Log spiral design for direction of arrival estimation

A four-armed spiral antenna can be used to perform broadband direction-of-arrival estimation using a single aperture. The accuracy of the estimate is degraded by the existence of unwanted modes, which arise from two sources: the network used to sense the individual mode information and energy that leaks past the primary radiation region of a particular mode into aliased models. A technique for minimizing the error induced by the leakage modes is described. The technique requires selecting a log spiral with a particular growth rate that causes the errors induced by these leakage modes to effectively cancel. A number of spirals have been built to test this technique, and it has been found to be effective. It reduced the root mean square (RMS) phase error in far-field patterns near the horizon to less than a quarter of that which would be seen in a nontuned spiral.<>