  A deterministic evaluation procedure for multi-port direction finding
antennas is proposed. It is based on a direction finding uncertainty parameter,
which describes how well different directions of arrival and polarizations are
distinguishable. By investigating a simple antenna array, it is shown that the
proposed parameter provides additional insight into the behavior of an antenna
system, when compared to established methods. Moreover, since the uncertainty
parameter is calculated from a set of far fields, it is applicable to port far
fields as well as Characteristic Modes. This finding is utilized to derive a
design guidance: Starting with a set of Characteristic Mode far fields, the
angular distribution of the uncertainty is investigated to verify that no
ambiguities are present. Different sets of far fields are compared and the
differences regarding their direction finding behavior are visualized and
explained using the uncertainty in conjunction with an estimate of the incident
field. To quantify these differences, a key performance indicator is introduced
that summarizes the direction finding capabilities over a selected angular
region. To demonstrate the design process, a multi-mode multi-port antenna with
three uncorrelated ports is developed, manufactured and measured.
