Phase comparison properties of the dual mode spiral with application to direction finding

A spiral antenna excited in any two adjacent radiation modes has built-in phase comparison properties useful in direction finding. The information derived from the phase comparison on the spiral allows unambiguous determination of direction of arrival of signals from anywhere in a hemisphere. The phase comparison properties are frequency insensitive when consideration can be confined to the two adjacent modes only. This paper will examine the phase comparison properties of the dual mode spiral and show how they can be applied to a very simple, passive direction finder system. In addition, the problem of a rotating azimuthal reference plane with change in frequency is treated and a suggestion for elimination of the problem is made.