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Antenna system is composed of transmitting antenna and receiving antenna. The former is a transmission mode converter that converts the radio frequency current or electromagnetic wave of guided wave mode into the space electromagnetic wave of diffuse wave mode, and the latter is a transmission mode converter of inverse transformation.
The transmission mode converter converts the radio frequency current or electromagnetic wave of guided traveling wave mode into the propagation mode converter of spatial electromagnetic wave of diffuse wave mode, and the transmission mode converter of inverse transformation. As the transmitting antenna used in the mode conversion of guided traveling wave to diffusion wave, the receiving antenna I used in the mode conversion of diffusion wave to guided traveling wave is called reciprocity theorem, except that the transmitting antenna's power bearing capacity and voltage bearing capacity are far greater than the receiving antenna's, both of them can be exchanged and the basic characteristic parameters of the antenna remain unchanged. Another important function of the antenna is to concentrate the energy of the electromagnetic wave, that is, to concentrate the energy in the transmitting direction while reducing the energy in other directions when it is used as the transmitting antenna; when it is used as the receiving antenna, more energy can be intercepted from the incoming waves in the receiving direction, while the incoming waves in other directions can be reduced by phase cancellation. This is the directivity of the antenna. Compared with the non directional antenna, the increase of the energy concentration is called the gain of the antenna. The extended meaning of antenna directivity is negative gain (attenuation) in non-communication direction, which can be used to describe another related performance index of antenna, i.e. sidelobe (interference) radiation rejection of transmitting antenna or incoming wave interference rejection of receiving antenna in non-communication direction.