Direct measurement of the radiation resistance of a dipole antenna in the whistler/lower hybrid wave regime

An electrically short dipole antenna radiating high-frequency waves is instrumented in such a way as to directly measure its radiation impedance when immersed in a laboratory plasma. By measuring both current and voltage at the radiating end, transmission line effects are largely eliminated, and extremely accurate results can be obtained. Initial results are presented for wave frequencies ranging from lower-hybrid to whistler, for density varying from 2 x 10(18) m(-3) down to 2 x 10(15) m(-3). The initial experiments are performed for omega/omega(ce) << 1. At the highest density, the impedance is approximately real, varying from 400 Omega at 30 MHz down to 100 Omega at 180 MHz, while at the lowest density, the impedance is almost entirely capacitive, at roughly 1.6 pF.