A three-element, superdirective array of electrically small, high-temperature superconducting half-loops at 500-MHz

Electrically small loop antennas become efficient radiators when constructed of high-temperature superconducting (HTS) materials since the surface resistance losses can be significantly reduced below the small radiation resistance of the loop. Recently, a significant improvement in radiation efficiency (30 vs 3%) was demonstrated for a small (0.04/spl lambda/ at 500 MHz) HTS half loop compared to a grid antenna. HTS materials can also improve the radiation efficiency of closely packed (i.e., < 0.2/spl lambda/ element spacing) superdirective arrays. A superdirective array produces a higher directive gain (compared to a uniform or tapered array excitation) by increasing the reactive currents of the closely spaced elements. For conventional metal elements, increased reactive currents result in increased I/sup 2/R losses and lower array efficiency. HTS elements substantially reduce I/sup 2/R losses and significantly improve the efficiency of a compact superdirective array. Results are presented for a three-element array composed of TiBaCaCuO half-loop antennas.<>