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How It Works

Current probe open to accept a wire

Current probe shown with calibration cell

Calibration cell mounted ready to test

This technology includes a nonferrous current probe based on a conductive toroidal coil. When placed around the leg or other body member, the probe acts as a transformer. As RF-induced current in the body generates a magnetic field, the probe’s magnetic pickup (Rogowsky) coil responds. The voltage induced on the coil is directly proportional to the time derivative of the magnet flux through the coil.

The coil is integrated with a wide-band transimpedance amplifier circuit with a design that keeps the sensitivity of the current probe substantially flat over a wide frequency range—60 Hz to 110 MHz. This frequency range makes the device useful with AM and FM transmitters as well as with RF heating equipment such as sealers, ovens, and welders (27.12 MHz).

Why It Is Better

This technology’s innovative design offers several advantages. Because it does not affect the impedance of the body, does not introduce an additional antenna, and has a flat response over a broad frequency range, this device provides a more accurate measurement. This accurate measurement of RF-induced current in the body is particularly useful in applications where various frequencies may be present (e.g., harmonics)—a capability not presently available in similar products. Its nonferrous core is inexpensive, lightweight, and provides for flexibility in product design, allowing the development of devices that can be worn comfortably around the ankle, leg, arm, wrist, chest, neck, etc.

Beyond its design advantages, this device directly measures RF-induced currents in the human body, making practical a direct calculation of specific absorption rate. IEEE safety standards for human exposure to RF (adopted by most regulatory bodies as the basis for legal statutes) specify SAR as the basic metric of safety (IEEE Std. C95.1-1991). Because measuring induced currents, more directly associated with SAR, was previously costly, inaccurate, or impractical, the standards set forth maximum permissible exposure (MPE) limits in terms of electromagnetic field (EMF) strengths. However, the standard itself noted that “absorption of electromagnetic energy from even the most uniform field can result in highly variable anatomical depositions of energy.” Because of this uncertainty—due to such variables as height, weight, body shape, and changing body position—the EMF safety limits are overstated by as much as a factor of 100. Moreover, in cases where RF shock or burn may be possible, the IEEE standard still states that:

Induced body currents should be measured by determining the RF current flowing to ground through the feet of the individual. Contact currents should be measured by determining the RF current through the hand in contact with the ungrounded surface.

Because this device can directly measure the actual EMF-induced current flowing at any point in the human body, both these induced and contact currents can be directly measured, as well as a SAR being calculated.

As a result, this device provides a low-cost means for companies with workers exposed to RF radiation to confirm unequivocally that employees are safe.