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.