So far, we have made measurements without saying anything about antennas, or about how power gets from the transmitter to the receiver. In our measurements, a 50-Ω load has taken the place of the antenna. However, instead of dissipating the power as heat, an antenna radiates power as electromagnetic waves. One thing that makes antennas interesting is that they necessarily involve both the voltages and currents that we study in circuits and the electric and magnetic fields that make up radio waves. This gives antennas a special place in the history of physics. They were the crucial components that Hertz developed in the 1880s to demonstrate that Maxwell's equations for electricity and magnetism are correct. In the 1960s, a special parabolic antenna allowed Arno Penzias and Robert Wilson at Bell Telephone Laboratories to discover the cosmic background radiation. That measurement earned them a Nobel Prize, and it gave an entirely new interpretation to the history of the universe.

An antenna is characterized by its impedance and pattern, which is a plot of where the power goes for a transmitting antenna. Traditionally, antennas have been analyzed as transmitters, and most antenna engineers think entirely in terms of transmitting antennas. If we know how an antenna transmits, we can use the reciprocity theorem to figure out how the antenna works in reception. The physical description of transmission and reception are actually quite different, and the physics of receiving antennas is in many ways as interesting as for transmitting antennas.