Electromagnetic field monitors measure the exposure to electromagnetic radiation in certain ranges of the electromagnetic spectrum. This article concentrates on monitors used in the telecommunication industry, which measure exposure to radio spectrum radiation. Other monitors, like extremely low frequency monitors which measure exposure to radiation from electric power lines, also exist. The major difference between a "Monitor" and a "Dosimeter" is that a Dosimeter can measure the absorbed dose of ionizing radiation, which does not exist for RF Monitors. Monitors are also separated by "RF Monitors" that simply measure fields and "RF Personal Monitors" that are designed to function while mounted on the human body.
Introduction
Electromagnetic field monitors, as used in the cellular phone industry, are referred as "personal RF safety monitors", personal protection monitors (PPM) or RF exposimeters. They form part of the personal protective equipment worn by a person working in areas exposed to radio spectrum radiation. A personal RF safety monitor is typically worn either on the torso region of the body or handheld and is required by the occupational safety and health acts of many telecommunication companies. Most of the scientifically proven RF safety monitors are designed to measure the RF exposure as a percentage of the two most common international RF safety guidelines: International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines and the U.S. Federal Communications Commission (FCC). The ICNIRP guidelines are also endorsed by the WHO. RF personal safety monitors were originally designed for RF Engineers working in environments where they could be exposed to high levels of RF energy or be working close to a RF source, for example working at the top of a telecommunication tower, or working on the rooftop of a building where transmitting antennas are present. Most international RF safety programs include the training and use of RF personal safety monitors and the IEEE C95.7 specifies what is a RF Personal Monitor. In some cases the RF safety monitor comes in a version or mode for the general public. These meters can then be used to determine areas where the public might be exposed to high levels of RF energy or used to indicate the RF level in areas where the general public has access.
Specification The specifications of a RF monitor determine the work environment where could be applicable. Wideband RF monitors can be used at a broader variety of base station sites than for example a narrowband, cellular RF monitor which is designed only to be used in the mobile telephone- and data networks. IEEE Std C95.3 states that "In the region between 1-100 GHz, resistive thermoelectric dipoles are used as sensors with a background of lossy material to reduce the effect of scattering from the body. Electrically short dipoles with diode detectors as sensors may cover a portion of this range". The results of monitors which do not incorporate "lossy material" to reduce the effects of scattering, are questionable on the body. The type of response is a basic feature of any RF personal monitor and can be expressed in two basic parameters:
Directivity: Some of them have an isotropic response, which means that they are able to measure RF fields from any space direction. Others, like radial field monitors, have a partial space coverage, and have to be worn in a specific way in order to provide a correct reading. Frequency response: Flat response: units that have a flat response for all the frequency range covered, i.e. the response does not change with frequency. Shaped response: contain frequency dependent sensors that automatically weight the detected RF fields in accordance with frequency-dependent RF exposure limits. It is common that RF personal monitors provide results as a percentage (%) of frequency-dependent limit values of a specific standard (sometimes called reference levels or MPE, maximum permissible exposure). It is important to be careful interpreting exposure during an alarm condition based on a % result; shaped response RF personal monitors will provide a result as a % of the standard, independently of the frequency, while flat response monitors will provide a result as a % of a particular value (not frequency-dependent), so it is important to know which is the particular value this % is referring to. Some RF personal monitors have different versions, shaped to each standard, so they will be more accurate, but can be used only for that standard. Others have a single version, so will be less accurate, but can be used for different standards. Usually, the alarm of most RF personal monitors is triggered by instant values, however, standard limits are specified as time-averaged values. Some RF monitors have the possibility to trigger alarms based on average values, which is a better indication of the real exposure situation (as an example, an instant value can be at 200% while the average being below 100%). As they are typically small, portable units, they are usually equipped with only a few LEDs for a rough field level indication (50%, 100%, etc). Nevertheless, some of them have a datalogger that allows to download the measurements, check for the exact values, and keep a history record of the exposures. Wavecontrol's WaveMon has available a GPS and altimeter to include position information to the data records. Other specifications that may be relevant, depending on the application are battery characteristics (lifetime, ways to change or recharge), dimensions, weight, and operating temperature. The following table shows different basic specifications of some RF monitors:
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