Superconducting Antennas

Abstract

A high-efficiency electrically and physically small superconducting antenna is reported. Antenna factors and matching circuit equivalents are developed. Theoretical predictions of efficiency and Q increase are derived on the basis of the superconducting surface resistance of lead. The antenna is a loop supsended through a ground plane and matched through a symmetrical coaxial cavity having a resonant frequency of 400 MHz. The loop is lead-plated and has an open area of approximately 1 cm2. Coupling to the system is achieved through a movable electric-field probe inserted into a slot in the wall of the matching cavity. A normal-state electrically small antenna of this size has a theoretical efficiency of less than 1% due to the fact that the loop resistance is much larger than the equivalent radiation resistance. Upon cooling to 4.2°K, the Q of the system increases to 20 000 from a room-temperature value of 156, indicating limitation by the radiation resistance only. The power radiation efficiency increases by a factor of 500, as measured by a fixed external receiving system. The VSWR of the system is maintained near unity in order to avoid the complications of mismatching.