The directionality of an antenna refers to its ability to radiate electromagnetic waves in a certain direction. For receiving antennas, directionality represents the ability of the antenna to receive radio waves from different directions. The directional characteristic curve of an antenna is commonly represented by a directional pattern.

　　A directional pattern can be used to illustrate the ability of an antenna to transmit or receive electromagnetic waves in various directions in space.

　　Concepts and oscillators of the basic principles of antenna radiation of electromagnetic waves

　　When a wire carries alternating current, electromagnetic radiation can be formed, and the ability of radiation depends on the length and shape of the wire. If the position of the wire is close, and the induced electromotive force generated by the two wires can almost cancel out, the radiation is very weak. If two wires are opened, the induced electromotive force generated by the two wires will have the same direction due to the same current direction, resulting in stronger radiation. When the length of the wire l is much smaller than the wavelength, the current of the wire is very small and the radiation is very weak.

　　When the length of the wire increases to be comparable to the wavelength, the current on the wire greatly increases, thus forming strong radiation. Usually, the straight wires that can generate significant radiation mentioned above are called oscillators.

　　Symmetrical oscillator

　　A oscillator with equal arm lengths is called a symmetric oscillator. Each arm has a length of one quarter wavelength. A oscillator with a total length equal to the wavelength is called a full wave symmetric oscillator. The oscillator that is folded together is called a folded oscillator.

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