Two-arm eccentric spiral antenna

This paper presents a free-space analysis of a planar two-arm eccentric spiral antenna. The eccentricity generates a circularly polarized wave with the main beam directed off-normal to the spiral plane. Input impedance and radiation characteristics are calculated from the current distribution generated by a method of moments solution. The radiation characteristics include power gain and axial ratio patterns. Three-dimensional (3-D) gain surface plots illustrate off-normal dependence of the eccentricity constant. The eccentric spiral antenna is found to have a significant increase in gain. This occurs when adjacent spiral arms rotating 360° have an electrical length increase of one wavelength. Thus, adjacent currents on one side of the spiral are nearly all in-phase. On the opposite side of the spiral, all neighboring currents are nearly out-of-phase. Evaluation of these currents result in a theory of operation for eccentric spiral antennas