A unidirectional semi-circle spiral antenna for subsurface radars

Specifications of an antenna for a subsurface radar are quite different from those of an ordinary antenna for telecommunication. The antennas for a subsurface radar are located very close to the interface between the air and the Earth´s surface. The antennas for subsurface radars have mutually conflicting requirements such as low-frequency operation for better penetration, broadband characteristics for finer resolution, small size for pinpointing the antenna location, and sufficient sensitivity for good signal-to-noise ratio of the image. It is found that a semi-circle spiral antenna with a cylindrical waveguide can meet most of the above requirements. A practical structure is proposed, and a method of designing such an antenna is presented. The experimental results show the usefulness of the developed antenna