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The nonlinear impedance of a satellite-borne, electric dipole antenna was measured at ELF/VLF at drive voltages between 10 mv and 100 v peak-to-peak. The magnitude of the impedance is in general agreement with the predictions of theoretical models of plasma-clad antennas. At low voltages the phase angle of the impedance is in agreement with linear sheath models, i.e., at low densities the current leads the voltage and as the density increases the phase angle approaches zero. However, at high drive voltages the current lags the voltage. Above 10 v p-p the phase angle often exceeds +90 deg.