Electric, magnetic and electromagnetic (EM) fields represent a natural phenomenon in the human environment, in which the human species has evolved and the human organism has become accustomed to their presence.
During the longest period of evolutionary development, the only sources of these fields were natural sources, such as: the Sun, cosmic radiation, atmospheric discharges, the Earth's natural electric and magnetic fields, etc.
However, the twentieth century brought a sudden expansion of artificial field sources created by human hands, which significantly changed the characteristics and levels of natural fields. The use of electricity and technological advances has enabled a number of useful innovations, which consequently generate a field or use the EM field for wireless data transfer.
Among many artificial sources of EM fields, there are radio and TV broadcasting antenna systems, mobile telephony base stations, wireless Internet access devices and many others. All of them inevitably radiate the EM field into our living and working environment, in the designated frequency ranges.
High frequency (HF) field sources are considered to be the sources which radiate in the frequency range from 100 kHz to 300 GHz. There is an additional division in this range into the following sub-bands:
• high frequencies fields (from 100 kHz to 30 MHz);
• very high frequencies fields (from 30 MHz to 300 MHz);
• ultra-high frequency fields (from 300 MHz to 3 GHz);
• super high frequency fields (from 3 GHz to 30 GHz) and
• extremely high frequencies fields (from 30 GHz to 300 GHz).
The Regulatory Agency for Electronic Communications and Postal Services (RATEL) is, inter alia, responsible for the rational and economical use of the radio frequency spectrum in the Republic of Serbia.
Radio frequency spectrum management in the Republic of Serbia is based on the Radio Frequency Allocation Plan and the Radio Frequency Allotment Plans for frequency bands.
Radio frequencies are used on the basis of an individual license, under the general authorization regime and for special purposes. All sources must radiate the EM field in the appropriate sub-band, which corresponds to its purpose.
What are the Sources of the EM Field?
In most cases, sources which generate HF fields are used for signal and data wireless transmission systems. Nowadays, the wireless transmission technology is dominant, therefor the communications and other various services cannot be provided without the sources of this type.
By increasing the number of HF sources. the intensity of the HF field in the surrounding environment is also increased.
The increase in the field intensity in our environment can also be influenced by the work of higher power transmitters (radar, radio and TV transmitters, mobile telephone base stations), located in vicinity, as well as by the operation of low-power devices that we use every day, such as mobile phones, microwave ovens, Wi-Fi routers, etc.
What are the Effects of Exposure to the EM Field?
The exposure to the field is an inevitable effect, which occurs when people stay in the area with a certain intensity of EM fields. During the exposure, the body absorbs part of the energy of these fields.
Estimating the daily amount of the absorbed energy is a complex process, since it depends on many factors, primarily on the effective radiated power of the source, the distance from the field source and the duration of the exposure. The field energy decreases drastically when the distance from the source is increased, which means that a person can absorb more radiated energy from devices in the immediate vicinity, such as a mobile phone, rather than from remote sources, such as roof antenna systems.
It should be noted that the degree of energy absorption is individual, as it depends on the tissue characteristics of an individual.
The HF fields change very quickly in time, thus causing the heating of the exposed tissue when the intensity of the field is high enough. It is therefore said that HF fields have a dominant thermal effect.
How is Exposure Assessment performed?
Each EM field source radiates its field at a specific frequency. Consequently, there is a permitted field limit defined for each frequency, within which the field is considered to have no harmful effects on the human body. This field value is called the reference level.
If the level of the field from all surrounding sources, that is, the cumulative value of the field level, is measured, then the so-called overall exposure is considered, described by Global Exposure Ration (GER) parameter.
One of the exposure assessment techniques uses the principle of determining the lower and upper limits of exposure, with real exposure to the field somewhere between these limits. This technique uses the minimum and maximum prescribed reference levels, which are defined by the legislation.
When measuring the electric field, the exposure limits are determined as follows:
GERlower = (Emeasurement / Eref. max)2 and GERupper = (Emeasurement / Eref. min)2.
The actual exposure to the cumulative field is somewhere between these boundaries.
How are the reference levels calculated?
The reference level for a given frequency is defined under the Rulebook on the Limits of Exposure to Non-Ionizing Radiation ("Official Gazette of the Republic of Serbia", no. 104/09). For each frequency, there is an appropriate formula for calculating the reference limit level.
For the frequency range of the sensors currently used in the EMF RATEL system (from 100 kHz to 7 GHz), the minimum electrical field reference level is Eref. min = 11 V/m, while the maximum reference level is Eref. max = 34.8 V/m.