Dual-Resonant Search Coil for Natural Electromagnetic Waves in the Near-Earth Environment

A dual-resonant search coil (DRSC) is designed as a wideband magnetometer for natural electromagnetic waves in the near-Earth environment. Since the frequency response of a search coil is determined by its resonant frequency, its bandwidth can be increased by using multiple resonances. Two resonances are obtained by capacitive coupling with negative mutual inductance between two coils. The resulting measurement band of the DRSC is from 10 Hz to 1 MHz for a sensor 100 mm in length, 25 mm in diameter, and 0.11 kg in mass. A theoretical analysis of the resonant frequencies and gain profile of the DRSC agree with measurements. The noise equivalent magnetic induction of the DRSC is 40 fT/Hz^1/2 at 3 kHz and 14 fT/Hz^1/2 at 140 kHz. This magnetometer is the first to realize two resonances using a single sensor and preamplifier.