Poynting Streamlines, Effective Area Shape, and the Design of Superdirective Antennas

A receiving antenna alters an incident field in such a way that the field is concentrated at the terminals of the antenna. The Poynting power flux density vector associated with the field carries to the antenna the power dissipated in the antenna structure and the load. Streamlines of the Poynting vector field can be used to understand electromagnetic energy flow near linear and aperture antennas. Poynting streamlines provide a way to understand and guide the design of superdirective antennas. Directivity-enhancing screens that attract Poynting streamlines and increase the aperture efficiency of a horn antenna to almost 200% are studied. Superdirective antennas generally have limited practical value due to poor radiation efficiency, narrow bandwidth, and extreme sensitivity to fabrication errors. We show that these limitations can be mitigated by using metal-only structures that are optimized for broadband operation. The tradeoff between peak achievable aperture efficiency and bandwidth is explored.