Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3 KHZ to 300 GHZ - Safety Code 6
Appendix V Measurements and Evaluation
A. RF Fields
The area around any RF source is generally divided into two zones: the near-field zone, and the far-field zone. More information on this subject can be found in Appendix III and references (15,18,21). In many RF safety surveys, the exposure levels have to be determined in the near-field zone of the source. Not infrequently, the field comprises RF radiation from several sources. Difficulties can be encountered in determining field strengths and power density of such fields, as outlined in references(15,21), and special care should be devoted to the selection of a survey instrument. Only instruments that are designed for operation in the frequency range required shall be used.
A.1 Basic Characteristics of Survey Meters
In surveying fields in the near-field zone of an antenna or in close proximity to a device, both electric field and magnetic field strengths shall be measured where possible. However, instrumentation for the measurement of magnetic fields at certain frequencies may not be commercially available. In this case, the electric field strength shall be measured. In the far-field zone, it is sufficient to measure any of the following parameters: electric field strength, magnetic field strength or power density. Many meters have indicators that are calibrated in power density units (e.g., mW/cm2 ), but the quantity actually measured may be the square of the strength of the electric or magnetic field. It must be remembered that power density measurements in the near-field zone are not meaningful for the evaluation of exposure levels. The information about the measured field parameter is normally provided in the instruction manual.
If the frequency range covered by one survey instrument is narrower than the frequency range of the fields generated by the RF sources in the vicinity of the site surveyed, as many instruments as necessary shall be used to determine the fields in the whole range of frequencies.
Since, in the majority of RF surveys, the orientation(s) of the electromagnetic field vector is not known, a survey meter having an isotropic detecting element is preferred.
If the only meter available is one having a single-axis detecting element, measurements of the total field can be performed by employing three mutually perpendicular orientations of the detecting element and calculating the resultant field from the following equations:
- E = [E12 + E22 + E32] (V.1) or
- H = [H12 + H22 + H32 ]½ (V.2) or
- W = W1 + W2 + W3 , (V.3)
where the subscripts 1, 2 and 3 refer to measurements in the three mutually orthogonal orientations.
In performing survey measurements in the near-field of an RF source, a meter suitable for operation in the near-field shall be employed. Special care is required to avoid perturbing the field by the instrument (e.g., the meter casing, but not the field probe), and by other objects or people in the vicinity.
When amplitude or frequency modulated and especially pulsed fields are surveyed, the meter response to such fields shall be evaluated to determine if it is capable of measuring these types of fields. A survey instrument shall be calibrated against a standard every three years and its operation checked at least once a year or after any repair that may affect its operation.
Exposure levels in the vicinity of RF sources having scanning (rotating) antennas may have to be determined with the antenna stationary, because of the limitations of the available measuring instruments. The exposure conditions when the antenna is in motion are then evaluated using methods described in Appendix III (Section C). More detailed information about measurements of potentially hazardous RF fields can be found in references(15,16,18,21).
A.2 Spatial Averaging
In conducting RF field surveys, locations accessible to people where maximum field strengths exist are identified. Even in the far-field of an RF radiator, the field strengths may vary over the cross-sectional (projected) area of a human body (approximately 0.6 m2) because of ground reflections and scattering from nearby objects. As a result, spatial averaging is required in most cases.
Frequently, exposure is in the near-field or in close proximity to reflecting objects where the fields are spatially non-uniform. A method for performing a spatially averaged measurement is as follows:
- determine the location of the maximum field.
- establish around the location of the maximum field a grid of points within approximately 0.35 m (width) x 1.25 m (height) surface area, at a reasonable distance (e.g., 0.5 m) above the floor or ground and perpendicular to it. These points should be uniformly spaced within the grid with the point of the maximum field included.
- measure the field strength in all points of the grid.
- calculate the average field. Note: A person performing measurements shall approach the exposure source from afar to avoid overexposure. In questionable situations, measurements may be performed with the output power of the source reduced, or the person may gradually approach the source while monitoring the field. The average field strength along a grid of n points may be calculated from the equation
where Fi is the rms field strength measured at the point i. An example of a measurement grid for the spatial averaging is given in Figure V-1.
Figure V-1 Example of a Grid for Measurements of a Non-Uniform Electric Field of 27 MHz and the Calculation of the Average Field
B. Specific Absorption Rate (SAR)
A very careful and well documented assessment of SARs has to be performed for conformity with the requirements of Sections 2.1.2 and 2.2.2. It should be remembered that the internal field within a human body, and thus the SAR, are not related to the external field in a simple way.
Determination of SARs for near-field exposures of humans is difficult and can be done only on simulated models of the human body under laboratory conditions. Both computational methods and measurements are feasible. To be valid, they have to be reliable and reasonably accurate.
