Absolute calibration of antennas at extremely low frequencies

To calibrate electric-field sensors at extremely low frequency (ELF) the measured terminal voltage at a receiving antenna is compared with the Analytically predicted for a precisely known incident field. That incident field is generated by an electrically very short thin linear radiator. Formulas are given for the field in the immediate vicinity of such a radiator above a perfectly conducting ground. Also given is an expression for the integral of this field along a vertical path starting at ground level. The expressions involve the magnitude of the current at the base of the radiator and methods are described for establishing that quantity precisely by measuring the base-to-ground voltage and the very small capacitance (about 20 pF) of the radiator. The methods for small capacitance measurement are also applied to linear and spherical receiving antennas, and experiments are described in which it was possible to predict the terminal voltage at a receiving antenna to within percent when the voltage applied to the transmitting radiator, located at a relatively short distance ( to 21 m) was known. Details of measurement circuitry are given including descriptions of an extremely high input impedance ( ) amplifier and a circuit for the compensation of cable capacitance. For a uniform incident field the effective height of both, a vertical linear receiving antenna and of a spherical antenna, is equal to the distance between ground level and the center of the conducting part of the antenna. Typical values for the natural, ambient ELF field in the frequency range 3 to 30 Hz are shown to be in the vicinity of 0.1 mV/m/Hz^1/2.