Eddy-current DC magnetometer as an ELF antenna

Abstract

A relatively simple magnetometer which can operate at ELF is described. Its relatively large sensitivity of O.lnT at a bandwith of 4 Hz (R. Rabinovici and B.Z. Kaplan, Proc. of IEEE (Lett.) 74,1049-1050, 1986) and its electronics low output impedance suggest that it can be used as an effective antenna in the latter frequency band. Of value are also the ruggedness of the instrument probe (Fig. 1) and its relatively small internal noise. Furthermore, the simple fact that the present magnetometer is sensitive to the field itself and not to a derivative (which is the case when induction coils are utilized) prevents some potential difficulties. The eddy current magnetometer is a direct application of eddy current effects in nonlinear ferromagnetic conductors in the presence of several alternating magnetic fields of different frequencies, (R. Rabinovici and B.Z. Kaplan, Proc. of IEEE (Lett.) 71, 682-683, 19832. It was shown that if a nonlinear magnetic conductor is magnetized by two alternating fields of close frequencies the new low-frequency product field appears only when a DC field is alsosuperimposed. Thus it is possible to build a DC magnetic field sensor, where;oufp.ut Signal frequency is the difference between the excitatiofilfrequencies. (B.Z. Kaplan and R. Rabinovici, Proc. of IEEE (Lett.) 73, 1147-1148, 1985). The new magnetometer possesses advantages which result from the fact, that eddy currents are employed as well as the nonlinearlity of the ferromagnetic coret only one ferromagnetic core is needed to cancel the excitation signal in the