Broad-Band, Noninvasive Radio-Frequency Current Probe

This circuit responds in approximately constant proportion to current over a wide frequency range.

An instrument that noninvasively measures alternating current over a broad frequency band (typically from about 0.3 to about 110 MHz) has been invented. This instrument could be especially useful for assessing radio-frequency hazards by measuring currents in various parts of humans or personnel exposed to radio-frequency electromagnetic fields.

The instrument includes a magnetic pickup coil connected to an active circuit that measures the current induced in the coil by the magnetic field of the current that one seeks to measure. The pickup coil is of a type known in the art as a Rogowsky coil or Rogowsky transformer. As such, the coil is essentially the secondary winding of a transformer with a toroidal core that is placed around the human limb or other object that carries the current that one seeks to measure. The current-carrying object acts, in effect, as the primary winding of the transformer. To avoid the weight, cost, and nonlinearity of a ferrous core, and to minimize the effect of the perturbation of the current to be measured, the coil in this circuit is wound on a nonferrous core.

The active circuit used to measure the current induced in the coil is similar to active-antenna circuits developed previously at NASA for measuring the magnetic components of electromagnetic fields at frequencies up to a few megahertz. The active circuit includes an operational amplifier. The virtual ground at the input terminals of this amplifier is used to present a low impedance to the coil, thereby making the series resistance of the coil circuit much less than the inductive reactance of the coil, even at the low end of the frequency range.

By basic principles of electromagnetism, the voltage induced in the coil is proportional to the frequency and to the current that one seeks to measure, while the inductive reactance of the coil is proportional to the frequency. The current in the coil, which is the current sampled by the amplifier, equals the ratio between the voltage induced in the coil and the total impedance of the coil circuit. Hence, at all frequencies for which the inductive reactance is the dominant component of the impedance of the coil, the coil current sampled by the active circuit is proportional to the current that one seeks to measure.

This work was done by John F. Sutton of Goddard Space Flight Center and Mark J. Hagmann of Florida International University. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Goddard Space Flight Center; (301) 286-7351. Refer to GSC-13985.

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