t tdt t P t t P where t1 and t2 are the start and stop time of the exposure the period t2 – t1 is the exposure range or band of frequencies in the electromagnetic spectrum within which a
Quantities
Magnetic flux density B tesla (Vs/m 2 ) T
Electric flux density D coulomb per square metre Cm 2
Electric field strength E volt per metre Vm 1
Magnetic field strength H ampere per metre Am 1
Current density J ampere per square metre Am 2
Power density S watt per square metre Wm 2
Specific absorption rate SAR watt per kilogram Wkg 1
Permittivity ε farad per metre Fm 1
Permeability μ henry per metre Hm 1
Mass density ρ kilogram per cubic metre kgm 3
Electric conductivity σ siemens per metre Sm 1
Constants
Velocity of light in free space c 2,998 × 10 8 ms 1
Permittivity of free space ε 0 8,854 × 10 12 Fm 1
Permeability of free space μ 0 4π × 10 7 Hm 1
Impedance of free space Z 0 120π (or 377) Ω
Terms and definitions
3.3.1 antenna antennas are conductive elements that radiate, and/or receive energy in the radio frequency spectrum
P avg time – averaged rate of energy transfer defined by:
P t where t 1 and t 2 are the start and stop time of the exposure (the period t 2 – t 1 is the exposure duration)
3.3.3 averaging time t avg appropriate time over which exposure is averaged for purposes of determining compliance
3.3.4 bandwidth range or band of frequencies in the electromagnetic spectrum within which a system is capable of receiving and transmitting
Basic restrictions for human exposure to time-varying electric, magnetic, and electromagnetic fields are established based on levels associated with known health effects, ensuring a high level of safety These restrictions can be quantified through metrics such as induced current density, in-situ electric field, specific absorption rate, or other relevant dosimetric quantities.
3.3.6 carrier frequency used to carry data by appropriate modulation of the carrier waveform
3.3.7 conductivity σ ratio of the conduction–current density in a medium to the electric field strength in the medium
J electromagnetic field-induced current per unit area inside the body
3.3.9 deactivator device which changes transponders so that they no longer respond
ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ electric field strength
E magnitude of a field vector at a point that represents the force (F) on an infinitely small charge
D magnitude of a field vector that is equal to the electric field strength (E) multiplied by the permittivity (ε)
EAS system which detects the presence of transponders, which is often used for anti-theft purposes
Exposure to electric, magnetic, or electromagnetic fields occurs whenever an individual is subjected to these forces, as well as to touch currents that are not generated by the body's physiological processes or other natural phenomena.
3.3.15 exposure level value of the quantity under analysis when a person is exposed to electromagnetic fields or touch currents
3.3.16 exposure requirements standard, recommendation, set of guidelines or limits or other document that defines exposure levels for guidance, assessment or compliance purposes
The far-field region of an antenna is characterized by an angular field distribution that remains largely unaffected by the distance from the antenna Also known as the free space region, this area exhibits a predominantly plane-wave nature, featuring a locally uniform distribution of electric and magnetic field strengths in planes that are perpendicular to the direction of propagation.
3.3.18 harmonics multiples of a principal frequency, invariably exhibiting lower amplitudes
3.3.19 induced current current induced inside the body as a result of direct exposure to electromagnetic fields
The interrogator module is essential for processing data protocols and serves as the interface for communication and data transfer with the transponder Commonly referred to as a reader, the interrogator plays a crucial role in facilitating these interactions.
B magnitude of a field vector that is equal to the magnetic field H multiplied by the permeability
H magnitude of a field vector in a point that results in a force (F) on a charge (q) moving with velocity (v)
[or magnetic flux density divided by permeability of the medium, see “magnetic flux density”]
The near-field region, located close to an antenna or radiating structure, is characterized by electric and magnetic fields that vary significantly from point to point, lacking a plane-wave nature This region is divided into two sub-regions: the reactive near-field, which is nearest to the radiating structure and contains most of the stored energy, and the radiating near-field, where the radiation field dominates over the reactive field, yet still exhibits a complex structure without substantial plane-wave characteristics.
Permeability (\( \mu \)) is a fundamental property of materials that describes the relationship between magnetic flux density (\( B \)) and magnetic field strength (\( H \)) It is defined by the equation \( \mu = \mu_R \mu_0 = \frac{B}{H} \), where \( \mu \) represents the permeability of the medium in henrys per meter (Hm\(^{-1}\)), \( \mu_0 \) is the permeability of free space, and \( \mu_R \) is the relative permeability of specific dielectric materials.
3.3.25 permittivity ε property of a dielectric material (e.g biological tissue) which defines the relationship between electrical flux density D and electrical field strength E It is commonly used as the combination
ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ ε is the permittivity of the medium expressed in farads per metre (Fm 1 ) ε0 is the permittivity of a vacuum İ R is the relative permittivity
The power per unit area, known as power density (S), is oriented normal to the direction of electromagnetic wave propagation In the case of plane waves, the relationship between power density (S), electric field strength (E), and magnetic field strength (H) is defined by the impedance of free space, which is approximately 377 ohms.
S= 2 77 2 377 where E and H are expressed in units of Vm 1 and Am 1 , respectively, and S in Wm 2
Many survey instruments report power density in specific units, but they actually measure electric (E) or magnetic (H) fields, or their squares It's important to recognize that the value of 377 Ω is applicable only in free space under far-field measurement conditions and does not pertain to inductive devices functioning in the reactive near field.
