The EM field testing and measurements are extremely important, considering the possible impact of the EM field on the human organism. The objective of testing is to determine the individual levels of the present EM fields, as well as the overall level, the so-called cumulative level of EM field.
When determining the possible impact of the source on the living environment and the population, the influence of the specific source is tested without the presence of other sources, but also the cumulative effect of all sources in the observed environment is measured.
So far, many techniques have been developed, which are used to determine the level of the EM field in the immediate vicinity of the source, the so-called near field zone, as well as in the far field area of the source. Only to professional staff and technical support members who maintain the observed sources have access to the near field zone, while the general population is always located in the far field area of the source.
For both zones, it is very important to evaluate the potential impact of the source, or their EM field, however the criteria for assessing the impacts in these two zones are different.
The theoretical calculation of EM field intensity is the first step of the assessment of the EM field impact. It requires knowledge of a significant set of technical characteristics of the source in order to accurately assess the level of EM fields that will be generated in the area of interest.
On the basis of theoretical calculations, it is possible to estimate the distribution of EM fields in the area surrounding the source, i.e the values of the EM field at arbitrary points and at arbitrary distance from the source. Therefore, this type of testing gives the highest degree of freedom and allows a very detailed assessment of the intensity of the EM field.
This type of testing is also the most demanding, both in terms of resources, and in terms of knowledge of the characteristics of the source and its environment, which significantly influences the propagation and distribution of EM fields.
Typically, this type of test is performed prior to the source set up, but also in cases where it is necessary to have an extremely precise and detailed image of the EM field in all relevant points.
EM Fields Measurement
Measuring the level of the EM field by using the appropriate measuring instruments is a relatively fast testing method, obtaining the values of EM fields that exist in real conditions, in which the source radiates the EM field. For the HF fields, the intensity of the electric field is always measured.
In many cases, it is not physically possible to place the measuring instrument at the desired place; therefore this type of testing gives the levels only for a limited part of the space of interest, which is also a limitation of this technique.
The existing measuring instruments can be classified into two categories:
• instruments that measure the level of EM fields in single frequencies or frequency subbands (frequency selective testing) and
• measuring instruments for broadband testing, which determine the cumulative contribution of all surrounding sources.
Frequency selective tests are carried out in situations where a detailed knowledge on the contribution of individual sources is necessary at the location of the test. If cumulative level of the EM field is required, then broadband measurements are more efficient, as they provide the measurement data faster.
Measuring the level of an EM field is a much simpler process than the theoretical calculation. The measured values give a real picture of the distribution of the EM field at the observed location, taking into account the influence of all the objects present in the environment. These objects can cause reflection and absorption of EM fields, which leads to changes in the distribution of the EM field level, compared to the free space.
Long-term Monitoring of the EM Field
This technique represents the most advanced approach of the EM field testing, with the goal to obtain information on the behaviour of the source over a longer period of time, or to determine the fluctuation in the level of the EM field.
Long-term monitoring techniques provide a detailed insight into daily changes of EM field level.
The EMF RATEL system represents the actual implementation of the long-term monitoring of EM field level, using measuring sensors that are joined in a single network, covering the territory of the Republic of Serbia.
This approach uses broadband measuring equipment, which measures the cumulative contribution of all active sources at a given location. If necessary, broadband measurements can be replaced by frequency selective measurements.
Electric Field Measurement
The obligation of a detailed measurement of the electric field when commissioning a certain source is decided by the authority in charge of the environmental protection. Measuring the field around the source is carried out by the firms accredited for this type of work and authorized by the ministry responsible for the environmental protection.
The method of measuring the level of the electric field usually consists of three phases. In the first stage, the so-called spatial level scanning is performed on the specific network of test points, in order to determine the point at which the field has the local maximum (the so-called "hot spot").
In the second phase, at the selected "hot spot" the electric field is measured at one or more points at specific heights. Based on the measured values, the average value of the electric field at different heights is determined, which is necessary for assessing the exposure of the entire human body.
In the third phase of the test, the assessment of the potential exposure to the electric field is performed.
The results of the continuous field monitoring provided by the EMF RATEL system may indicate the need to perform a detailed measurement of the electric field in a particular zone.
Zone of Interest for Continuous Monitoring of EM Fields
The EMF RATEL system envisages the sensors for measuring the EM fields to be installed in the zones of increased sensitivity.
Increased sensitivity zones cover the areas where more people stay for longer periods and, in particular, the areas where kindergartens, schools, universities and hospitals are located.
Individual sensors are placed at adequate locations so that the measured values of the cumulative EM field, generated by the sources in the surrounding area, represent the maximum expected value in the given zone.
Why is it customary to measure only the electric field?
Usually, the locations where the measurements are made, as well as areas of interest, are at a greater distance from the source of the EM field (a few tens or hundreds of meters). Thus, it is usually said that such places are located in the far field zone of the source.
In the theory of EM fields, it is known that in the far field zone there is a direct connection between the electric (E) and the magnetic field (H), given by the formula:
E = Z H,
where Z is called characteristic impedance of the space. In case of air, this impedance has a numerical value of Z = 377 Ω.
Since it is easier to produce a device for measuring the electric field, and in view of the fact that there is a direct connection with the magnetic field, it is usual to measure only the electric field in the far field zones.
Measured uncertainty of the sensor
The devices with which measurements are made have appropriate real characteristics, so the measured value differs from its true value. The difference between the measured and the true value is expressed in percentages, in appropriate mathematical manner, and it is then called the measurement uncertainty of the device.
This difference can be negative, in case the measuring device shows a lower value than the correct one, or it can be positive, in case the measuring device shows a higher value than the correct value.
Based on the knowledge of the measurement uncertainty, it is usually said that a measuring device measures the value which is in the interval around the true value. Practically, the measured value is within the limits below and above the true value, where the boundaries are determined by the measurement uncertainty of the measuring device.
Measurement uncertainty depends on the parameters of the practical realization of the measuring device and each manufacturer is obliged to give uncertainty for his measuring device.
The measurement uncertainty of the sensor elements currently used in the EMF RATEL network is 22%.