GL-01 — Guidelines for the Measurement of Radio Frequency Fields at Frequencies from 3 KHz to 300 GHz

4. Measurement

4.1 Preliminary Considerations

Before carrying out a survey of potentially hazardous EM fields, it is important to determine as many of the known characteristics of the sources of these fields as possible. This will permit a better evaluation of the expected field strength and, consequently, a more appropriate selection of test instruments and test procedures.

A checklist of source characteristics should include:

1. type of RF generator and the output power;
2. carrier frequency(ies);
3. modulation characteristics (e.g. peak and average values, waveform, signal duty factor, pulse width, pulse-repetition frequency, etc.);
4. intermittency (e.g. scanning beams, operational duty factors).
5. number of sources. If more than one source is present, are some or all of the signals coherent? Are intensities likely to add linearly or will they create interference patterns (standing waves etc.); and
6. spurious frequencies including radiated harmonics.

A checklist of field characteristics may include:

1. distance of source to test site;
2. type of antenna and properties including gain, beam-width, elevation and azimuth patterns, orientation, physical size with respect to the distance of the area being surveyed (i.e. near field, etc.);
3. polarization; and
4. existence of absorbing or scattering objects likely to influence the field distribution at the test site.

A review of such a checklist is a necessity if the surveyor is to avoid some simple, but often surprising situations. For example, it is necessary to know the location of the source and RF propagation path during surveys with hand held probes. Only then can an appropriate assessment of the effect of the presence of the surveyor's body be made, and measurement errors avoided. Another example common in leakage situations is the possibility that the levels of the EM fields may be hazardous to the surveyor and may produce malfunction in the instrument electronics if it was not designed for operation in the presence of such fields.

Evaluation of Expected Field Strength

If the fields are far fields or radiating near fields of an antenna, then the material on theoretical calculations of exposure fields in Section 4.2 can be used to obtain field strength estimates. General references on antennas and hazard surveys are useful.

Field enhancement due to ground reflections could increase by as much as a factor of four times and even more if focusing effects are present. On the other hand, it should be recognized that such fields measured in the absence of a person may be misleading relative to hazards. For example, a person exposed in front of a reflecting plane reduces the magnitude of the standing wave.

In the case of low frequencies or small aperture antennas, the existence of potentially hazardous reactive near fields becomes relevant. Since these fields cannot be calculated with accuracy, measurements of E and H are usually required. However, one can always utilize the general property (see Ref. [8]) that reactive fields predominate at distances d close to sources where 2πd/λ<<1. Reactive near field amplitudes diminish as 1/d² or faster, whereas radiation far field amplitudes diminish as 1/d. General texts can sometimes be used to estimate E and H field values at these lower frequencies, and specific literature on the propagation characteristics of various broadcasting and communication antennas can be used to estimate either near or far fields from these sources.

Determination of Type of Instrument Required

Although many instruments designed for the measurement of electromagnetic fields are broadband in nature, none of them cover the entire frequency range of interest and all parameters of potential interest. Some general considerations in the selection of an instrument include the following:

1. Frequency. Frequencies must be determined in advance so that proper instruments and measurement methods can be selected. The presence of several frequencies dictates the use of a broadband device with true Root Mean Square response.
2. Response Time. It is usually desirable to begin a survey using an instrument with a response time (integrator time-constant) of one second or less (the "fast" setting on some commercial instruments). This enables a coarse measurement or the detection of pulse-modulated or intermittent fields (e.g. those created by a scanning radar beam). A "peak hold" feature on some survey instruments can provide an accurate indication of moderately fast bursts of RF energy (duration greater than several milliseconds). Once a high field strength zone is located, a slower time constant (3 seconds or more) should be used to obtain the time averaged value of the field strength. If the hazard meter still indicates that an intermittent field exists, other means of recording and averaging should be used. Data logging systems are available specifically for use with RF hazard meters.
3. Peak Field Limitations. Knowledge of the peak field limitations of the instrument is necessary to protect probes from damage in some low-duty-factor pulsed fields, such as those associated with radars.
4. Polarization. Knowledge of the polarization of the fields enables a surveyor to use a non-isotropic probe for hazard surveys. In the absence of such knowledge, an isotropic probe is highly desirable both for ensuring accuracy and ease of performance of the survey in a reasonable period of time.
5. Dynamic Range. The maximum anticipated field strengths should be estimated before measuring emissions from an RF source. A survey instrument capable of withstanding continuous exposure to field strengths (E² or H²) of at least ten times the predetermined value should be chosen in order to avoid destruction of the probe sensing elements or the high resistance leads connected to those elements. In addition, adequate sensitivity is required to ensure a reasonable signal-to-noise ratio when the minimum expected field strengths are being measured.
6. Near Field Measurement Capabilities. If a leakage situation exists or if the fields in close proximity to a source are to be measured, care must be taken to select a suitable instrument.

