Characteristic Mode Analysis of a Metallic Ring Antenna
The normalised surface current distributions and far field patterns of the first four eigencurrents of a metallic ring antenna based on the antenna described in  are calculated in FEKO.
|The first four modal currents and associated far fields of a ring antenna with four slots bridged with excitation ports.
Mode J3 of the ring is a non-resonant mode with inductive behaviour in the frequency band of interest. A mode resonates when the eigenvalue is zero and the characteristic angle is 180 degrees. A positive eigenvalue indicates a mode that is storing magnetic energy and a negative eigenvalue one that is storing electric energy . The characteristic angle of mode J3 is below 180 degrees and its eigenvalue is positive across the frequency band.
|Characteristic angle plot for the ring antenna showing resonating and non-resonating modes are present.
Antonino-Daviu et al. show in  how the antenna can be modified to turn mode J3 into a radiating mode. Capacitive loading is added to move the modal resonances closer in frequency thereby resulting in a MIMO antenna.
 E. Antonino-Daviu, M. Cabedo-Fabres, M. Gallo, M. Ferrando-Bataller, and M. Bozzetti, “Design of a multimode MIMO antenna using characteristic modes”, EuCAP 2009.