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Characteristic Mode Analysis of a Metallic Ring Antenna

Characteristic mode analysis is carried out to give insight into the radiation properties 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 [1] are calculated in FEKO.

first 4 modal currents and associated far fields of metallic ring antenna
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 [1].  The characteristic angle of mode J3 is below 180 degrees and its eigenvalue is positive across the frequency band.

characteristic angle plot for metallic ring antenna
Characteristic angle plot for the ring antenna showing resonating and non-resonating modes are present.

Antonino-Daviu et al. show in [1] 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.

[1] 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.