What is the symbol of the straight line below the radio wave line?

This is a communication symbol. The reason why the straight line below the electric wave line is the symbol of alternating current is to indicate the direction and magnitude of current. In alternating current, the direction of current will change with time, and the wavy line indicates the periodic change of current. The curved part above the wavy line indicates the forward flow of current, while the straight part below indicates the reverse flow of current. The design of this symbol enables us to intuitively understand the periodic change and direction of current.