Keywords: the principle, production and adjustment of eight wood antennas
Due to the strong anti-interference resistance of FM broadcast, the source of the sound quality and audio, has become the preferred edition of the majority of broadcast enthusiasts, but limited to the line of view transmission characteristics of the VHF band electromagnetic wave, the radius of the frequency modulation broadcast program is smaller, Improve long-distance listening effects, it is necessary to give a more excellent eight-wood receiving antenna with a relatively excellent performance. Eighthwat antennas is a very wide range of classic orientation antennas, full name "eight wood / Yutian antennas", English name YAGI, from the 1920s, Japanese motor engineering, eight wooden show, studying short beam with his student Yutian New Tailang At the time of invention. Relative to the basic half-wave symmetry, high gain, the direction of direction, anti-interference, the direction of direction, the structure is simple, the material is easy, the price is low, the price is small, the stool is small, soft and firm, convenient . Usually the eight wood antennas consists of an excitation vibrator (also known as the main vibrator), a reflected vibrator (also known as reflector) and several leading vibrators (also known as the guide). Compared to the lower side of the reflector, the leader is shorter, and the leader is shorter, and the number located on the other side of the main scorch is, the number of all the vibrators is the number of cells of the antenna, such as a pair. The eight-way antenna of the five units includes a master vibrator, a reflector, and three leaders, as shown in Figure 1. The main vibrator is directly connected to the feed system. It belongs to active vibrators. It belongs to passive vibrators. All of the vibrators are in the same plane, and fixed in parallel to a molter in the center of a transverse vibrator according to a certain spacing. On the beam.
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Principle Brief, the principle of FM receiving eight wooden antenna orientation, can be derived in detail according to electromagnetic theory, but it is more complicated, and ordinary readers are not easy to understand. Here only is a simple analysis: We know, close to the antenna electrical indicator The associated wavelength λ, the length of the wire slightly longer than the λ / 4 integer times is inductive, and the length is slightly shorter than the wire of the λ / 4 integer times in capacitive. Since the main vibrator L uses a half-wave symmetry or half-wavelength of λ / 2, the center frequency is in a resonant state, and the impedance is presented as a pure resistance, and the reflector A is slightly longer than the main vibrator, exhibits sensibility. It is assumed that the distance between the two is λ / 4. When the signal is received, the electromagnetic wave over the front of the antenna will first reach the main vibrator, and generate the inductive electromotive force ε1 and the induction current I1, and then the electromagnetic wave re-reaching the reflector after the distance of λ / 4. Generate an induced electromotive force ε2 and induction current I2, due to the distance between the spatial difference λ / 4, the ε2 is hysteresis 90 °, and since the reflector is hysteresis 90 °, the I2 is hysteresis 180 °, reflection The magnetic field H2 generated by the sensing current I2 generates radiation to the main vibrator is also hysteresis 90 ° than I2. According to the inductive electromotive force ε1 'generated on the main vibrator according to the electromagnetic induction, 90 °, that is, ε1' ratio ε1 hysteresis 360 ° That is, the inductive electromotive force ε1 of the inductive electromotive force ε1 'generated by the reflector is inhabited, and the antenna output voltage is both the sum of the antenna output voltage.
The antenna geometry is considered to design a pair of receive antennas. We always hope that it can have high efficiency and gain, sufficient bandwidth, and strong signal selection and anti-interference capabilities, while matching the feeder impedance, The stationbox and reduce signal loss. However, the geometric parameters of the antenna have an impact on its electrical performance, and often contradict each other, mutually implicit, design and adjustment, it is necessary to take care of the other, to combine the actual use, it is clear, and it is necessary to sacrifice some secondary. Performance. Due to the gain of the eight-timber antenna and the axial length (from the reflector to the last guide distance), the number of units, the vibrator length, and the pitch are closely related, the longer the axial direction, the number of units is actually the leader, the direction The higher the gain, the farther the action distance, but after more than four leaders, the improvement effect is not obvious, and the volume, weight, production cost increase, the material intensity requirements are more stringent, while leading to The working band is narrower. Under normal circumstances, 6 to 12 units are sufficient, and the antenna gain can reach 10 to 15 dB. For high gain requirements, it can be solved by an antenna array. The length of the guide is usually (0.41 ~ 0.46) λ, the more the number of cells, the best length of the guide, and if the operating frequency band is wide, the length of the leader should also be made short. The spacing of the leader is generally taken (0.15 ~ 0.4) λ, and the antenna gain will be rapidly declining, but the spacing of the first leader B and the main vibration should be slightly smaller than other spacing, such as b≈0.1λ, Gain will increase. In general, the length of the reflector A and the spacing of the main scorch have little effect on the antenna gain, and the front and rear radiation ratio has a large influence, the reflector length is usually (0.5 ~ 0.55) λ, with The spacing of the main vibrator is (0.15 ~ 0.23) λ. The reflector has a long or pitch to effectively suppress rear radiation, but the input impedance is low, it is difficult to match the feeder, so it is necessary to take compromise. For some use occasions requiring higher rear radiation ratio, two reflectors can be installed up and down in the vertical direction of the antenna, or simply use the reflective network. Sometimes, in order to improve the low frequency end characteristics of the antenna bandwidth, two lengths of reflectors are arranged at different distances at different distances in different distances, of which shorter is close to the main vibrator. If you want to improve the high frequency end characteristics of the antenna, you can adjust the length of the shorter. The length and spacing of the guide in the multi-eight wooden antenna can be equal, it can be equal, thereby dividing the uniform structure and the uneven structure of two forms, the leader of the uneven structure, the farther the length of the main vibrator, the larger the interval, the larger the interval, The working frequency band is expanded to the high frequency end direction and is adjusted to relatively flexible. The higher the antenna gain, the narrower the bandwidth, sometimes the extension band, and two excitation vibrators can also be used as double excitation, or directly select the composite directive antenna. Considering that the electrical indicators of the eight wooden antenna are relatively stable in the frequency band, and the high-end changes are faster, the frequency is initially designed to be slightly higher than the center frequency. Further, the larger the metal pipe used in the vibrator, the lower the characteristic impedance, the larger the antenna bandwidth, the vibrator diameter is usually (1/100 ~ 1/150) λ, of course, the overall mechanical characteristics of the antenna will also be considered when the actual choice is actually selected. The thickness of the vibrator also affects the practicality of the vibrator because the speed of the electric wave travels in the metal is different from the vacuum, and the actual production length should reduce a shortening coefficient on the theoretical value, and the wire is shortened. The larger the coefficient, the smaller the length of the vibrator, and the impedance characteristics also have a certain effect.
