Yagi antenna principle analysis _ Yagi antenna production process

As an electromagnetic transducing element, the position of the antenna in the entire radio communication system is very important, and the quality directly affects the distance between the transceiving distance and the interlinking effect. It can be said that without the antenna there is no radio communication. As a classic directional antenna, Yagi antenna is widely used in HF, VHF and UHF bands. It is called "Yagi/Uda antenna". This article first introduced the principle of the Yagi antenna, followed by the process of self-made Yagi antenna, and the specific following Xiaobian together to find out about it.

The principle of yagi antenna's directional work can be deduced mathematically in detail based on electromagnetic theory, but it is cumbersome and complex, and ordinary readers are not easy to understand. Here only a qualitative and simple analysis is performed: we know that the wavelength λ is closely related to the electrical specifications of the antenna. Wires that are slightly longer than λ/4 are inductive, and wires that are slightly shorter than λ/4 are capacitive.

Since the main oscillator L uses a half-wave symmetric oscillator or a half-wave folded oscillator with a length of about λ/2, it is in a resonant state when operating at the center frequency, and the impedance exhibits a pure resistance, while the reflector A is slightly longer than the main oscillator and is inductive. Assume that the distance a between them 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 the induced electromotive force ε1 and the induced current I1, and then the distance λ/4 after the electromagnetic wave Reaching the reflector, the induced electromotive force ε2 and the induced current I2 are generated, and because the space differs by a distance of λ/4, ε2 lags 90° behind ε1, and because the reflector is inductive I2 lags 90° behind ε2, I2 lags behind ε1 by 180°. °, the reflector induces current I2 to produce radiation. The magnetic field H2 formed by reaching the main oscillator lags behind I2 by 90°. According to the law of electromagnetic induction, the induced electromotive force ε1′ produced by the H2 on the main oscillator lags behind H2 by 90°, that is, ε1′ is greater than ε1. The hysteresis is 360°, that is, the induced electromotive force ε1′ generated by the reflector in the main transducer is in phase with the induced electromotive force ε1 directly generated by the electromagnetic signal source, and the output voltage of the antenna is the sum of both.

Similarly, it can be deduced that, for a signal from a certain point behind the antenna, the induced electromotive force generated by the reflector in the main vibrator is directly opposite to the induced electromotive force generated by the signal, and it serves to cancel the output. The directors B, C, D, etc. are all slightly shorter than the main transducer and the impedance is capacitive. Assuming that the spacing between the transducers b, c, and d is also equal to λ/4, the signal from the director can also be pushed out by the above method. Plays a role in enhancing the antenna output. In summary, the reflector can effectively eliminate the back lobe of the antenna pattern and together with the director enhances the sensitivity of the antenna to the forward signal, so that the antenna has strong directionality and improves the antenna gain. For the launch status, the derivation process is also the same. In the actual production process, Yagi antennas operating at different center frequencies, having a certain bandwidth, a certain impedance value, and a good end-fire pattern can be obtained by careful design and proper adjustment of the length and spacing of each oscillator.

The Yagi antenna structure is shown in Figure 1. It consists of an active transducer, a reflector, and a number of directors. The reflector, which is slightly longer than the active transducer, acts as a reflected energy, and the director, which is slightly shorter than the active transducer, plays the role of guiding energy. The reflectors and directors on both sides of the active oscillator make the original bi-directional radiation become unidirectional radiation to increase the gain of the antenna. Yagi antenna is simple in structure, easy to feed, and has high gain. It is widely used in VHF/UHF bands.

1, antenna size

The number of units, the length, and the unit spacing of the Yagi antenna have a great influence on the antenna gain, front-back radiation ratio, and bandwidth. The theoretical calculation of the antenna size of Yagi is more complicated. In most cases, preliminary selection is made using some approximate formulas and empirical data, or a modification is made on the basis of a finished antenna, and then the relevant data is finally determined after repeated adjustment through experiments.

The determination of the size of the Yagi antenna needs to be considered in terms of tradeoffs among various performance indicators of the antenna. The length of the antenna reflector is 35 cm (0.5λ, wavelength λ=70 cm), the lengths of the three directors are equal, both are 31 cm (0.44λ), and the length of the active transducer is temporarily 34 cm (0.486λ). Also determine in antenna adjustment.

