Design of Multi-Arm Multi-Mode Spiral Antennas for Directional Beams Using Equivalent Array Concept

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Abstract
Multi-arm spiral antennas can be considered as antenna arrays where the spiral arms constitute the array elements. The proximity of the arms over the full length of the spiral and their direct coupling make such arrays unique in the sense that they operate with very strong mutual coupling. As a consequence, the array performance is dominated by the array configuration, rather than the array elements. In particular, the phase mode concept that is used successfully for the analysis of circular arrays can be used also for investigation of these arrays. For symmetric multi-arm spirals the phase mode excitation is also symmetric and used with Butler matrix type feeds in applications such as the direction finding, where the beam rotation and scanning is achieved with an aid of inter-mode phase shifts. In such applications, however, mostly the first two phase modes are utilized and mode purity has been the dominant requirement, rather than the antenna gain. In this paper the concept of phase modes is reviewed first and the relationship of these modes with azimuthal modes in microstrip antennas is demonstrated. It is then used to develop an equivalent array concept to relate to radiation by multi-arm spirals to that of other symmetric arrays supporting similar phase modes. Stacked microstrip arrays resonating in corresponding TMn1 azimuthal modes are then selected as the equivalent array to represent the mulit-mode spirals, and used to determine appropriate antenna parameters to design for highly scanned beams and high gain. Representative measured results are presented to indicate the feasibility of the array design.

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