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The antenna itself is an oscillator, but it is different from the ordinary LC oscillation circuit. It is a deformation of the ordinary oscillation circuit. The electric field is concentrated in the two plates of the capacitor, and the magnetic field is distributed in the limited space of the inductor coil. Obviously, electromagnetic waves cannot radiate to a wide space. If the oscillating circuit is expanded so that the electromagnetic field is distributed over a large space, this creates conditions conducive to radiation; therefore, the modulated high-frequency signal current from the transmitter is sent to the antenna from the feeder and passed through The antenna converts high-frequency current energy into corresponding electromagnetic wave energy. After the energy of the electromagnetic wave radiating to space is radiated from the transmitting antenna, it will propagate forward in all directions on the ground surface.

If a wire is placed in the alternating electromagnetic field, because the magnetic field lines cut the wire, a certain alternating voltage, electromotive force, is excited at both ends of the wire, and its frequency is the same as the transmission frequency. If the wire is connected to the receiver through a feeder, the current of the modulated signal can be obtained in the receiver. Therefore, this wire plays the role of receiving electromagnetic wave energy and converting it into high-frequency signal current energy, so this wire is called a receiving antenna. Whether it is a transmitting antenna or a receiving antenna, they are all energy converters, and "reversibility" is a characteristic of general energy converters. The same antenna can be used as both a transmitting antenna and a receiving antenna. Communication equipment generally uses a single antenna for both transmission and transmission. Therefore, the same antenna not only affects the effective energy output of the transmitting system, but also directly affects the performance of the receiving system.

The reversibility of the antenna is not only reflected in the fact that the transmitting antenna can be used as a receiving antenna, the receiving antenna can be used as a transmitting antenna, and the parameters when the antenna is used as a transmitting antenna are maintained with the parameters when used as a receiving antenna No change, this is the reciprocity principle of the antenna.

For the convenience of discussion, the antenna is often analyzed as a transmitting antenna, and the conclusions obtained are also applicable to the case where the antenna is used as a receiving antenna.