The antenna itself is an oscillator, but unlike the ordinary LC oscillator circuit, it is a distortion of the ordinary oscillator circuit. LC is the oscillation circuit of the transmitter. The electric field is concentrated in the two plates of the capacitor, while the magnetic field is distributed in the finite space of the inductor coil, which obviously cannot radiate to the broad space. 

    If the oscillation circuit is expanded, the electromagnetic field is distributed in a wide range of space, which creates conditions conducive to radiation. Then, from the transmitter, the modulated by high frequency signal current feeder to the antenna, and the antenna high frequency current energy is transformed into the corresponding electromagnetic energy, to space radiation electromagnetic energy from the transmitting antenna radiation, will spread along the surface of all direction forward. If a wire is placed in the alternating current field, it is the same as the frequency of the transmitting voltage, which is excited by the magnetic wire cutting wire. 

    If the wire is connected to the receiving machine through the feeder, the current of the modulated wave signal can be obtained in the receiver. Thus, this wire ACTS as a receiver of electromagnetic energy and is converted into a high frequency signal current energy, so this wire is called a receiving antenna. Whether it is a letter antenna or a receiving antenna, they are all energy converters, and "invertibility" is the characteristic of the general energy converter. The same antenna, which can be used as a transmitting antenna, can also be used as a receiving antenna. The communication equipment is usually collected and sent together with an antenna. 

    Therefore, the same antenna is related to the effective energy output of the transmitting system and directly affects the performance of the receiving system. Antenna the reversibility of performance not only in the transmitting antenna can be used as a receiving antenna, receiving antenna can be used as a transmitting antenna, and perform the antenna used for transmitting antenna parameters, and used as the parameters of the receiving antenna in remains the same, this is the reciprocity principle of antenna.