RF Hotspots

Conductive objects immersed in relatively weak RF fields can exhibit localized RF field strengths substantially greater than the ambient values only a short distance from the object. Examples include metal curtain rods, stair railings and metallic guy wires. When these localized RF fields are found to exceed applicable exposure limits, how should these apparently excessive fields be interpreted in terms of compliance? One approach to this issue is assessing the magnitude of the localized specific absorption rate (SAR) that could occur when an individual comes into direct contact with the object. Under touching conditions, the maximum RF current will flow between the object and the individual.

This methodology was developed for a contractual effort to the National Association of Broadcasters. Subsequently, in a separate study of contact currents conducted at the Sutro Tower in San Francisco, narrow-band measurements of current were performed when touching various objects in the immediate neighborhood of the tower. These currents were then used to compute values for the SAR in the wrist of a person making contact with the objects. The wrist represents the point of minimum cross-sectional area and the current flowing in this region will result in the greatest local current density, and, hence, greatest SAR. By taking into account the conductive cross section area of the wrist, the electrical conductivity of the tissue and the measured current, the wrist SAR was able to be estimated and compared to limits recommended by the IEEE C95.1-1991 SAR standard for the extremities. In this case, a conventional, small aperture current transformer is placed about a copper pipe probe configuration that is held and brought into contact with the object of interest. The output of the current transformer is detected with either a narrow band or broadband instrument to measure the RF current flowing between the object and the hand of the observer.

  
With the advent of new contact current meters, the large aperture current transformer fits directly around the wrist (or ankle or arm) to measure the contact current or induced body current as illustrated here in an investigation of compliance with contact current limits at a broadcast site. Metering electronics provide a direct readout of the true rms current.