Determination of Antenna Displacement from a Single Cut Quasi-Farfield Pattern for Cylindrical Mode Filtering Applications

In the quasi-far-field, the tangential orthogonal electric field components, e.g., in the phi direction and z direction (in the cylindrical coordinate system), are decoupled, which means that only the co-polarized field component is sufficient to transform a single cut antenna pattern to the cylindrical mode domain. When the antenna pattern is measured with a displacement from the center of rotation, a phase coherent mathematical translation of the pattern to the center of rotation promotes mode separation between the modes of the antenna and those of the chamber reflections. A mode filter can then be applied to remove effects due to chamber multipath reflections. In this scheme, an accurate measurement of the displacement distance from the rotation center is essential. Conventionally, this is obtained by using a physical measure, e.g. ruler, laser tracker, etc. In this study, we investigate several techniques to automatically retrieve the requisite offset distance using the vector pattern data, including using: the unwrapped phase response, through time domain transformation, and by examining mode concentration within the spectrum domain. Each method is studied for its robustness, limitations and efficiency in accurately determining the displacement distance.