Input impedance input impedance is an important characteristic indicator of antenna, which is mainly determined by the self-impedance of active vibrator and the mutual resistance between several passive vibrators adjacent to it. The distant leadership is negligible because the main scorpion is weakly coupled. Usually, the main vibrator has two forms of half-wave symmetry and half-wavelerged vibrators. In a separate resonant state, the input impedance is a pure resistance, and ZIN = 73.1 Ω, nominal 75Ω, half-wavelength-assisted vibrator. ZIN = 292.4Ω, nominal 300Ω, is four times that of semi-wave symmetrical vibrators. When the leader is added, the reflector is passively ivoted, due to the electromagnetic coupling between each other, the impedance relationship becomes more complicated, the input impedance is significantly reduced, and the pitch of the eight wooden antennas is the lower the impedance. In order to increase the input impedance, improve the antenna efficiency, so that the main vibrator is in the form of a half-wavely converted vibrator, which can also increase the bandwidth of the antenna. As long as appropriate, the length of the oscillator is appropriate, the diameter ratio of the two conductors and the spacing thereof, and combined with the adjustment of the reflector and the size of several leading vibrators, the input impedance can be converted to the value equal to or close to the feed line characteristic impedance. It is particularly worth mentioning that although most radio communication machine antenna ports and the coaxial cable characteristic impedance are designed to 50 Ω, and the radio television reception and transmission of the coaxial cable characteristic impedance is 75Ω, but for anyone, people always You can make a satisfactory standing wave ratio by impedance debugging, within the required frequency range. Therefore, it is not very important to pay attention to the specific value of the eight wooden antenna input impedance, which is mainly based on the standing wave ratio on the feed line. Size selection or test adjustment. If the coaxial cable feed is selected, in order to ensure the symmetry of the antenna and match the impedance of the feeder, it must be added to the feeder and antenna interface, such as a half-wave U-shaped ring matching, transformer. Matcher, etc., otherwise the high frequency signal is attenuated in the transmission. Due to the half-wave U-ring matching, only one λ / 2 coaxial cable, the structure is simple, the application is wide, and the specific wiring method is shown in Figure 2.
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The adjustment of the antenna is influential due to the leader array, and the experimental adjustment is an essential step in the use of eight wood antennas. Be sure to take the antenna when debugging, leave the ground height two or more, so as not to affect the impedance and elevation angle of the antenna. When eating eight wooden antennas, the antenna plane in which the vibrator can be both parallel and vertical, as long as the same polarization is maintained as long as the antenna of the transmitting end is maintained, and the horizontal polarization wave is received, and the vertical polarization wave is received, which is At sufficient isolation, it is also possible to set up two pairs of mutually vertical lead antennas, which is very convenient to use. To avoid more complicated phase relationship, reduce the difficulty of adjustment, usually the transverse beam is vertical. Since the length of each vibrator is about half wavelength, the midpoint of the vibrator is just in the zero point of the electric wave induction signal voltage, so the midpoint of the vibrator can be directly fixed with metal bolts and aluminum beams, which is not necessary to insulate, so it is convenient to discharge sensing Static electricity. If the main vibrator uses a semi-wave symmetrical vibrator, where the feeder is in contact with the beam, it is necessary to maintain good insulation, and the midpoint is still in communication with the beam. The metal beam is perpendicular to the electric field polarization in the lateral direction, so that there is no significant effect on the antenna radiation field. In addition, it should be noted that due to the general set of outdoor environments, the air-blown sun, the interface is easily oxidized, the transmission of the signal is easily oxidized, and the reception effect is deteriorated, and the reception effect is deteriorated. Waterproof tape is processed in advance, but also pay attention to lightning protection.
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Although the shortcut of the selection, only a few metal vibrators and a beam, the structure is not complicated, but if you want to do fine, if you don't have enough grasp if you want to do it, you can The size given by the engineering theory book is given, or with some ready-made design software, such as foreign YAGI (download address http://www.ve3sqb.com/), etc. to receive the Hebei Radio Traffic Music Channel 99.2 MHz program For example, just enter the frequency, the number of cells, the number and the vibrator diameter can be obtained, as shown in Figure 3, ensuring that you can also create a pair of YAGI smoothly. Theoretical theory, only practice can really know, how, don't grab it, try it!
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