Characteristic Mode Analysis (CMA)
- Characteristic Mode Analysis
The first four modal currents calculated for
a rectangular plate.
Characteristic Mode Analysis (CMA) is the numerical calculation of a weighted set of orthogonal current-modes that are supported on a conducting surface. The sets of characteristic near fields and far fields associated with these characteristic currents can provide insight into the radiating properties of structures, allowing for a systematic approach to antenna design and placement.
Characteristic modes are obtained by solving a particular weighted eigenvalue equation that is derived from the Method of Moments (MoM) impedance matrix. FEKO has a built-in solver that calculates these modes, with no need for post-processing by the user. The eigenvalues, modal significance, characteristic angles, currents, near fields and far fields can be visualised in POSTFEKO.
One of the challenges in CMA is being able to track the modes over a discrete frequency range. The modes may change with frequency, causing the mode index to switch if no tracking is used. This makes it difficult to interpret graphs like the eigenvalues or characteristic angles versus frequency.
FEKO performs mode tracking over frequency. Mode indexes are assigned according to the ranking of the modes at the lowest frequency of interest. At each frequency, FEKO orders the modes based on their ranking at the lowest frequency. Since mode rankings change over frequency, more than the requested number of modes are calculated. If a mode requires the calculation of many higher order modes to be accurately represented, a much finer mesh could be required for CMA than what would be sufficient to solve the same problem with the method of moments solver.
Characteristic angle plot with no tracking. Characteristic angle plot with tracking.
The first six characteristic modes of a rectangular metallic plate are calculated. Cartesian graphs of the modal significance and eigenvalues plotted against frequency and the near field and far field patterns associated with the first few modes are shown.
In the second example characteristic mode analysis is performed on a motor vehicle. The near and far fields associated with the first four characteristic modes are displayed.
Rectangular plate eigenvalues. Rectangular plate modal significance values. Rectangular plate modal currents and near fields. Rectangular plate modal currents and far fields. Near and far field patterns for the first four characteristic modes of a car body.