An RFID system utilizes electromagnetic fields to read data stored in transponders, enabling efficient data retrieval Certain combinations of systems and transponders also support the transfer of new or updated information, allowing for read/write capabilities.
3.3.28 read decoding, extraction and presentation of data from formatting, control and error management bits sent from a transponder
3.3.29 read/write transponder transponders that are capable of having their data repeatedly modified are called read/write transponders
The reference value, also known as the maximum permissible exposure or action value, is a conservatively derived measurable quantity that ensures compliance with basic restrictions or limits Meeting this reference value indicates adherence to the underlying basic restrictions; however, failing to meet the reference value does not automatically mean that the basic restrictions are violated Instead, it signifies that further evaluations or actions are necessary to demonstrate compliance.
3.3.31 root-mean-square (rms) effective value or the value associated with joule heating, of a periodic electromagnetic wave
The rms value is obtained by taking the square root of the mean of the squared value of a function
Or in its equivalent form for a series of discrete parts ¦ ( )
NOTE Although many survey instruments indicate rms, the actual quantity measured is root sum square (rss)
(equivalent field strength) The value rss is obtained from three individual rms field strength values, measured in three orthogonal directions combined disregarding the phases
3.3.32 root-sum-square rss effective value or the value associated with joule heating, of a periodic electromagnetic wave
The rss value is obtained by taking the square root of the sum of the squared value of a function
Or in its equivalent form for a series of discrete parts ¦ ( )
SAR time derivative of the incremental electromagnetic energy (dW) absorbed by (dissipated in) an incremental mass (dm) contained in a volume element (dV) of given mass density (ρ ) á ạ ă ã ©
SAR is expressed in units of watts per kilogram (Wkg 1 ).
NOTE SAR can be calculated by: ρ σ E i 2
The specific absorption rate (SAR) is influenced by the conductivity (\(\sigma\)) of body tissue, measured in S/m, and the density (\(\rho\)) of body tissue, expressed in kg/m³ Additionally, the specific heat capacity (\(c_i\)) of body tissue is quantified in J kg\(^{-1}\) K\(^{-1}\) The initial time derivative of temperature in body tissue, denoted as \(\frac{dT}{dt}\), is measured in K/s.
3.3.34 touch current contact current electric current passing through a human body or when it touches one or more accessible parts of an installation or of equipment transponder
A transponder is a transmitter/receiver pair housed in a single package that responds to an external interrogating signal It is commonly known by various terms, including tag, electronic tag, electronic label, electronic bar-code, RFID chip, and code plate.
3.3.36 wavelength wavelength (λ) of an electromagnetic wave is related to the frequency (f) and velocity (Ȟ) of the wave by the expression f
= Ȟ Ȝ where Ȟ is the velocity of the wave in ms –1
NOTE In free space the wave velocity is the velocity of light in free space, c
4 Measurements and calculations for equipment evaluation
Introduction
This clause provides a three-stage method of exposure evaluation The stages vary in complexity and the one most suitable for the equipment and the exposure situation should be used
Evaluations are made either against basic restrictions or against derived reference values
Reference value parameters are directly measurable and serve as the simplest evaluation method Basic restriction parameters offer a more fundamental assessment of exposure but are challenging to measure directly, necessitating calculation and numerical modeling techniques Evaluations against these basic restrictions are detailed in sections 4.4 and 4.5, which present increasing levels of sophistication and complexity Section 4.4 considers the non-uniformity of fields, while section 4.5 incorporates the non-uniformity of human tissue and its properties.
Subclause 4.6 contains a method to demonstrate compliance for contact and limb currents
This shall be used in all cases
Evaluation against reference values
General
This subclause describes the method for determining compliance of a system to field strength reference values
The measuring instrumentation must be appropriate for the specific purpose and cover the frequency range of emissions from the unit under test When using broadband instruments, their bandwidth should encompass the emitted frequency range It's important to note that the measuring equipment may exhibit a frequency-dependent response that aligns with the established limits All measurement devices must be calibrated with traceable results from an accredited laboratory While the test site for product compliance evaluations should also be calibrated, this may not be feasible for measurements taken at installation sites In cases where the test site is uncalibrated, precautions should be taken to mitigate external influences that could impact results, and all such effects, along with any uncertainties, should be documented in the evaluation report.
Instrumentation used to measure exposure levels may be commercially available or specifically designed for purpose IEC 61786 and IEC 61566 provide information about such measurements and equipment
To accurately characterize exposure conditions, it is essential to utilize multiple instruments such as broadband meters, oscilloscopes, or spectrum analyzers While using several instruments may lead to some overlap in frequency ranges, it is important to minimize unnecessary overestimation of levels Additionally, spectral information is crucial for assessing compliance with frequency-dependent levels.
Measurements must utilize instruments that can accurately assess both frequency domain and time domain characteristics of the signal For time domain measurements, it is essential to analyze the frequency content for comparison with reference values.
When assessing emissions, it is crucial to evaluate the frequency range and any time-varying modulation, as well as the duration of exposure These factors are essential for time-averaged exposure assessments Additionally, calculating the instantaneous maximum field strength is important for comparing with limits for pulsed sources It may also be necessary to aggregate the field levels at each frequency to comply with the relevant exposure requirements.
When assessing emissions, the wavelength relative to a person's position is crucial in deciding if separate measurements for electric and magnetic fields are needed In the near field, measuring only magnetic fields may suffice.
