While discussing transmitter and receiver circuitry we did not have to know much about antennas or propagation. It sufficed to know only that a voltage applied to the terminals of a transmitting antenna causes a proportional voltage to appear very shortly thereafter at the terminals of a receiving antenna. To be more exact, it was sufficient to know that everything between the terminals of the two antennas is equivalent to a linear two-port network. Here we will consider the transmission through this propagation link.

When an ac source (transmitter) is connected to an antenna (practically any metal structure) the resulting current has a component that is in phase with the applied voltage. The impedance of the antenna therefore has a real part, a resistance, and draws power from the source. If the antenna is efficient, most of the power flows away from the antenna in the form of (energy-bearing) electromagnetic waves and only a small fraction of the power will be dissipated by ohmic heating of the antenna itself. The impedance will also generally have a nonzero imaginary part, a reactance. If the reactance is zero at the operating frequency the antenna is said to be resonant, just as an RLC circuit is purely resistive at its resonant frequency. An external tuning network (an antenna tuner) can be used to cancel the reactance and also transform the resistance to a value that matches a receiver's input impedance or to a value that draws a desired amount of power from a transmitter.