In free space, a similar principle occurs. The sender (a transmitter) can send an alternating
current into a section of wire (an antenna), which sets up moving electric and magnetic
fields that propagate out and away as traveling waves. The electric and magnetic fields
travel along together and are always at right angles to each other, as shown in Figure 1-3 .
The signal must keep changing, or alternating, by cycling up and down, to keep the electric
and magnetic fields cycling and pushing ever outward.