When a high-frequency current flows through a conductor, an electric field and a magnetic field occur in the surrounding space. According to the distribution characteristics of electromagnetic field in space, it can be divided into near area, central area and far area.
When the length L of the wire is much smaller than the wavelength λ, the radiation is weak; when the length L of the wire is increased to be comparable with the wavelength, the current on the wire will be greatly added, and thus a strong radiation can be formed.
The transmitting antenna uses this property of the radiation field, so that the transmitted signal can fully radiate to space after passing through the transmitting antenna. How to make a conductor a useful radiator guide system? Here we first analyze the situation on the transmission line. In order to transmit only energy without radiation on parallel two-line transmission lines, it is necessary to ensure that the structure of the two lines is symmetrical, and the magnitude and direction of the current at the corresponding points on the lines are opposite. Interval <π. In order for the electromagnetic field to be effectively radiated, it is necessary to destroy this symmetry of the transmission line. For example, if the two conductors are separated at a certain angle, or one of them is removed, the symmetry of the conductor can be destroyed and radiation will occur. .
The conductors at the open-ended transmission or spaced terminal π / 4 are separated in a straight shape. At this moment, the current on the terminal conductor is no longer in phase but in phase, so that the radiation fields of the conductor at the spatial point are superimposed in phase, forming a useful Radiation system. This is the simplest and most basic unit antenna, called a half-wave symmetrical dipole antenna, with a characteristic impedance of 75Ω. The electromagnetic wave is radiated from the transmitting antenna and will be transmitted to all sides. If a symmetrical vibrator is placed in the direction of the electromagnetic wave transmission, an induced electromotive force will occur on the antenna vibrator under the effect of the electromagnetic wave. At this moment, the antenna is connected to the receiving device, and a high-frequency current will occur at the input of the receiving device. In this way, the antenna plays the receiving effect and converts electromagnetic waves into high-frequency current. That is to say, the antenna plays the effect of receiving the antenna at the moment. The quality of the receiving effect depends on the directionality of the antenna and the half-symmetrical oscillator. Match with receiving equipment.