Source and field reconstruction with field splitting

The research activity has concerned diagnostics for antennas and other intentional or un-intentional radiating structures (e.g. for EMI considerations). The diagnostics effort is based on measured field, and on information on the device under test (DUT) wrapping volume; the latter is used to reconstruct equivalent currents that radiate the same field as measured. Current reconstruction is based on a previous work. The activity has involved testing of the existing approach -not reported in this thesis document-, and its extension. The main emphasis has been on relating the polarization of the radiated field to the (equivalent) currents on the DUT, especially to allow the designer a clear view of what the origin could be of possible deviations from expected or desired performance. In this sense, the geometry of the current reconstruction surface plays a key role: one would like to do this reconstruction on a closely wrapping surface to maximize the information content. The basic idea is to split the measured field into two polarization components (co- and cross-polarization), and then observe the corresponding equivalent currents; for example, the sources of cross-polarization could be directly tracked this way. However, this approach cannot be applied in a straightforward manner for two different reasons. In the first place, it is well known that polarization (co-polarized and cross-polarized orthogonal components of the radiated field) is not defined uniquely, and the definition of co- and cross-polarization depends on utility for the specific use. This problem can be handled in a conventional way, but requires attention, and in view of the subsequent use, and in particular for the definition of the cross-polarized field. Second, and more important for the present purpose, there is no guarantee that a purely polarized field could be radiated by a set of currents having spatial support on the wrapping surface of the DUT (e.g., a purely polarized field might require a larger antenna). Also, one is normally interested in co- and cross-polarizations levels only in the main radiation lobe, or a portion thereof. This has prompted the definition of an “improved” field. This “improved” field has a given polarization purity only in a portion of the main beam, via a windowing process that sets the level of cross-polarization and the extent of the (angular) region where the improvement is enforced. The improvement is in any case a perturbation of the actual measured fields, that potentially worsens the current reconstruction process; therefore, trade-offs have been studied in several cases between the information conveyed by the current reconstruction via improved field, and the accuracy in reproducing the measured field. The concept is more general, though, and can indeed be used to “improve” a prototype DUT whose measured performances are below the level of expectation, e.g. by lowering the side lobes. This can be recognized as a linear synthesis (“field synthesis”) procedure, whose results is a useful indication for the designer on what parts of the DUT have to be modified and on how to get a better performance. Issues about the extent of the “improvement” (distance from actually measured pattern) and physical realizability in the volume occupied by the DUT are still to be considered. Conversely, and to some extent counter-intuitively, the polarization splitting is simpler when two polarizations are specified at the source level; this is what commonly happens for dual-polarized antennas. In this case it can be seen that the field splitting in polarization leads to a stable reconstructions of the related currents.