We’ve seen that an electromagnetic wave is self-sustaining, independent of charges or currents. However, charges and currents are needed at the source of an electromagnetic wave. We’ll take a brief look at how an electromagnetic wave is generated by an antenna.

FIGURE 31.25 is the electric field of an electric dipole. If the dipole is vertical, the electric field $\vec{E}$ at points along a horizontal line is also vertical. Reversing the dipole, by switching the charges, reverses $\vec{E}$. If the charges were to oscillate back and forth, switching position at frequency f, then $\vec{E}$ would oscillate in a vertical plane. The changing $\vec{E}$ would then create an induced magnetic field $\vec{B}$, which could then create an $\vec{E}$, which could then create a $\vec{B}\text{,}\dots$, and an electromagnetic wave at frequency f would radiate out into space.

FIGURE 31.25 An electric dipole creates an electric field that reverses direction if the dipole charges are switched.

This is exactly what an antenna does. FIGURE 31.26 shows two metal wires attached to the terminals of an oscillating voltage source. The figure shows an instant when the top wire is negative and the bottom is positive, but these will reverse in half a cycle. The wire is basically an oscillating dipole, and it creates an oscillating electric field. The oscillating $\vec{E}$ induces an oscillating $\vec{B}$, and they take off as an electromagnetic wave at speed $v_{\text{em}}=c$. The wave does need oscillating charges as a wave source, but once created it is self-sustaining and independent of the source. The antenna might be destroyed, but the wave could travel billions of light years across the universe, bearing the legacy of James Clerk Maxwell.

FIGURE 31.26 An antenna generates a self-sustaining electromagnetic wave.