Antenna impedance is the impedance that can be measured at the antenna connector and that an antenna exhibits at a given frequency. It is also referred to as the footpoint impedance or footpoint resistance.
Since antennas can be considered open resonant circuits, antenna impedance is complex and is composed of a real and an imaginary component and changes with frequency.
In equivalent circuit terms, the antenna impedance can be represented as a series connection of the radiation resistance, the loss resistance and the reactance. The latter is capacitive or inductive depending on the frequency. At resonant frequency, the two imaginary components cancel each other out and the footpoint resistance corresponds to the real component of the radiation resistance and the loss resistance. The antenna impedance is characterized by the inductive reactive component at wavelengths below the resonant frequency, and by the capacitive reactive component at wavelengths above the resonant frequency.
Antenna impedances are shown as locus curves in Smith charts for better interpretation.