Measurements will be conducted to assess the unperturbed field strengths The human body can notably influence electric field measurements, so it is essential to mount the instrumentation on a non-conductive support Additionally, utilizing a fiber-optic coupled remote read-out unit or similar methods to distance the operator from the measurement area may be beneficial for certain electric field assessments.
When testing a unit, ensure that the power adjustment is set to maximum or according to the manufacturer's setup instructions Additionally, position the unit at a sufficient distance from nearby objects to prevent any interference with the field.
Direct measurement for comparison against reference values
Field strengths will be measured around the unit under test at a specified distance X, as outlined in Table 1 A preliminary scan can be conducted to identify the locations of maximum electromagnetic field strength at this distance The determination of field strength will be carried out accordingly.
Spatial measurements for comparison against reference values
This subclause presents alternative methods to section 4.2.1, enabling the implementation of a measurement grid pattern across the typical exposed volume to reduce the number of required measurements.
For the equipment types outlined in this document, the torso is the primary area for assessment, utilizing the grid shown in Figure 1 The grid's positioning may vary based on the unit's typical usage, but its layout and dimensions must remain consistent In rare instances where exposure primarily affects the head, the grid in Figure 2 should be employed to ensure a more conservative outcome.
The measurement methods outlined in the previous section are applicable, utilizing the grid patterns illustrated in Figures 1 to 11 and Table 1 The specific grid position relative to the unit under test will vary based on the typical equipment configuration Alternative grid positions may be employed as long as they accurately represent the unit's normal usage.
The measured values at each of the grid points shall be recorded and compared with the appropriate derived reference values This result shall be recorded
In certain exposure requirements, reference values are determined using spatially averaged measurements across the entire body of the exposed individual It is essential to calculate the appropriate linear or quadratic average (rms) of the measured values and compare this with the relevant derived reference values, ensuring that the results are documented accordingly.
Spatially averaged results can be relevant for whole body exposure; however, significant variations in individual values near or exceeding reference levels may suggest that localized basic restrictions are not satisfied In these instances, it is essential to employ more complex methods from other subclauses to ensure compliance with basic restrictions.
For frequencies above 300 MHz and when the measurement is substantially in the far field, measurements can be taken of E-field as above.
Modelling and analysis including field non-uniformity
The methods described in this subclause are an option to be used instead of 4.2.1 and 4.2.2
For evaluating both near and far fields, using calculated fields for comparison with reference values is permissible There are various commercial software packages available for modeling field patterns It is essential to validate the model through comparative field measurements, ensuring that the comparison falls within acceptable measurement and modeling uncertainties.
This subclause addresses the non-uniform fields typical of the equipment discussed in this document at specified frequencies Evaluations are conducted within the near field, which extends several meters from the unit.
To achieve accurate interpolation between measurement points, it is essential to use a finer grid size that aligns with the spatial variation of the field, surpassing the grid size utilized in section 4.2.
Field modeling is an effective method for analyzing complex field patterns, supported by various commercial software packages To ensure accuracy, the model must first be validated through comparative field measurements, with results falling within acceptable measurement and modeling uncertainties.
Table 1 – Dimensions and distances for Figures 1 to 11
Informative dimensions cm a,h a/b/c X Z Height Width Depth
Single unit in the floor f 5 15 85 60 100 40 80
Single unit in the ceiling 6 15 85 210 300 60 100 40 80
The dimensions of the hand-held unit range from 100 to 200 cm², encompassing the majority of equipment, though some may exceed this range The combined size of the head grid and Z dimension totals 175 cm, aligning with the height of an average person The X distance indicates a typical mounting distance on a countertop, with closer distances likely subject to occupational exposure levels For scanning the human body, an X distance of no more than 3 cm is recommended for close scanning devices Equipment not fitting these categories may adopt the nearest appropriate category or a new configuration based on similar principles Some units are installed at a minimum depth below the floor surface, which can be added to the Z dimension if specified in the installation documentation The grid positions and dimensions represent the center of the probe, with the grey circle illustrating the probe's position relative to the grid While some units may be circular or oval, their approximate dimensions align with the provided rectangular sizes.
Front view Side view Top view
Front view Side view Top view
Figure 3 – Single floor standing antenna
Front view Side view Top view
Figure 4 – Dual floor standing antenna
Front view Side view Top view
Front view Side view Top view
Front view Side view Top view
Figure 7 – Combined floor and ceiling antennas
Front view Side view Top view
Figure 8 – “Walk-through” loop antenna
Figure 9 – Counter or desk mounted antenna
Figure 10 – Vertical, wall or frame mounted antenna
Specific absorption rate (SAR) measurements
General
There are three main methods to directly measure SAR in human body phantoms:
– internal electric field strength measurements for evaluation of localised SAR;
– internal temperature measurement for evaluation of localized SAR;
– calorimetric measurement of heat transfer for evaluation of whole-body-averaged SAR.ã
IEC 62209-1 and IEC 62209-2 offer extensive guidelines on SAR measurement procedures These standards can serve as a foundation for device evaluation, as long as the employed method is well-documented and yields results with a known uncertainty, or one that inherently overestimates exposure.