The spacing of the director is chosen to have two kinds of variable pitch and equal spacing. The cell pitch can be selected between 0.1λ and 0.34 in. When the distance between the director is large, the antenna gain is high; when the distance is small, the antenna has good frequency band characteristics. The pitch of this antenna director is 0.2λ. It should be noted that the spacing between the first director and the active oscillator is smaller, typically 0.14λ. The distance between the reflector and the active transducer is also 0.2λ. See Table 1 for the length and spacing of each unit of the antenna.

2. gamma matching

The first thing to resolve when connecting an antenna to a feeder is impedance matching. The so-called impedance matching is to transform the input impedance of the antenna to the characteristic impedance (usually 50Ω) of the feeder connected to it, so that the power of the radio output can be completely transmitted from the antenna.

Yagi antenna matching methods come in many forms. Figure 2 is a schematic diagram of a gamma matching connection. The core wire of the coaxial cable is connected to the γ bar via a variable capacitor. The cable shield is connected to the center of the active transducer. The short bar connects the active transducer to the γ bar and can move. Adjusting the variable capacitor capacity and shorting bar position enables the antenna to match. Gamma matching is an unbalanced type and can be directly connected to a coaxial cable. It is a very convenient matching method for amateur radio enthusiasts.

3, antenna production

The required materials for antenna production are shown in Table 2. All vibrators use Φ3mm copper electrodes. The crossbars can be made of 15mm, 15mm, and 70cm square tubes or aluminum alloy materials. First, cut 6 copper rods according to the size of Table 1 and make a hole mark in the corresponding position of the square tube. Use a Φ3mm drill bit and use a bench drill to pierce the square tube in the five holes of the square tube so that the copper electrode just fits into the crossbar. In order to facilitate adjustment and dismantling, a hole can be drilled on the top of the vibrator. A nut is welded and the screw is tightened to fix the vibrator. See Figure 3. Note that it is best to use a bench drill to drill a hole in a square pipe. It is difficult to control the direction with a pistol drill, which can easily cause the tilt of the vibrator.

Find a piece of 60mm&Times;15mm, 1mm thick, bent at right angles and drilled holes. The long side is fixed on the cross bar, 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 20 mm. Remove the paint between the iron plate and the square tube to ensure good contact. The shorting 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. A hole was made in the middle of the two aluminum plates, and screws were placed on the active and gamma rods. The distance between the two copper bars was adjusted to 20 mm, as shown in FIG. 4 . Finally, the capacitor was soldered to the core of the BNC socket and the γ bar, and the antenna fabrication was completed.

4, antenna adjustment

There are many factors affecting the performance of the Yagi antenna, and the Yagi antenna adjustment is also more complex than other antennas. Under amateur conditions we mainly adjust two parameters of the antenna: resonance frequency and standing wave ratio. That is, the resonant frequency of the antenna is adjusted to be around 435 MHz, and the standing wave ratio of the antenna is made as close to 1 as possible.

Set the antenna about 1.5m away from the ground and connect it to a standing wave table to start measurement. To reduce the measurement error, connect the antenna to the standing wave table and the radio to the standing wave table as short as possible. The antenna can be adjusted in three places: the capacity of the trimmer, the location of the shorting bar, and the length of the active transducer. The specific adjustment steps are as follows:

(1) Fix the shorting bar at a distance of 5~6cm from the crossbar;

(2) The frequency of the transmitter is adjusted to 435MHz, and the capacitance of the ceramic chip is adjusted to minimize the standing wave of the antenna.

(3) Measure the standing wave of the antenna from 430 to 440 MHz, every 2 MHz, and plot or list the measured data.

(4) Observe whether the frequency (antenna resonance frequency) corresponding to the minimum standing wave is near 435MHz. If the frequency is too high or too low, replace the active oscillator with a longer or a few millimeters longer to re-measure the standing wave;

(5) Change the position of the shorting rod a little and finely adjust the capacitance of the ceramic tile so that the antenna standing wave is as small as possible near 435 MHz.

Each time the antenna is adjusted to adjust one place, it is easy to find out the law of change. Due to the high frequency of operation, all adjustments should not be too large. For example, the capacitance of the trimmer capacitor connected in series with the γ rod is about 3 to 4 pF. Changing the zero point method (pF) will cause a large 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 influence on the measurement of the standing wave. This is something we should pay attention to in the adjustment process. Figure 5 shows the results of a standing wave (SWR) measurement after the antenna is adjusted.