The role of antenna is to transform the guide waves on the transmission line to the electromagnetic waves spread in the unbounded medium (usually free space), or the opposite transformation.  
 
At the transmitting end, the modulated high -frequency oscillating current (energy) generated by the transmitter (energy) input the transmitting antenna through the feed equipment (the feed equipment can directly transmit current waves or electromagnetic waves with different frequencies and forms). The transmitting antenna will high -frequency current will be high -frequency current Or guide wave (energy) to transform into radio waves -free electromagnetic wave (energy) radiation to the surrounding space; on the receiving end, radio waves (energy) transform into high -frequency current or guide wave (energy) by receiving antenna transmitted to the feed equipment to transmit to the feed equipment to transmit to the feeder equipment to transmit to Receiving machine.  
 
From the above process, it can be seen that the antenna is not only a device for radiation and receiving radio waves, but also an energy converter, an interface device of the circuit and space.  
 
When the conductor is high -frequency current, electric fields and magnetic fields are generated in its surrounding space. According to the distribution characteristics of the electromagnetic field in space, it can be divided into near, middle areas, and district.  
 
The electromagnetic field in this area can leave the conductor to spread to the space. Its change is lagging behind the current voltage on the conductor. At this time The electromagnetic field in this area is called the radiation field.