The following subclauses provide some general information and some specific information relevant for SAR evaluation of devices covered by this standard, to assist when using
Internal electric field strength measurements
The evaluation of Specific Absorption Rate (SAR) is conducted through E-field measurements using a miniature probe, which is automatically positioned within a liquid-filled phantom model that simulates the human body or specific parts, such as the head, exposed to an electromagnetic field By analyzing the measured E-field values, it is possible to calculate the SAR distribution and determine the maximum mass-averaged SAR value.
To achieve three-dimensional SAR distributions in exposed human phantoms using the E field probe technique, it is recommended to utilize an automatic probe positioning system, such as an industrial robot.
The measurement equipment shall be calibrated as a complete system Sensitivity, linearity and isotropy of the probe system shall be determined in the tissue equivalent liquid
Measurements in close vicinity to media interfaces result in errors due to boundary effects
The effects of probe size can be quantified based on the distance from the surface using a waveguide calibration setup Once quantified, these effects can be compensated to reduce errors effectively.
Internal temperature measurements
The evaluation of Specific Absorption Rate (SAR) through temperature measurement involves using a temperature probe placed in a liquid-filled phantom model that simulates the human body or specific parts, such as the head, when exposed to an electromagnetic field By analyzing the increase in temperature, the local SAR distribution can be determined using the formula \( t c T \).
The equation \$\Delta (2)\$ describes the relationship between the temperature rise (\$ǻT\$) during a small time interval (\$ǻt\$) and the specific heat capacity (\$c_i\$) of body tissue, measured in J kg\$^{-1}\$ K\$^{-1}\$ Accurate temperature-rise measurements must begin at thermal equilibrium This equation is valid only when heat diffusion effects can be ignored; otherwise, an integral equation that accounts for heat diffusion must be used.
Common equipment for measuring temperature in exposed body models includes high-resistance thermistor probes and optical probes These temperature probes feature small tips that enable high spatial resolution Typically, the temperature resolution of these probes ranges from 0.005 K to 0.1 K, which in turn limits the sensitivity of Specific Absorption Rate (SAR) measurements.
0,03 W/kg In order that the probe does not perturb the electromagnetic field, it is constructed using high-resistance thermistors connected to high-resistance leads or by using fibre optics
To accurately assess the three-dimensional Specific Absorption Rate (SAR) distribution or local peak SAR within a phantom model, it is essential to utilize a temperature probe that is maneuvered inside the phantom This should be done using a positioning system akin to that employed for electric field probes.
Infrared imaging devices, such as thermographic cameras, can effectively assess temperature rise in both phantom models and real human surfaces To accurately determine the 2-dimensional temperature rise and Specific Absorption Rate (SAR) distribution, the phantom must first achieve a uniform temperature After a brief exposure period, the phantom is positioned in front of the thermographic camera to capture an immediate image that maps the temperature rise profile Additionally, temperature profiles within the phantom can be obtained by making specially prepared cuts to separate the phantom.
Temperature sensors exhibit lower sensitivity compared to E-field probes, necessitating the use of high-power exposure sources for effective SAR evaluation through temperature measurement This approach ensures a significant temperature increase within a short time frame, which is essential for accurate testing.
Calibration of temperature-measurement equipment includes, beside the general probe calibration, a careful evaluation of heat diffusion processes
Calorimetric measurements of heat transfer
Calorimeters allow the measurement of the whole-body averaged or partial-body averaged
The Specific Absorption Rate (SAR) for human body models exposed to electromagnetic fields is calculated by measuring the total energy absorbed, denoted as \$\Delta W\$, in relation to the mass \$m\$ over a specific exposure time \$\Delta t\$ The averaged SAR is derived from the equation that relates these variables.
To maintain thermal equilibrium with the environment, exposure times should generally be limited to several minutes While heat diffusion within the phantom does not significantly affect the averaged value, it is crucial to minimize thermal radiation and convection losses by carefully controlling environmental conditions and managing exposure duration.
Whole-body averaged Specific Absorption Rate (SAR) measurements utilize human-size calorimeters, while smaller calorimeters are suitable for partial-body averaged SAR or scaled-down whole-body phantoms After exposure, the phantom is placed in a calorimeter container to achieve thermal equilibrium in a known mass of water, typically over several hours The calorimetric twin-well technique employs two calorimeters and identical body models, allowing measurements under less controlled thermal conditions Although calorimeters provide highly accurate averaged SAR values, they do not offer insights into the three-dimensional SAR distribution within the body.
Phantom models and fluid
The phantom model must be sufficiently large to effectively assess the Specific Absorption Rate (SAR) effects from the device Ideally, the maximum size of the phantom should correspond to a real human model, whether for the entire body or the specific body part under evaluation.
The fluid utilized in the evaluation must possess characteristics that accurately represent the body for the device's operational frequency Typically, these fluids are specifically formulated for this purpose Any deviations from the anticipated body characteristics should be documented and factored into the uncertainty assessment.
Numerical evaluations for comparison against basic restrictions
General
When measured values surpass reference values, compliance can be shown by comparing them to basic restrictions This can be accomplished through straightforward analytical or numerical modeling, as detailed in the subsequent subclauses It is not required to evaluate exposure using all subclauses in section 4.4.
Evaluations using homogeneous models
To model dosimetric quantities for comparison with basic restrictions, simplified body shapes with uniform conductivity, such as disks, cubes, prolate spheroids, or homogeneous human body shapes, are utilized For detailed information on these shapes, refer to Annex B The dimensions should align with those specified in Table 2, unless otherwise indicated by the exposure requirements relevant to the limits.
