Animation showing a half-wave dipole antenna receiving power from a radio wave. The antenna consists of two metal rods each one-quarter of the wavelength long, attached through a parallel transmission line to a resistance equal to the characteristic impedance of the antenna, representing the receiver. The electromagnetic wave, coming from the right, is represented by its electric field (E, green arrows) (it should be kept in mind that the drawing only shows the field along one line, while the radio wave is actually a plane wave and the electric field is actually the same at every point on a plane perpendicular to the direction of motion). The wave’s magnetic field is not shown.
The oscillating electric field exerts force on the electrons in the antenna rods (black arrows), causing them to move back and forth between the ends of the antenna rods, charging the ends of the antenna alternately positive (+) and negative (−). Since the antenna is a half-wavelength long at the radio wave’s frequency, it excites standing waves of voltage (V, red) and current in the antenna. The voltage along the antenna elements is represented graphically by a band of red whose thickness at any point is proportional to the magnitude of the voltage. The waves of radio frequency voltage and current travel down the transmission line and are absorbed in the resistor. In this animation the action is shown slowed down drastically; the radio waves received by dipoles actually oscillate back and forth at tens of thousands to billions of cycles per second.