The Yagi antenna is an end-fire antenna that is arranged in parallel by an active vibrator, a passive reflector, and several passive directors.
In the 1920s, this antenna was invented by Yagi Hideo and Toda Taiga of Tohoku University in Japan, called “Yagi Antenna”. Yagi antenna is the most important category of HD antennas at present.
Yagi antenna is very easy to use. It has good directivity and has a higher gain than a dipole antenna, hence called high definition Yagi antenna. It is especially good for measuring direction and long-distance communication.
If equipped with an elevation and azimuth rotation control device, it is possible to communicate with the radio stations in all directions including the spacecraft as desired, which is not available from the upright antenna.
A typical Yagi antenna should have three pairs of vibrators, and the entire structure is “king”. The active vibrator connected to the feeder, or the main vibrator, is among the three pairs of vibrators, a reflector that is slightly longer than the active vibrator.
It acts on one side of the active vibrator to attenuate the electric wave from this direction or the electric wave emitted from the antenna. It is slightly shorter than the active vibrator.
A director, located on the other side of the active vibrator, enhances the waves that are transmitted from this side or that are emitted in this direction. There can be many directors, each of which is slightly shorter than the one adjacent to it and close to the active oscillator.
The more the directors, the sharper the direction and the higher the gain, but in fact, after more than four or five directors, this “benefit” increase is not obvious, but the volume is large, the dead weight is increased, and the material strength is required.
Problems such as improvement and cost increase have become increasingly prominent. Usually, a pair of the five-unit arbor (three directors, one reflector and one active oscillator) is sufficient.
Each director and reflector is made of a metal rod. Regardless of the number of “units”, all vibrators are fixed in parallel on a “gird” at a certain spacing. The girders are also made of metal.
An active oscillator is a key unit. There are two common forms: folding vibrators and straight vibrators. The direct oscillator is actually a one-half-wavelength dipole, and the folded oscillator is its deformation.
The place where the active vibrator meets the feeder must be well insulated from the main beam, and the midpoint of the folded vibrator still communicates with the beam.
An important feature of the antenna is the “input impedance.” In the resonant state, the antenna is connected to the feeder end as a resistor.
The common feeder impedance is 50Ω. If the input impedance of the antenna is also 50Ω, then the “match” is achieved, and the signal output by the radio can all be transmitted from the antenna; if it is not “matched”, some of the power will be reflected back to the radio amplifier circuit.
The Yagi antenna input impedance of a half-wavelength dipole antenna is about 67 Ω, and the input impedance of a one-half wavelength folded oscillator is four times higher than the former.
When the director and reflector are added, the impedance relationship becomes complicated. In general, Yagi is much lower than the impedance of only the basic vibrator, and the impedance of each unit of Yagi is high, and the impedance is low, and the antenna efficiency is lowered.
According to the data, the impedance is the lowest when the distance between the director and the main oscillator is 0.15 wavelength, and the impedance is high at 0.2-0.25, and the efficiency is improved. At this time, the impedance varies between about 5-20 Ω.
The characteristic impedance of the classic folded oscillator Yagi antenna is about 300Ω (the pitch of the vibrator is about a quarter of a wavelength) such as a common TV receiving antenna.
When the pitch of the folded oscillator is narrow, or when the “long side” diameter of one-half wavelength is larger than the diameter of the “short side” of the two quarter-wavelengths, the input impedance is high.
Our telecom output is designed to be 50Ω with a 50Ω cable as the feeder. How can the Yagi antenna achieve impedance matching with the feeder? Obviously, this issue cannot be ignored. Then there are a variety of matching methods.
The short-wave band Yagi’s commonly used “hairpin” matching is to connect a U-shaped conductor at the feeding point. It acts as an inductor and forms a parallel resonance with the capacitance of the antenna itself to improve the antenna impedance. “Gamma” matching, the famous HB9CV antenna and so on.
The easiest way is to wrap the feeder near the antenna feed into a coil of about six or seven turns about 15 cm in diameter. I think the principle of matching this with the hairpin should be the same.
There is another problem to note: the Yagi antenna is a “balanced output”, and its two feed points exhibit the same characteristics for “ground”, but the usual transceiver antenna port is “unbalanced” and the core is a Hot end, the outer conductor is grounded.
Although we can also blindly connect the feeder core wire to one of the two feed points of the antenna, the other point connects the outer conductor layer of the feeder, but this will destroy the original directional characteristics of the antenna, and it will also be on the feeder.
Produce unnecessary emissions. A good eight wood, there should be a “balanced-unbalanced” conversion.
A friend asked, is the antenna erected parallel with earth or perpendicular to the earth?
The answer is – it is better for both parties to receive and send the same “pose”. When the vibrator is horizontal, the electric wave emitted by the electric field is parallel to the earth, and is called “horizontally polarized wave”.
The electric wave emitted when the vibrator is perpendicular to the ground is a “vertically polarized wave”.
Both transceivers should maintain the same polarization. In the U/V band, people use a large number of upright antennas.
Of course, the Yagi antenna should be subject to a small majority, so that the vibrator is perpendicular to the earth.
The short-wave band Yagi antennas are mostly erected horizontally, and it is impossible for such a giant to realize a vertical frame!
Our factory has more than 100 kinds of Yagi antenna configuration solutions, suitable for different countries and regions.
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