When using a piece of equipment, the standard dimensions may not always be suitable In such instances, alternative dimensions can be employed if they are justifiable The field values applied in the model can either be measured or modeled, as previously mentioned.
The tissue conductivity used should be as described in Annex B, Table B.6
The comparison of numerical results from www.bzfxw.com against an analytical solution is conducted using simple cases, such as a disk or layered cylinder/sphere with uniform conductivity in a uniform magnetic field By averaging the relevant model cells, the numerically calculated current density should align with the analytically derived value within a 20% margin, serving as a validation check for the computational software employed.
Model validation is typically a one-time requirement, as it does not need to be repeated with each use This validation process can often be handled by the software provider.
In certain very simple cases, such as circularly symmetrical fields, numerical integration of an analytical expression may be possible using less complex and lower cost software packages
The modeling results can be expressed as induced current density, in-situ electric field, or specific absorption rate (SAR), which are defined over suitable averaging sizes for both localized and whole-body assessments.
The maximum value over the modelled space (averaged according to the exposure requirements being used) should be compared with the appropriate basic restriction
Exceeding the relevant basic restrictions in this comparison may allow for compliance demonstration by considering tissue non-uniformity and shape through computational dosimetry, as detailed in section 4.5.
Table 2 – Dimensions and distances for simplified body shapes
Body/Torso (cm) Head (cm) h w d h w d
The distances for X and Z should correspond with those specified in Table 1
Figure 12 – Disk model Figure 13 – Cubic model w h
Special case of inductive near-field exposure 100 kHz to 50 MHz
This is a special case for near-field exposure (even under the assumption of uniform field) for sources at 100 kHz to 50 MHz
At frequencies below 100 kHz, the dosimetric quantities are induced current density and in-situ magnetic field, with some guidelines extending these measures up to 10 MHz Above 100 kHz, the specific absorption rate (SAR) becomes the primary dosimetric quantity, while the magnetic field is treated as part of a plane electromagnetic wave's interaction with the body However, in cases of non-uniform exposure within the inductive near field of a source exceeding 100 kHz, this method may lead to an overestimation of SAR, as the electric field component is significantly smaller than what is typically assumed for a plane wave.
SAR remains a valuable dosimetric quantity, allowing for the assessment of compliance with fundamental restrictions on both SAR and induced current density Even in frequency ranges without specific limitations on induced current density, it is possible to derive localized SAR using the relationship involving resistivity and conductivity.
The Specific Absorption Rate (SAR) is defined by the equation SAR = (4), where J represents the root mean square (rms) value of the induced current density, E denotes the rms value of the in-situ electric field, ı indicates the electric conductivity, and ȡ refers to the density of body tissue.
The relationship between Specific Absorption Rate (SAR) and the H-field can be determined through a uniform-field-in-a-ring model that connects current density (J) and magnetic field strength (H) This model serves as an extension of reference values at higher frequencies, specifically for near-field magnetic exposure scenarios.
Due to fundamental limitations on the induced current density, J, at frequencies up to 10 MHz, it is crucial to ensure that the instantaneous field remains below the established basic restrictions when applying SAR time-averaging.
Frequencies > 50 MHz
Currently, there are no straightforward modeling techniques for the near field, while commercially available software can effectively model far fields, allowing for comparison with reference values.
When reference values are surpassed in the far-field, analytical methods can be utilized to determine localized Specific Absorption Rate (SAR) Conversely, if these values are exceeded in the near-field, it is essential to directly evaluate compliance with basic restrictions While straightforward analytical techniques may exist for this assessment, the presence of a person or body part complicates the situation.
The radiation characteristics of an antenna are influenced by its head In such cases, numerical modeling is commonly employed, treating the antenna and body as components of a coupled system.
Localised SAR (100 kHz to 10 GHz)
Certain exposure guidelines establish localized maximum Specific Absorption Rate (SAR) values, often defined as SAR averaged over 10 g or 1 g of contiguous tissue, which may be represented as a cube or an undefined shape Effective power calculations can be performed for localized tissue volumes, with the most straightforward approach assuming that all transmitted power is absorbed by the averaging mass of tissue.
SAR Max is the basic restriction or limit for localised exposure
M Tissue is the mass of contiguous tissue, used for the averaging
P Max is the maximum power delivered to the antenna, assuming all power is absorbed by the mass of contiguous tissue, independent of its shape
For a specific absorption rate (SAR) maximum of 2 W/kg, averaged over 10 grams of tissue, any device emitting less than 20 mW from its antenna will remain within the safe SAR limit.
Modeling the maximum power absorption by tissue can be enhanced by considering environmental refractions and reflections It is essential to validate the proportioning model through comparative measurements, as highlighted in previous sections.
Evaluations using non-homogeneous models for comparison against basic
General
Compliance can be demonstrated through computational dosimetry, which utilizes advanced body models with millimeter resolution, typically derived from MRI data or anatomical diagrams These models accurately represent tissue conductivities, including those of the heart and Central Nervous System (CNS) tissues like the brain and spinal cord The guidelines do not mandate a specific method or model, as multiple techniques are valid and precise Ongoing research in this field will yield new methods and information, which can be utilized as long as they adhere to the established criteria.
Induced currents, in-situ electric fields, and SAR and power density distributions are calculated based on the model's resolution Utilizing anatomically based models allows for specific results tailored to various tissue types, such as CNS tissues (brain and spinal cord), relevant to the exposure type and requirements being analyzed.
