Discussion of series and parallel resonance phenomena in the input impedance of antennas

The series and parallel resonances of an antenna's input impedance are explained through the theory of characteristic modes (CM). We are showing for the first time that the parallel resonance phenomenon corresponds to the interaction of at least two nearby CM, each having eigenvalues of opposite sign. Additionally, the series resonance phenomenon is shown to be mainly due to the resonance of a single characteristic mode. Analysis of a simple wire dipole antenna, an edge-fed patch antenna, and a loop antenna are used to validate this finding.