillator is also 0.2λ. The length and spacing of each element of the antenna are shown in Table 1.
2. γ matching
The first thing to be solved when the antenna is connected to the feeder is the impedance matching problem. The so-called impedance matching is to transform the input impedance of the antenna to the characteristic impedance value of the feeder connected to it (usually 50Ω), so that all the power output by the radio can be emitted from the antenna.
There are many forms of matching methods for Yagi antennas. Figure 2 is a schematic diagram of γ matching connection. The core wire of the coaxial cable is connected to the gamma rod through a variable capacitor, the cable shielding layer is connected to the center of the active vibrator, and the shorting bar connects the active vibrator and the gamma rod and can move. Adjusting the capacity of the variable capacitor and the position of the shorting bar can make the antenna reach a matching state. γ matching is an unbalanced type, which can be directly connected to a coaxial cable. It is a very convenient matching method that amateur radio enthusiasts love.
3. Antenna production
The materials required for antenna production are shown in Table 2. All the vibrators are made of Φ3mm copper welding rods, and the crossbars can be 15mm & TImes; 15mm, 70cm long square tubes or aluminum alloy materials. First, cut 6 copper rods according to the dimensions in Table 1, and make punch marks on the corresponding positions of the square tubes. Use a Φ3mm drill bit, and use a bench drill to punch the square pipe through the five holes of the square pipe, so that the copper electrode can just be inserted into the cross bar. To facilitate adjustment and disassembly, drill a hole above the vibrator, weld a nut, and tighten the screw to fix the vibrator. See Figure 3. Note that it is best to use a bench drill to drill the square pipe. It is not easy to control the direction with a pistol drill, and it is easy to cause the vibrator to tilt.
Find a piece of 60mm&TImes;15mm, 1mm thick iron sheet, bend it at a right angle and drill a hole. The long side is fixed on the crossbar, and the short side is equipped with a BNC socket. The vertical distance from the center of the socket to the active vibrator is about 20mm. File off the paint between the iron sheet and the square tube to ensure good contact. The short-circuit bar can be made of two pieces of aluminum or copper with a size of 30 mm & TImes; 10 mm and a thickness of about 1-2 mm. Punch a hole in the middle of the two aluminum sheets, install a screw, and clamp it between the active vibrator and the γ rod, and adjust the distance between the two copper rods to 20 mm, as shown in Figure 4. Finally, solder the ceramic capacitor on the core wire and γ rod of the BNC socket, and the antenna production is completed.
4. Antenna adjustment
There are many factors that affect the performance of the Yagi antenna, and the adjustment of the Yagi antenna is also more complicated than other antennas. Under amateur conditions, we mainly adjust the two parameters of the antenna: resonant frequency and standing wave ratio. That is, adjust the resonant frequency of the antenna to around 435MHz, and make the standing wave ratio of the antenna as close to 1 as possible.
Set up the antenna about 1.5m from the ground, connect the standing wave meter to start measurement. In order to reduce the measurement error, the cables connecting the antenna to the standing wave meter and the radio to the standing wave meter should be as short as possible. The antenna can be adjusted in three places: the capacity of the fine-tuning capacitor, the position of the shorting bar and the length of the active oscillator. The specific adjustment steps are as follows:
(1) Fix the short-circuit bar at a distance of 5-6cm from the crossbar;
(2) Adjust the frequency of the transmitter to 435MHz, and adjust the ceramic capacitor to minimize the standing wave of the antenna;
(3) Measure the standing wave of the antenna from 430 to 440 MHz at every interval of 2 MHz, and plot or list the measured data.
(4) Observe whether the frequency corresponding to the minimum standing wave (antenna resonance frequency) is around 435MHz. If the frequency is higher or lower, you can replace it with an active oscillator that is a few millimeters longer or shorter to measure the standing wave again;
(5) Change the position of the shorting bar slightly, and fine-tune the ceramic capacitor repeatedly to make the antenna standing wave as small as possible near 435MHz.
When adjusting the antenna, adjust one place at a time, so that it is easy to find the law of change. Due to the high operating frequency, the adjustment range should not be too large. For example, the adjusted capacity of the fine-tuning capacitor connected in series to the γ rod is about 3~4pF. Changing a few fractions of picofarad (pF) will cause a great change in the standing wave. In addition, many factors such as the length of the crossbar and the position of the cable will also have a certain impact on the measurement of the standing wave. This is what we need to pay attention to during the adjustment process. Figure 5 is the measurement result of the standing wave (SWR) after the antenna is adjusted.
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