If such modelling techniques are used, appropriate validation is required This can be provided by peer review, appropriate published reference citations or comparison against other reviewed or referenced models.
Anatomical body models
The International Commission for Radiological Protection has defined a "standard man" [ 7] as 1,76 m tall Based around this, models that fulfil the following criteria are suitable for use:
• height (from top of head to base of heel): 1,76 m ± 8 %;
• representative of the inhomogeneous structure of the human body;
• realistic dielectric properties of tissues;
• data resolution better than or equal to 10 mm steps
Anatomical body models, derived from medical imaging data or cross-sectional diagrams, represent the human male form These datasets can be scaled to meet specific criteria, and in cases of localized exposure, it is permissible to model only the affected body parts.
Annex B and various references in the bibliography detail example body models, many of which are tailored to specific institutions or authors but remain applicable if certain criteria are satisfied A notable publicly accessible example is the Visible Human Project from the National Library of Medicine in Bethesda, MD, USA, which has also led to commercially available datasets derived from its information.
2) A voxel is a volume unit corresponding to the smallest element of a tridimensional space to which individual characteristics such as colour or intensity can be attributed
In modeling scenarios where multiple tissue types exist within a single voxel, it is common to utilize an average of the tissue properties Alternatively, one may opt for the worst-case tissue type, which yields the highest induced current densities, in-situ electric fields, or specific absorption rate (SAR) distributions relevant to that voxel If the voxel or averaging area includes the necessary tissue type, it is permissible to assume it consists solely of that tissue type for modeling purposes.
Calculation/modelling method
Numerous computational methods have been developed for calculating induced current densities, in-situ electric fields, and SAR distributions in anatomically accurate models of the human body Key techniques include the finite element method (FEM), finite integration technique (FIT), impedance method, scaled frequency finite-difference time-domain (FDTD) method, and scalar potential finite difference (SPFD) method For more information, refer to Annex B.
All methods are applicable under uniform field exposure conditions Quasi-static methods, which assume a constant phase of the incident field across the modeled body, are effective at lower frequencies (up to approximately 30 MHz) when body dimensions are small relative to the wavelength For higher frequencies, methods that utilize solutions to Maxwell's equations, such as the FDTD method, are favored.
When modeling the non-uniform vector magnetic fields typical of lower frequency EAS/RFID devices, it is crucial to ensure that the methods used effectively address the field's non-uniformity.
Typical resolutions for whole body models range from 6 mm to 10 mm for coarse representation, while finer modeling of specific tissue types requires resolutions of 2 mm to 4 mm These results can also be averaged at a lower resolution if needed, such as when exposure requirements specify averages over 1 cm² It is important to ensure that the averaging area does not extend beyond the outer boundary of the body being evaluated unless otherwise defined in the exposure requirements.
Position of the body in relation to the unit under evaluation
The body's position relative to the evaluated unit must adhere to the principles outlined in section 4.2.3 While exact alignment may not always be achievable due to variations in body models, the dimensions specified in Table 1 should be maintained within a tolerance of ± 10% Alternative positions may be utilized, provided they accurately represent the unit's actual use, and this must be explicitly detailed in the evaluation report.
Measurement of limb and touch currents
Limb currents and touch currents can be effectively measured using a clamp-on current transformer applied to an arm or leg Alternatively, for currents flowing through the legs, stand-on current meters can serve as a substitute for the clamp-on current transformer.
As an alternative to measurement, it is possible to directly model some of the currents using the techniques given in 4.4 and 4.5
General
This clause provides methods to evaluate the emitted field on installed equipment When performing this type of evaluation it is necessary to take account of the following points
The assessed equipment can generate fields that exceed the reference values set by exposure requirements, as long as it adheres to the basic restrictions However, direct measurement of these basic restrictions is typically not feasible.
• There are significant effects due to background noise and other sources in the environment
Certain equipment may contribute to the overall measured exposure value, potentially leading to levels that exceed the appropriate exposure requirements However, the exposure from the equipment specifically addressed by this standard may remain below those required levels.
Field measurements
Measurement where persons spend significant periods of time
This subclause addresses exposure requirements for areas where the general public spends considerable time It recognizes that exposure from the equipment covered by this standard is typically brief, as the public usually encounters it for short durations This section is specifically for evaluating exposure when the system is not in direct use; if not specified, refer to section 5.2.2.
The evaluation of the level will be conducted using the method outlined in section 4.2.1, specifically at a distance of 2 meters from the equipment If the equipment's location alters the distance to areas where people frequently spend time, this actual distance should be utilized and documented in the results Additional measurements may be required if workers are exposed at distances different from those of the general public Should the measured values exceed the reference values for comparison, further evaluation will be necessary It is important to note that exposure guidelines often have distinct reference values for workers compared to the general public, so the correct comparison values must be applied for accurate exposure assessments.
Detailed measurements for non-transitory exposure
The measurement of the level will be conducted according to the method and distance specified in section 4.2, tailored to the equipment and exposure guidelines in use Should the measured value exceed the reference values for comparison, further evaluation may be required.
Additional evaluation
Exceeding reference values may result from the cumulative effect of multiple sources at a location rather than a single piece of equipment Alternatively, it could stem from one compliant piece of equipment In such instances, further evaluation of all equipment contributing to the exposure is necessary, as outlined below.
