“SHAPED” FREQUENCY RESPONSE – WHAT IT IS
AND WHY IT MATTERS
Standards for public exposure to electromagnetic fields (EMF)
Discussions on standards and guidelines emerged as early as the late 1950s and since then, there have been diverse approaches taken by various national and international authorities and agencies in drafting standards and guidelines related to the exposure to RF energy. Standard-setting organisations in collaboration with governments have developed acceptable exposure standards for RF energy. Federal health and safety agencies, such as the Environmental Protection Agency, Food and Drug Administration (FDA), the National Institute for Occupational Safety and Health and the Occupational Safety and Health Administration are also involved in monitoring and investigating issues related to RF levels and RF exposure. The most recent standards recommend separate safe exposure levels for the general public and for RF trained workers.
The various initiatives undertaken around the world by the scientific community to firstly understand, then quantify and finally limit the exposure by humans to RF energy have all drawn the same conclusions, evident in the limits themselves, namely that the human body reacts differently to different frequencies. At the lower end of the spectrum the human body, due to its physical size becomes resonant and thereby more easily absorbs radiated energy. This is in the region of FM radio and classic television. These transmitters also typically radiate very high levels of RF to achieve the wide area coverage from a single transmitter, all placing those working on these structures at risk of RF overexposure.
RF exposure safety guidelines and limits such as the FCC (NCRP), Safety Code 6 or the limits set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) are all shaped over frequency for this reason and therefore personal protective equipment for working in these areas, namely personal RF monitors, must also have a shaped probe response to accurately measure to these established limits.
Shaped frequency response – what it is and why it matters.
Shaped frequency response sensors have inbuild frequency dependant filtering components within the actual probe itself to accurately assess RF exposure across frequency in accordance with the shaped limits. Devices having a flat response are merely field probes and report, for example, just the E field in V/m. Such devices are not suitable for RF exposure safety as they are not measuring in accordance with the established safety limits by either only measuring one of the field components like the E field, or not being shaped, thereby disregarding the research requiring that the body influence be considered.
It is often not enough to simply know the magnitude of the field strength. Shaped frequency response is extremely valuable when measurements are required at sites where there are multiple transmitting systems operating at different frequencies with potentially different exposure limits. A shaped probe response allows for the accurate combined assessment of multiple sources of RF exposure giving a total combined exposure in percentage of the actual exposure limits. This differentiates mere field probes from personal RF monitors, which must have a shaped response, and measure both field components, namely E and H to correctly report the cumulative exposure in percentage of the limits.
Why use a personal RF monitor with a shaped frequency response?
When transmitting antennas are mounted on, for example, rooftops it is very easy for those working near these to exceed the permissible exposure levels. These levels far exceed those in areas accessible to the public, on the walkways etc below the rooftop. Any persons accessing these areas, such as telecommunications contractors, window washers, waterproofing specialists, HVAC technicians or the likes should be aware of the risks involved and be paying attention to posted signage indicating the various areas to be avoided. As many of those working in these areas will know, once a building is hosting transmitting antennas, there are many added over time, often making it almost impossible to move about with the various cable trays and the likes. This then also leads to the entire rooftop being deemed as an area where the RF exposure limits will be exceeded and the onus rests on the workers themselves to know where to and where not to stand or work as having all the transmitters turned down to acceptable levels is near impossible. This is where a personal RF monitor with a Shaped frequency response measuring both E and H fields becomes critical as it is constantly assessing all the sources of RF present in terms of the safety limits and presenting the user with a single combined readout as a percentage of the limits. Also, as this combined level varies rapidly whilst moving about, or due to traffic variations on the systems an inbuilt alarm should begin to warn once levels exceed 50% of the limits. This allows the user to know they are still safe, and can continue to work there, but they are close to exceeding the limits, which would be once 100% is indicated. Once 100% is indicated the user needs to either move from the specific area or have the dominant transmitters powered down to levels allowing safe work in those areas.
For more download our introduction to RF here.