Measuring RF Levels At Multi-Signal Sites

Anguish is a familiar response to FCC rulings, and industry concerns were in ample supply when the FCC issued its guidelines for human exposure to RF emissions back on Aug. 1, 1996. The release was first greeted with silence and then with a cacophony of comments ranging from general acceptance to disagreement with specific elements of the guidelines.

That is not surprising, considering the difficulty that today's multi-emitter transmission sites present to accurate RF emissions measurements. Multiple emitters dramatically complicate the measurement process, and it is quite easy to make measurements that appear accurate but are, in fact, quite the opposite. However, the shaped response probe, when used with an accurate RF radiation measuring instrument, provides true RMS detection and allows the operator to quickly determine compliance with FCC radiation exposure standards.

A COMPLEX SITUATION
The human body is a thermal entity that responds proportionally to RMS energy levels. The major human exposure standards used throughout the world, such as IEEE C95.1-1991 and National Council on Radiation Protection (NCRP) Report 86, are based on controlling the RMS level of exposure an individual receives. This level is averaged over the whole body during a period of time, typically six minutes. The FCC originally planned to adopt the IEEE standard but essentially adopted the NCRP standard instead.

The instruments used to make these measurements increasingly rely on digital circuitry, relegating the RF (analog) portion of the measurement only to the probe. Both in appearance and ease of use, these instruments are a vast improvement over their analog predecessors. However, it is easy to assume that because they display values in digits rather than with an analog meter, they are fundamentally more accurate.