Surface Waves and Mode Selection on Multielement Spiral Antennas

The solution of Maxwell's equations for the idealized sheet of spiral antennas has been extended to a cos nφ input excitation. At large distances the current density is the combination of a circularly polarized surface wave that vanes as exp (− jnφ ) and the frequency independent current distribution previouslr worked out. The frequency independent component is almost entirely in the exp ( jnφ ) mode. Half the input power goes into the exp ( jnφ ) radiation pattern, and almost half goes into the surface wave. This explains the remarkable mode selection properties of multielement spiral antennas that were observed a few years ago.