Eigenmode Analysis of Small Multielement Spherical Antennas

A numerical eigenmode solver is used to analyze the behavior of two-, four-, and six-arm small multielement spherical antennas. The resonant frequency and radiation Q-factor of the natural modes of the shorted antenna structure are determined from eigenmode simulations using perfectly matched layer outer boundaries. The radiation resistance of each eigenmode at resonance is determined using a multiarm feed geometry that excites the modes individually. The impedance response of the antenna when driven on a single arm can then be described as a multipole RLC network, where the component values of the network are directly related to the parameters and relative excitation amplitudes of the various eigenmodes of the antenna. A good match between the modal analysis and the full harmonic simulation is observed, and the analysis also predicts the far-field cross-polarization levels arising from energy radiated by the higher-order modes. The analysis provides physical insight into the multiresonant impedance behavior of these antennas, clearly illustrating their ultimate performance limits and yielding ideas for improved designs. The analysis can be applied towards understanding the behavior of other multielement antenna structures.