An antenna is said to be resonant if its input impedance is entirely real, i.e.
Zin = R + j*0. |
In this case the voltage and current are in phase at the antenna's terminals. This property makes the impedance matching of an antenna to a transmission line and receiver easier, as the imaginary part of the impedance does not need tuned out.
In addition, when viewing the frequency plot of S11 for an antenna, there is often a large decrease in the magnitude of S11 around the resonanct frequency, indicating that power is radiated well around this frequency.