Characteristic mode analysis

Background

When an electromagnetic wave is scattered by an object, currents are induced on said object which subsequently re-radiate electromagnetic energy. The structure of the currents (and fields) are unique to the physical dimensions of the scatterer and incident frequency of radiation. From this perspective, a scatterer can be viewed as a parasitic antenna that radiates electromagnetic radiation in the same manner that the original incident wave was radiated.

Principle

The principle behind characteristic mode analysis is based on optimizing the currents in any given structure by minimizing the stored power and maximizing the total radiated power. The characteristic modes of any structure can be solved by first calculating the impedance matrix of a structure. This is most commonly done by solving for the impedance matrix of the scatter, referred to as the Z matrix, using the boundary element method. Once the impedance matrix is solved, the characteristic modes for the scatterer can be solved through the weighted eigenvalue equation:

In this equation, R and X represent the real and imaginary parts, respectively, of the Z matrix, λ n represents the characteristic eigenvalues, and J n represents the characteristic currents or eigencurrents of the scatterer. It is important to note that both R and X are real and symmetric matrices. This means that all λ n and J n must also be real.

The eigenvalue has a unique meaning. The larger the absolute value of the characteristic eigenvalue, the more stored energy the scatter contains. When an eigenvalue is zero the mode is resonant and does not store any power. The following equations must also be satisfied for each characteristic eigencurrent when m ≠ n :

This particular identity states that each eigencurrent and radiated eigen far-fields are fully orthogonal to every other eigencurrent and eigen far-field. In 2007, it was first noted that due to this unique identity, characteristic modes facilitate extremely low correlation and hence will maximize MIMO performance. From this initial idea, a resurgence of interest in characteristic modes for the design of MIMO antennas took place. Since 2007, characteristic modes have been used to design chipless RFID tags, WiFi antennas, low frequency multi-band chassis antennas, UAV antennas, HF ship antennas, microstrip patch antenna designs, and dielectric resonator antenna designs.