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A free electronic newsletter covering news and other topics for those interested in RF safety issues.
LIVE, Web-Based RF Safety Training
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Contents
There are many parameters that define an RF personal monitor. Some of the newest models are rich in features while others provide basic functionality—they simply sound an audible alarm when field levels above a preset limit are detected. Although many of these features are important, the performance and rightfully, the value, all depends on the performance of the sensors. For if the sensors are not detecting field levels accurately, then all the other features are of little value. Sensors are designed to detect either the electric (E) field or the magnetic (H) field. They can have either flat frequency response or shaped frequency response. See the following two sections for details. Shaped frequency response versus flat response The sensors can be designed with a goal to detect all frequencies with equal sensitivity, which is referred to as "flat frequency response, or to vary sensitivity as a function of frequency in accordance with a particular standard. These sensors designs are referred to as "shaped" frequency response. The following graphs illustrate the value of shaped frequency response in a multi-signal environment. In this very simple example, there are two emitters on a rooftop. One operates in the VHF communications band where the Maximum Permissible Exposure (MPE) limit is 1.0 mW/cm². The other emitter operates at about 900 MHz where the MPE limit is 3.0 mW/cm². Since cellular frequencies are a little lower than 900 MHz and pager frequencies are a little higher and many two-way services operate in the VHF band, this is representative of a typical wireless site. Most rooftops have more than two emitters but the analysis is the same. In both cases the field level measured at a particular spot on the rooftop is 1.5 mW/cm². The problem is if you measure this with either a survey instrument or an RF personal monitor that has a flat frequency response, you can get a value for field strength but you cannot determine compliance with the standard. This is because it is not possible to determine the amount of energy contributed by each emitter at the point of measurement.
A device with a shaped frequency response would interpret these two scenarios (subject to measurement uncertainty, of course) as 150% of standard and 83% of standard. So, in the first scenario, the field levels are well above the MPE limits while in the second scenario, the field levels are within the MPE limits. Yet, the total field strength is the same! The only way to determine compliance under these conditions with a flat response sensor is to shut off all but one of the emitters. While that may be possible if only two emitters are present, it is not feasible if there are several, which is often the case. Thus, RF personal monitors with a flat frequency response are of little value in most multi-signal environments. A flat response monitor can be useful if all the emitters operate at frequencies where the exposure limits are the same. Microwave band RF personal monitors, which were the first RF personal ever developed, are still useful for applications where all the emitters operate in the microwave region. But, the increasing complexity of the RF environment is diminishing the value of these monitors. Sensor type: E-field, H-field, or both In the far field, electric field and the magnetic field levels are equal. Therefore, either an E-field or an H-field sensor should yield the same results in the far field. The distance between the source of energy, such as the antenna, and the far field is dependent on several factors. The two most important are frequency and the type of the antenna. The major measurement practices standards use 300 MHz as the cutoff point for measuring both fields. Although it is not stated in this way, the logic behind this part of the standard is that under all but the most unusual circumstances you will invariably be in the far field at frequencies above 300 MHz. In reality, you can make measurements at frequencies as low as 50 MHz with little compromise in accuracy providing that you remain at half a meter away (about 20 inches). This is because the definition of far field conditions include that not only the fields must be equal in magnitude but the phases must be correct. Yet phase error has little impact on the sensors used in RF personal monitors and RF survey instruments. Below 50 MHz it becomes more likely that the E-field and the H-field will be of different magnitudes when you are anywhere near enough to the antenna to be concerned about RF radiation. At these frequencies, the electric field is far more important biologically. Many of the newest standards have relaxed exposure limits for the magnetic field below 100 MHz. This is because RF heating efficiency, or SAR rates, drop off dramatically below the human resonance region. At the lower frequencies, electrostimulation (shocks and burns) is more of a concern than whole body heating. And the potential for induced currents and contact current is proportionate to the strength of the electric field. The magnetic field has limited effect on the body. See Health Effects, Body Heating, and Shocks and Burns for more information. Therefore, a good electric field sensor does the job biologically although it may not guarantee compliance with a standard that has the same exposure limits for both the E-field and the H-field. An H-field sensor alone is of little value. RF personal monitors that contain both an electric field sensor and a magnetic field sensor appear to provide the ideal solution. The problem is that the designs currently on the market use a simple dipole for the electric field. And dipoles do not work on the body at frequencies below about 30 MHz. The only electric field sensors that work well on the body at these lower frequencies employ surface area detection rather than dipole detection. See Sensor Designs for more information. Important Specifications for RF Personal Monitors An RF personal monitor's accuracy is primarily dependent on the design, construction, and calibration of its sensors. Many of the same parameters that are considered when looking at a probe are also important in RF personal monitors. These include:
Other Important Features for RF Personal Monitors RF personal monitors are more than sensors and control circuits. How do you know that you have entered an area above a certain level? Can you review the exposure levels at a later date? How do I wear the monitor and make sure that it doesn't fall off or get damaged in the rain? These are all important questions.
The are currently two types of RF area monitors available:
The simple fixed monitors first became available in late 1980's. Advanced area monitoring systems first became available about 2000. Simple Fixed Monitors The original RF personal monitors can best be described as the RF equivalent of a smoke detector. In fact, these monitors use an integrated circuit designed for smoke detectors in combination with RF sensors. The RF sensors are similar to the sensors in probes and personal monitors. These 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. These RF area 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 possible with many models. Advanced Area Monitoring Systems A new type of area monitor started to appear about three years. 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 (without the display) including data logging 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 is subject to debate. Applications for area monitors The original simple fixed monitors were developed to monitor for waveguide leaks in the military's transportable shelters. These small "buildings" have metal walls that reflect energy should a leak occur. Operators are confined in 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. It is quite prone to developing leaks. And the 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:
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 the potential for human exposure while the monitor will not see 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 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 it gets a lot more complicated. The market and applications for the new advanced monitoring systems are just developing.
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