• adjusting the position of some pieces of equipment, where possible, to minimise the overall exposure;
• tests repeated on individual pieces of equipment (or similar equipment) in a controlled test environment;
• comparison of equipment settings (currents, voltages etc.) with information provided in the user guide or installation guide for the equipment;
• comparison of the equipment against the Technical Documentation, including any evaluation results covered by this standard;
6 Exposure from sources with multiple frequencies or complex waveforms
Devices discussed in this document operate on one or more discrete frequencies, with other frequencies suppressed by over 30 dB In such cases, exposure evaluations can be conducted at the declared operating frequency or frequencies without the need to assess all other frequencies However, if this condition is not met, an exposure evaluation must be performed for all applicable frequencies.
Devices that function across multiple frequencies may not utilize all of them at the same time The impact of non-simultaneous operation is not cumulative, allowing for individual assessment of each frequency or those not in simultaneous use When employing time averaging, it is essential to account for all frequencies transmitted throughout the averaging duration.
When exposed to multiple frequencies simultaneously, it is important to consider the potential additive effects of these exposures Additionally, accurately assessing devices that emit complex, non-sinusoidal waveforms may be challenging In such instances, these waveforms can be analyzed as a Fourier series of related frequencies, which may also contribute to their cumulative impact.
The biological and health effects of electromagnetic fields vary significantly between low and high frequencies At low frequencies, these effects arise from electrical stimulation, while at higher frequencies, they are primarily related to thermal considerations It is essential to conduct separate calculations for each effect, ensuring distinct evaluations for thermal and electrical stimulation impacts on the body Additionally, different exposure requirements necessitate specific calculation methods for combining multiple fields, which should be utilized when assessing compliance.
Different exposure requirements utilize specific methods for assessing short duration pulses, such as calculating an effective frequency through Fourier techniques or employing a weighted peak approach For waveforms that feature a sinusoidal carrier frequency within short duration bursts (with a minimum of five carrier cycles), compliance can be determined by comparing the maximum peak amplitude to the peak value specified in relevant guidelines Additionally, it is essential to ensure that the rms amplitude of the waveform adheres to the rms limit outlined in these guidelines The appropriate technique for compliance evaluation must align with the specific exposure requirements being applied.
To determine the total exposure from multiple sources, the summation formulae for simultaneous exposure to multiple frequencies applicable to the exposure requirements under consideration should be used
Exposure requirements often utilize induced current or in-situ electric fields as fundamental restrictions, applicable up to 100 kHz (or 10 MHz) and specific absorption rate (SAR) from 100 kHz It is essential to evaluate exposure independently for the frequency ranges of 0 kHz to 100 kHz (or 0 MHz to 10 MHz) and 100 kHz to 300 GHz Additionally, any values at frequencies that fall within both ranges must be included in both exposure assessments.
When assessing exposure from multiple sources, it's essential to consider their time relationship For instance, in a "listen before talk" spectrum management system, sources are synchronized to prevent simultaneous emissions, allowing only one to transmit at a time Additionally, when using time averaging for exposure assessments, all active sources during the averaging period must be taken into account.
When assessing exposure from multiple sources, it is crucial to consider the spatial relationship between the sources and the exposure position Typically, equipment is evaluated at a fixed distance; however, the actual exposure position may be farther away when multiple sources are involved As the distance from a source increases, the exposure diminishes significantly, often following an inverse second or third order relationship.
Current exposure guidelines lack a method for effectively combining basic restriction proportions with reference values This creates challenges in accurately summing exposure from multiple sources, especially when some are evaluated against basic restrictions while others rely on reference values.
Annex C provides a straightforward and conservative method for combining exposure from various sources, particularly when some are evaluated against basic restrictions and others against reference values However, this method is not suitable for demonstrating non-compliance.
General
Uncertainty represents a statistical assessment of the quality of evaluation results The true value of the evaluated item may vary from the assessed value by an amount that falls within the established uncertainty, corresponding to a specific confidence level.
Uncertainty, while not an error value to be added or subtracted, may need to be factored into the overall evaluation of a product or exposure situation according to certain guidelines or standards For further details on how to incorporate uncertainty when necessary, refer to Annex D.
Different regions, countries and even agencies treat the inclusion of uncertainty in different ways Also different methods and assessors will have different uncertainties
This clause outlines the uncertainty associated with various evaluation methods and establishes a reasonable overall level of uncertainty for equipment assessment Evaluations must be conducted using a representative sample of the equipment, excluding any production variations.
Evaluating uncertainties
Individual uncertainties
Individual uncertainties should be assessed and expressed as equivalent standard deviations in the final result It is essential to evaluate how these individual uncertainty values influence the overall outcome, as not all will have a direct proportional effect Whenever feasible, the effective uncertainty impacting the result should be utilized.
The uncertainty shall be evaluated based on the accumulated standard deviation expanded by a factor of 1,96, to obtain a 95 % confidence level
8.3 Examples of typical uncertainty components
Typical uncertainty components are merely examples; other components may exist that are not included here, and some of the mentioned examples may be absent or insignificant in the overall assessment.
