RF Area Monitors
There are currently two types of RF area monitors available:
Simple Fixed Monitors
Advanced Area Monitoring Systems
The simple fixed monitors—Narda Safety Test Solutions’ SMARTS series—first became available in late 1980s. Advanced area monitoring systems became available around 2000.
The original RF area monitors can best be described as the RF equivalent of a smoke detector. In fact, these monitors used an integrated circuit designed for smoke detectors in combination with RF sensors. The RF sensors are similar to those used in probes and personal monitors. These area monitors are not isotropic, however. They are designed to be mounted on the ceiling or a wall. At best, these simple fixed monitors sense over an entire hemisphere.
The monitors normally operate from battery power, although low-voltage DC operation is possible with some models. They have internal audible alarms that sound when the detected field strength exceeds a preset limit. Remote alarm indications are also possible with many models.
A new type of area monitor started to appear a few years ago. At least two of the designs were developed for the Italian market. Italy has adopted some extremely low public exposure limits—far below the ICNIRP public limits. Some communities have deployed solar- and/or battery-powered systems that use conventional isotropic field probes and circuitry similar to survey instruments, including data-logging capabilities and a wireless modem. Everything is packaged in a weatherproof housing. These systems are designed to make measurements continuously. Alarm thresholds are set up so that the system calls a central monitoring station should field levels exceed a preset level and/or if there are any problems with the system. Logged data is downloaded periodically. Exactly what is done with the data and how useful it is subject to debate!
The original simple fixed monitors were developed to monitor for waveguide leaks in the transportable communications and radar shelters used by the military. These small “buildings” have metal walls that would reflect energy should a leak occur, and also confine equipment operators to a relatively small space. Any waveguide can leak, and the rough handling that can occur during the movement of these shelters makes leaks even more likely. The biggest concern is the rubber-coated flexible waveguide, which is quite prone to developing leaks. Even in the event of a leak, that rubber coating will still hold air, so those that depend on using a loss of pressure as a sign of a leak may not recognize the potential hazard.
Of course, transportable shelters were only the first application for these monitors. Today, most of the applications fall into two broad categories:
Detecting an acute hardware failure, such as a waveguide leak.
Detecting that a system is on as a redundant measure to other controls and indicators.
The biggest limitation with using these simple fixed monitors is determining when to use one and where to mount it. Since field strength falls off rapidly with the distance from the source, it is still possible to have a significant leak and potential for human exposure while the monitor isn’t sensing field levels above its preset alarm threshold. The ideal solution is to make sure the monitor is closer to the potential source of energy than to where people might be. This is often difficult to achieve, but it is the only placement that guarantees complete protection. There are other mounting schemes, but this gets into much more complicated scenarios.
The market and applications for the new advanced monitoring systems are just developing.