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When a high-frequency current is applied to a conductor, an electric field and a magnetic field will be generated in the surrounding space. According to the distribution characteristics of electromagnetic field in space, it can be divided into near area, middle area and far area. Let R be the distance from a point in space to the conductor. The area in time is called the near area. The electromagnetic field in this area is closely related to the current and voltage in the conductor.

The area in is called the far area. In this area, the electromagnetic field can leave the conductor and propagate into space. Its change will lag behind the current and voltage on the conductor for a period of time. At this time, the electromagnetic wave that propagates is no longer directly related to the current and voltage on the conductor. The electromagnetic field in this area is called the radiation field.

It should be noted that when the length L of the wire is much smaller than the wavelength λ The radiation is very weak; When the length L of the wire is increased to be comparable to the wavelength, the current on the wire will be greatly increased, and thus strong radiation can be formed.

The transmitting antenna makes full use of this property of the radiation field to make the transmitted signal radiate into space after passing through the transmitting antenna. How to make the conductor an effective radiator guide system? Here we first analyze the situation on the transmission line. On the parallel two-wire transmission line, in order to ensure that only energy is transmitted without radiation, it is necessary to ensure that the two-wire structure is symmetrical, the current magnitude and direction at the corresponding point on the line are opposite, and the distance between the two lines is < π. In order to effectively radiate the electromagnetic field, it is necessary to break the symmetry of the transmission line. For example, by separating the two conductors at a certain angle or removing one side of them, the symmetry of the conductor can be broken and radiation can be generated.