Study of the Limitation of Characteristic Modes Using the Far-Field Based Reconstruction Technique for Complex Antenna Structures

This paper presents the study of the characteristic mode simulation and the limitation of the far-field based reconstruction technique for complex antenna structures. The characteristic modes can also be reconstructed based on the radiated far fields obtained from simulations or measurements. Traditionally, the characteristic modes are usually calculated by using the method of moments (MoM) to determine the impedance matrix and associated eigenvalues and eigenvectors in order to calculate the current distribution on antenna structures and associated radiated far fields. In an inverse approach, the characteristic modes can also be reconstructed based on the radiated far fields obtained from simulations or measurements. The characteristic modes of a relevant simplified antenna structure and the related far fields are used to approximate characteristic modes of more complex antenna structures. It has been shown in the literature that characteristic modes can be reconstructed with good accuracy for simplified antenna structures with small perturbation. However, when the antenna structures are more complex or the structure perturbation is larger over the important part of antenna structures associated with dominant characteristic modes, the far field based reconstruction of these modes may not be sufficiently accurate due to significant mode perturbation. In this paper, the limitation of the far-field based reconstruction technique is investigated. The parametric studies of the structure perturbation are performed in terms of scaling and location on antenna structures to determine the limitation of the method. The proper perturbation remaining dominant modes are discussed as well.