There are two general approaches in computational methods(22). One involves the use of an analytical technique for calculation of distribution of absorbed energy in simplified tissue geometries such as plane slabs, cylinders and spheroids. The other uses a numerical formulation for analysing the coupling of radiofrequency energy to human bodies. Examples of numerical methods for SAR calculations are the impedance method, the method of moments and the finitedifference time domain (FDTD) technique. Detailed representations of the complex geometry and composition of the human body have been made available using data from computerized tomography and magnetic resonance imaging scans. Recent advances in computers (memory and speed) and in the FDTD technique have led to the development of a tool for analysis of SAR in the human head from various cellular telephones(23,24,25). This numerical tool allows a detailed modelling of anatomically relevant human inhomogeneities, such as those in the head, that are difficult to model experimentally. Software for numerical calculation of local and regional SARs is commercially available, but at the time of writing, there is not enough information to discuss the calculation accuracy.
Measurement methods have been developed for determination of SAR in experimental animals and models made of tissue-equivalent synthetic material(26,27). Such simulated models are referred to as phantoms. Measurement methods are used to verify the accuracy of numerical calculations. There are two basic methods for SAR measurements. One is to use a temperature probe to measure the temperature change caused by the heat produced by the absorbed RF energy, and then calculate SAR from(15)
where is the temperature rise (in ºC) within the time interval (in seconds), and c is the tissue (or phantom material) specific heat capacity, in J/kgºC. Calculations of SARs from temperature rise can be done only if the temperature rise is linear with time. This method is appropriate for local SAR measurement when the exposure levels (irradiating fields) are intense enough so that the temperature rise is not influenced significantly by heat transfer within and out of the body. The second basic method for SAR determination is to measure the electric field inside the body with implantable electric field probes and then calculate the SAR from
where σ is the tissue conductivity (S/m), E is the rms electric field strength induced in the tissue (V/m) and p is the mass density (kg/m3). This method is suitable only for measuring SAR at specific points in the body and for low values of SAR where the absorbed energy is insufficient to cause a detectable change in temperature. Instrumentation for this type of SAR measurement method usually includes an implantable electric field probe, a phantom and a computer controlled system for positioning the probe(28,29). This instrumentation has recently become commercially available and has been used to test portable transmitters for compliance.
More information about various methods of SAR determination can be found in references(15,21,30,31,32,33,34). New methods may become available after publication of this document.
C. Contact and Induced Currents
C.1 Contact Current
An RF field induces an alternating electric potential on ungrounded or poorly grounded conducting (metallic) objects such as cars, trucks, buses, cranes and fences. When a person touches such objects, RF current flows through the person to ground (Figure V-2). The amount of the current depends on the object (its size, shape), the frequency and strength of the field and the person's impedance. The impedance in turn depends on the person's height, weight and body composition (ratio of the lean to fat body mass), type of contact (surface area of contact, i.e., finger or grasp, skin wet or dry), and the type of footwear. The impedance also varies with the frequency of the RF field.
Figure V-2 Typical Situations Where Currents Can Be Perceived by Persons Touching Ungrounded or Poorly Grounded Conducting Objects
Contact current flowing through the person is perceived at a certain level; at a higher level it becomes painful and at a still higher level may cause an injury (e.g., local burn, respiratory tetanus, heart effects). Below a frequency of about 100 kHz the perception is of a tingling, prickling feeling in the finger or hand touching the object. At higher frequencies heat is perceived. Thresholds for perception and pain under various conditions have been measured and more information can be found in references(35, 36).
Currents below the limits set in Section 2.1.3 (Table 3) for RF and microwave exposed workers will not cause pain, but may be perceived. Since females are more sensitive35, 36), the lower percentile (more sensitive) was selected for setting the limits. Currents below the limits set in Section 2.2.3 (Table 7) will not be perceived. In this case extrapolated data for children were used35, 36). Contact currents are evaluated using an electronic circuit representing the impedance of the average human body in grasp contact with an insulated, conductive object, energized by an RF field. More information about the equivalent human impedance can be found elsewhere 37, 38).
An evaluation for compliance with the limits for contact currents (Tables 3 and 7) should be made with an appropriate instrument. Contact current meters are commercially available.
C.2 Induced Current
Even though a person may not be touching a metallic object, RF currents which are induced in the body by RF fields may flow through the body to ground.
Induced current through both feet can be measured by using a clamp-on current probe or a low profile platform con si sting of two parallel conductive plates isolated from each other and one located above the other. If the latter is used, the platform is placed on the surface where the person stands, and a person or a human equivalent antenna is placed on the upper plate of the platform. A voltage drop on a low-inductance resistor placed between the plates provides a measure of the induced current.
An evaluation for compliance with the limits of induced currents should be made with an appropriate instrument. A person or a human equivalent antenna should stand upright on the platform of the induced current meter. Induced current meters and human equivalent antennas are commercially available.
|
