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A free electronic newsletter covering news and other topics for those interested in RF safety issues.
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Contents This section covers specifications for RF Survey Instruments.
RF Personal Monitors are covered on another page. Specifications and What They Mean Equipment suppliers usually try to accurately describe the performance specifications of their equipment. But, the specifications are often confusing to most people and sometimes they are misleading. Most specifications that deal with RF sensors are particularly difficult to understand. Probe Specifications The accuracy of a survey instrument or a personal monitor is almost entirely driven by the accuracy of the sensors. Many sensor specifications are expressed in the logarithmic ratio of dB. A parameter that has a 1.0 dB tolerance means the value could be off by 26%. In contrast, even a simple meter should be accurate within a maximum of 5%. Here are some of the most important probe specifications, what they mean, and reasonable values to expect in a good probe:
Meter Specifications Unlike a probe with many parameters to look at, there are only two parameters that impact accuracy in a meter. Everything else should be considered "features" that, while nice, normally have no impact on accuracy. Some manufacturers do not even publish a meter accuracy specification, although they have them for internal purposes. This is because probe parameters dominate when looking at the accuracy of a complete survey set (meter and probe).
How Specifications Impact Results People familiar with many types of scientific instruments come to expect accuracy specifications of ±5% or less. They are often shocked when they figure out that RF field measurements involve accuracies with a great deal more error or uncertainty. The top professionals that make RF surveys normally factor in measurement uncertainty. For example, broadband measurements with the best shaped probes available result in a measurement uncertainty of ±3 dB. In other words, when all the variables are considered and every technique available to the surveyor is used properly, the actual field strength ranges from 50% to 200% of the value indicated on the instrument. Other errors can be far higher. If you make measurements with a probe that is "out of square law" , which is described in Measurement Artifacts, it is easy to overestimate the actual field strength by up to ten to one. At first glance, an overestimation seems fine. However, if inaccurate measurements cause you to impact your operations unnecessarily the cost of the overestimation may be very large. In contrast, if you use probe correction factors when making single frequency or narrow band measurements, the uncertainty is much less than 3 dB. |
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