Measurement of ELF field strength

This article reviews the measurement of the absolute magnitude of electric and magnetic fields at extremely low frequencies (ELF), 3 Hz to 3 KHz, when the dimensions (
L
) of any practical receiving antenna are an infinitesimal fraction of the wavelength (
L < \lambda/1000
) and the incident electric and magnetic fields are, in general, not related to each other by the wave impedance of free space. Various sensor systems and techniques for their calibration are discussed.