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# Yagi antenna - The adjustment methods

2019-02-16
TAG：Yagi antenna - The adjustment methods

The Yagi antenna, as a classic directional antenna, is widely used in HF, VHF and UHF bands. The Yagi antenna is an end-fire antenna composed of an active element, a passive reflector, and several passive directors arranged in parallel.

1, Working principle of the Yagi antenna

Taking a three-element (the sum of the number of directors, reflectors, and active elements) Yagi antenna as an example, the reflector is slightly longer than half a wavelength, so it is inductive, and its induced current lags behind the induced electromotive force by 90°. The active element is located one-quarter wavelength in front of the reflector, and its induced electromotive force leads the induced electromotive force of the reflector by 90°, and because the active element is resonant, the induced electromotive force of the active element is in phase with the current. Therefore, the active element current leads the reflector current by 180°, and the magnetic field induced by the reflector current lags the current by 90°, and the induced electric field on the active element due to the magnetic field lags behind the field itself by 90°. Finally, the induced electromotive force of the reflector on the active element is in phase with the induced electromotive force of the active element, and the two are superimposed and enhanced. The directors also enhance the induced electromotive force of the active element based on a similar principle.

2, Adjustment method of the Yagi antenna

There are many factors that affect the performance of the Yagi antenna, and its adjustment is more complicated than that of other antennas. The two main parameters to adjust are the resonant frequency and the standing wave ratio of the antenna. 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 away from the ground, and start measuring it with a standing wave meter. To reduce measurement errors, the cable connecting the antenna to the standing wave meter and the radio to the standing wave meter should be as short as possible. There are three places to adjust: the capacitance of the fine-tuning capacitor, the position of the short-circuit rod, and the length of the active element. The specific adjustment steps are as follows:

(1) Fix the short-circuit rod about 5-6cm away from the crossbar;

(2) Adjust the ceramic capacitor to minimize the standing wave of the antenna when the transmitter frequency is adjusted to 435MHz;

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

(4) Observe whether the frequency corresponding to the minimum standing wave (the resonant frequency of the antenna) is around 435MHz. If the frequency is too high or too low, replace the active element with one that is slightly longer or shorter by a few millimeters and measure the standing wave again.

(5) Slightly change the position of the short-circuit rod, and repeatedly fine-tune the ceramic capacitor to minimize the standing wave of the antenna around 435MHz.

When adjusting the antenna, only adjust one place at a time to make it easier to identify the pattern of changes. Because the operating frequency is relatively high, the amplitude of all adjustments should not be too large. For example, the capacitance of the fine-tuning capacitor connected in series to the gamma rod is adjusted to be about 3-4pF, and changing a few picofarads (pF) will cause a significant change in the standing wave. In addition, many factors such as the length of the crossbar and the position of the cable can also have a certain impact on the measurement of standing waves, which should be noted during the adjustment process.

3, The outdoor Yagi antenna

The Yagi antenna series suitable for outdoor use includes anodized aluminum elements, solid elements, low-loss series 400 coaxial cable, and sturdy and durable installation hardware. With a 3 dBi gain and a weather-resistant anodized aluminum structure, it is very suitable for wireless radio frequency identification (RFID), detection control, and data acquisition (SCADA).The Yagi WiFi antenna with an antenna cover has high gain and a 45° beamwidth. It is also particularly suitable for directional and multipoint IEEE 802.11b, 802.11g, and 802.11n wireless LAN, Bluetooth, public wireless hotspot applications, and other systems using the 2.4GHz ISM frequency band. This antenna uses a unique design and can be installed with vertical or horizontal polarity. Moreover, this antenna is enclosed in an anti-UV antenna cover with a standard fire rating, which allows it to operate in all weather conditions.The antenna is very suitable for directional and multipoint IEEE 802.11a/n WiFi and wireless LAN applications, as well as other systems operating in the 5.8GHz ISM frequency band. As a lightweight Yagi antenna with excellent performance, it has a 30-degree beamwidth and can be installed in either vertical or horizontal polarization

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