What Is a Yagi Antenna?

The Yagi antenna is an end-fire antenna composed of an active oscillator (usually a folded oscillator), a passive reflector, and several passive directors arranged in parallel. In the 1920s, this antenna was invented by Hideki Yagi and Taiji Uda of Tohoku University, and it was called "Yagi Uda antenna", or "Yagi antenna" for short.

Yagi Antenna

The Yagi antenna is an end-fire antenna composed of an active oscillator (usually a folded oscillator), a passive reflector, and several passive directors arranged in parallel. In the 1920s, by

English: Yagi-Uda antenna; Yagi antenna

Explanation: An end-emission array composed of an excited unit, a reflective unit, and one or more directing units. Note: In fact, the reflection unit can be composed of some units or a reflection surface.

The working principle of the Yagi antenna is as follows (take three-element antenna reception as an example): the director is slightly shorter than the half wavelength, the main oscillator is equal to the half wavelength, and the reflector is slightly longer than the half wavelength. Oscillator pitch is a quarter wavelength. At this time, the director is capacitive to the induction signal, and the current is 90 ° ahead of the voltage; the electromagnetic waves induced by the director will radiate to the main oscillator, and the radiated signal will lag 90 ° to exactly offset by a quarter-wavelength path. Because of the "lead" caused earlier, the two phases are the same, so the signals are superimposed and strengthened. The reflector is slightly longer than one-half wavelength, is inductive, and the current lags 90 °. In addition, the radiation to the main oscillator lags 90 °. The two add up to exactly 180 °, which offsets the effect. When one direction is strengthened and one direction is weakened, there is strong directionality. The same is true of the firing state.

Yagi antenna antenna principle

The active oscillator is a key unit. There are two common forms: folded oscillator and straight oscillator. The straight oscillator is actually a half-wavelength dipole oscillator, and the reduced oscillator is its deformation. The place where the active vibrator is connected to the feeder must maintain good insulation with the main beam, while the midpoint of the folded vibrator is still in communication with the beam.

Imitation of an antenna, but always needs to be adjusted appropriately. What to tune? Why tune? This requires us to understand the principle of the antenna.

An important feature of an antenna is its "input impedance". In the resonance state, the antenna is connected to the feeder as a resistor. The common feeder impedance is 50. If the input impedance of the antenna is also 50, then matching is achieved, and all signals output by the radio can be transmitted from the antenna; if it is not matching, part of the power will be reflected back to the radio's power amplifier circuit .

The input impedance of the half-wavelength dipole antenna is about 67, and the input impedance of the half-wavelength folded dipole is 4 times higher than the former. When directors and reflectors are added, the impedance relationship becomes complicated. In general, the impedance of Yagi is much lower than that of only the basic vibrator, and the larger the distance between each element of Yagi is, the higher the impedance is. On the contrary, the impedance becomes lower and the antenna efficiency decreases. According to some 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 when 0.2-0.25, and the efficiency is improved. At this time, the impedance range is between 5-20.

The characteristic impedance of a classic folded vibrator Yagi antenna is about 300, (the vibrator spacing is about a quarter of a wavelength), such as a common TV receiving antenna. When the folded pitch of the folded oscillator is narrow, or the diameter of the "long side" of the half wavelength is larger than the diameter of the "short side" of the two quarter wavelengths, the input impedance is high.

Yagi antenna transmission

The output of our communicator is designed according to 50, with 50 cable as feeder. How can Yagi antennas achieve impedance matching with feeders? Obviously, this issue cannot be ignored. So there are various matching methods. The short-wave band Yagi's commonly used "hairpin" matching is a U-shaped conductor connected in parallel at the feed. It acts as an inductor and forms a parallel resonance with the antenna's capacitance to increase the antenna impedance. There are also classic "Gamma" matching, the famous HB9CV antenna and more. The simplest way is to wind the feeder near the antenna feed into a coil of about six or seven turns with a diameter of about 15 cm and hang it there. I think this should be the same as the principle of hairpin matching.

There is also a problem to pay attention to: Yagi antenna is "balanced output", its two feed points show the same characteristics to "ground", but the usual transceiver antenna port is "unbalanced", the core wire is The hot end, the outer conductor is grounded. Although we can also blindly connect the feeder core wire to one of the two feeding points of the antenna and the other point to the outer conductor layer of the feeder line, this will destroy the original directional characteristics of the antenna, and it will also be on the feeder line. Generate unnecessary emissions. A good pair of Yagi should have a "balance-unbalance" transition.

Notes on Yagi antennas

A friend asked, is it better for the antenna of the antenna to be parallel to the earth or vertical to the earth? The answer is that both the receiving and sending parties should keep the same "posture". When the vibrator is horizontal, the electric field emitted is parallel to the earth, which is called "horizontal polarized wave." The sending and receiving sides should maintain the same polarization. In the U / V band, people use upright antennas a lot. Of course, Yagi antennas should obey the majority, so that the vibrator is perpendicular to the ground. Short-wave band Yagi antennas are mostly installed horizontally, and such a behemoth may not be possible to achieve vertical installation!

A friend asked, what effect does the diameter of the vibrator have on the antenna performance? The answer is that the diameter affects the length of the vibrator, and the larger the diameter, the shorter the length. The larger the diameter, the lower the antenna Q value, and the larger the operating frequency bandwidth.

Another friend asked, is the folded oscillator "laying flat" on the beam, how many sides are on a plane with other oscillators? Or is it that the face of the vibrator is perpendicular to the girder, and only its long side and other vibrators are kept on the same plane? The classic folded vibrator Yagi antenna is the former. According to the above-mentioned working principle, if the folded oscillator lies flat between the director and the reflector, the folded oscillator has two sides "plugging in", and the phase relationship is more complicated?

But then again. Many of the achievements of amateur radio, especially various antennas, have been obtained through practical experiments. "Success" or "unsuccessful" is often determined by their own satisfaction and "compared to the past." Some of the antennas introduced again in this issue are some 50 feeders that are directly connected to the folded resonators, and the folded resonators lie flat and stable among the "brothers". What is the best? Let's try it yourself. Connect a standing wave table, try to adjust the length of each oscillator, the distance between each unit, and how to match, etc., there may be new discoveries.

By the way, be sure to set up the antenna carefully when debugging, at least two or three meters away from the ground, and open wider around. .