• Size and shape of the measurement probe (including the relative coil positions)
• The actual position of the probe in relation to the planned measurement point
• Calibration or stated accuracy of the measurement instrument
• Interaction between the equipment under evaluation and the measurement system
• Effect of the environment during the measurement
• Variation in the anatomical model size and weight from the standard man
• Uncertainty in the values of tissue electrical properties
• Accuracy of the modelled parameters of the equipment being assessed
The overall maximum uncertainties as given in Table 3 are reasonable
Table 3 – Maximum total evaluation uncertainties
The evaluation report shall contain or have appended to it, the following minimum information:
• trade name or product reference of the unit under evaluation;
• contact details for the submitting organisation or person;
• contact details for the evaluating expert and/or organisation;
• identification of the subclauses of this standard used;
• description of the unit under evaluation, including emission type, power adjustment, output levels and any modulation effects;
• description of the spatial positioning of the unit under evaluation and the exposure position(s), including evaluation points;
• detailed description of the method used (including, where appropriate, published references and clauses from other international standards used);
• summary and explanation of the results
In the case of evaluations made by measurement the following shall also be included
• power adjustment on the unit under evaluation (if applicable);
• details of the measurement equipment, instrumentation and any special fixtures and ancillary equipment used;
• description of the measurement environment, ambient temperature, humidity, and any background levels which could affect the results;
• diagram of the measurement points;
• measured levels and frequencies at each measurement point
In the case of evaluations done by numerical modelling, the following shall also be included:
• explanation of the model representing the shape and emission of the unit under evaluation and, if appropriate, the results of any validation;
• summary of the body model and tissue parameters used (including published references or data sources where appropriate);
• summary of the body model position and the position of the specific tissues under consideration, in relation to the unit under evaluation;
• explanation of the modelling/calculation method used (including published references where appropriate)
EAS and RFID systems are designed for brief public use, as lingering individuals can obstruct passageways, prompting movement by other users or security personnel The average walking speed through these systems is around 1 m/s (approximately 3.6 km/h), with a detection zone extending up to 1 meter along the path Consequently, the typical exposure duration is about 1 second.
In cases where a customer or user is slow or hesitant, the exposure duration within the system may extend to around 10 seconds However, in rare situations, a customer might find themselves stuck in a queue, leading to an exposure duration that could last a few minutes Overall, the longest exposure times in these systems are typically considered brief.
Electronic article surveillance (EAS) systems utilize electromagnetic fields to deter unauthorized item removal, making them essential in retail environments like shops, supermarkets, and boutiques to combat theft Additionally, EAS systems play a crucial role in libraries by monitoring book removals and are employed in hospitals to prevent baby abductions.
EAS systems consist of two primary components: the detection system and the detected transponders The detection system is made up of two sub-elements: current-generation and processing electronics, along with field-generation and detection antennas Notably, the antennas in the detection system are the primary source of electromagnetic fields.
Antenna systems are typically identifiable as flat panels, loops, or pillars located near store exits, creating a "detection zone" where the primary magnetic fields are present The detection electronics are often hidden within a cupboard or behind a fascia at the checkout, or they may be integrated directly into the panels Transponders are strategically placed throughout the store, attached to various articles.
Transponders, often found as detachable tags or labels, create a slight disturbance in detection systems Typically passive, these devices lack a power source but may include minimal electronic components like diodes Most transponders are designed for single use, accompanying products out of the store, resembling paper labels However, some higher-cost variants are more durable, encased in robust plastic capsules and secured to items with pins, which checkout staff remove during the purchase process.
The magnetic field disturbances generated by transponders are significantly smaller—by four to five orders of magnitude—than those of the primary detection field, and they are comparable in strength to the disturbances caused by everyday items like keys, eyeglass frames, belt buckles, and tin cans.
A small number of the re-useable transponders have a battery on-board, and do produce an
Active transponders emit signals, but their magnetic fields are usually two to three orders of magnitude weaker than the primary detection field These transponders are engineered to ensure that they do not produce any output when located outside the detection or deactivation zones.
User -changeable settings are typically absent in most systems, as the system fields are usually established during manufacturing or by a trained technician during installation and servicing These fields are set to nominal levels, measured using meters for magnetic fields, electrical currents, or voltages in the drive coil Certain systems feature electronics that automatically re-optimize the fields if they are disrupted by large metal objects near the panels Additionally, some systems can alter the field pattern during the detection cycle.
EAS systems are designed to detect field perturbations, which means that out-of-band emissions are tightly regulated Consequently, measuring field values outside the specified operational frequency bands is unnecessary, as they contribute minimally to overall exposure.
EAS systems are distinct from identification systems, such as entry-permit systems, as they operate using a single data point—specifically, the presence or absence of a signal Despite this difference, the underlying technologies of EAS and identification systems are frequently interconnected.
A.2.2 Types of EAS system and operating principles
There are four main types of EAS system, with characteristically different field types:
System type Dominant field Typical frequencies
Non-linear magnetic Magnetic (H) 10 Hz to 20 kHz
Resonant inductive (electronic or magnetic) Magnetic (H) 20 kHz to 135 kHz Resonant radio frequency inductive Magnetic (H) 1 MHz to 20 MHz Non-linear microwave Electromagnetic 0,8 GHz to 2,5 GHz
The initial three system types utilize inductive fields, where the operating zone is significantly smaller than the wavelength of electromagnetic (EM) radiation This results in a field that is predominantly magnetic, with minimal propagating field The current-carrying loop embedded in the detection panel serves as the field-generation element.
Microwave systems primarily utilize a short-range microwave field generated by a helical coil antenna or a similar device The power levels of this field are low, and while it does propagate, its coverage area is limited by the directivity of the antenna.
In brief, the four types of system operate as described below