Applying wire antenna theory to cored search coils


  • B. Z. Kaplan,

  • U. Suissa


It seems conceivable that the magnetic field pattern around ferromagnetic objects of high permeability is similar to the electric field pattern around conductive objects, provided that the shapes of the magnetic and electric objects are identical. As a result, we expect the mmf behavior in the gap between two ferromagnetic limbs immersed in a magnetic field to be similar to the open-circuit voltage of a short electric dipole antenna immersed in a related electric field. Experimental setups have been developed in order to demonstrate the affinity between the electric and magnetic cases exists both in uniform and nonuniform surrounding field (electric and magnetic). The experimental results conform closely with the results obtained by the theoretical evaluation. We have thus discovered that the true counterpart of a short electric dipole is a geometrically similar magnetic structure, where ferromagnetic limbs of high permeability replace the conductive limbs of the short dipole. This is contrary to the majority of the literature on the subject, which regards a small wire loop as consisting of the entity dual to a short electric dipole. The present results enable us to treat practical magnetic sensors for ELF and ULF in terms of electric antenna theory. It is also shown that the present developments are of value in interpreting the results of measurements performed by employing ELF and ULF cored search coils in probing nonuniform magnetic fields.