If possible, one should estimate the maximum expected field levels in order to facilitate the selection of an appropriate survey instrument. In many cases it may be best to begin by using a broadband instrument capable of accurately measuring the total field from all sources, including reflections. If the total field does not exceed the relevant exposure guideline in accessible areas, and if the measurement technique employed is sufficiently accurate, this would mean compliance with that particular guideline, and further measurements would be unnecessary.

When using a broadband survey instrument an average exposure level may be determined by slowly moving the probe in first a horizontal and then a vertical direction. An average can be estimated by observing the meter reading during this scanning process. A maximum field reading is also desirable, and, if the instrument has a "peak hold" feature, this can be obtained by observing the peak reading according to the instrument instructions. Otherwise, the maximum reading can be determined by simply recording the peak during the scanning process.

The term "hot spots" has been used to describe locations where peak readings occur because of local field distortions or other perturbations in the field and such readings are often found near conductive objects.

In many situations there may be several RF sources. For example, a broadcast antenna farm or multiple-use tower could have several types of RF sources including AM, FM, and TV, as well as land-mobile and microwave transmitters. In such a situation it is generally useful to use both broadband and narrowband instrumentation to fully characterize the electromagnetic environment.

Broadband instrumentation could be used to determine what the overall field levels appeared to be, while narrowband instrumentation would be required to determine the relative contributions of each signal to the total field.

At frequencies above 300 MHz it is usually sufficient to measure only the electric field (E), or the mean squared electric field, in the far field. However, at lower frequencies both the electric (E) and magnetic field (H) shall be measured.

In many situations a relatively large sampling of data will be necessary to spatially resolve areas of field intensification that may be caused by reflection and multipath interference. Areas that are normally are accessible to the general public should be examined in detail to determine exposure potential.

If narrowband instrumentation and a linear antenna are used, field intensities at three mutually orthogonal orientations of the antenna must be obtained at each measurement point. The values of E² or H², will then be equal to the sum of the squares of the corresponding orthogonal field components.

If an aperture antenna is used, it should be rotated in both azimuth and elevation until a maximum is obtained. The antenna should then be rotated about its longitudinal axis and the measurement repeated so that both horizontally and vertically polarized field components are measured.

When making measurements, procedures should be followed which minimize possible sources of error. For example, when the polarization of a field is known, all cables associated with the survey instrument should be held perpendicular to the electric field in order to minimize pickup. Ideally, non-conductive cable (e.g. optical fibre) should be used, since substantial error can be introduced by cable pick-up.

Interaction of the entire instrument (probe plus readout device) with the field can be a significant problem below approximately 10 MHz, and it may be desirable to use a self-contained meter for measuring electric field at these frequencies. Also, at frequencies below about 1 MHz, the body of the person making the measurement may become part of the antenna, and error from probe/cable pickup and instrument/body interaction may be reduced by supporting the probe and electronics on a dielectric structure made of wood, styrofoam, etc. In this connection, it is also desirable to remove all unnecessary personnel from an area where a survey is being conducted in order to minimize errors due to reflection and field perturbation.

In areas with relatively high fields, or pulsed fields with high peak powers, it is a good idea to occasionally hold the probe fixed and rotate the readout device and move the connecting cable while observing the meter reading. Any significant change usually indicates pickup in the leads and interference problems. When a field strength meter or spectrum analyzer is used in the above environments, the antenna cable should occasionally be removed and replaced with an impedance- matched termination. Any reading on the device indicates pickup or interference.

Substantial errors may be introduced due to zero drift. If a device is being used which requires zeroing, it should frequently be checked for drift. This should be done with the probe shielded with metal foil, with the source(s) shut off, or with the probe removed from the field.

4.2 Procedures

4.2.1 General Considerations

Prior to making measurements one should estimate the expected field strength and determine the type of instrument required, as discussed in Section 4.1. Some additional approaches and equations for calculating field strength in various situations are given below. The measurement procedures to be used may differ, depending on the source and propagation information available.

Technical Considerations of RF Source Characteristics

Although the prediction of power density levels in the vicinity of RF sources is complicated by many factors, useful estimates can be made. The quality of such calculations will depend on the analytical approach used as well as on the accuracy of the values of the peak power, pulse duration, pulse repetition rate, antenna radiation patterns, antenna placement, and scanning rates that are used in theoretical computations. Corrections for near field effects may also be appropriate. The operating parameters listed below must be specified adequately so that the true average radiated power from the antenna, and resulting power density at a distant point can be calculated.

For all sources (pulsed or CW) the antenna type and size, gain, antenna pattern including E and H plane beam widths and sidelobe distribution, antenna height above ground, operating frequency, antenna beam orientation (all possible cases) and the attenuation of the transmission line that connects the RF generator to the antenna must be known or estimated. For the calculation of the expected power density levels of pulse-modulated sources, the maximum possible values of peak power, pulse duration, and pulse repetition rate which closely approximate, but do not exceed the maximum rated duty factor of a transmitter should be used. In the case of multiple sources, the contribution of each source must be considered when estimating the combined effect.

Antennas - On Axis

The space around an antenna can be sub-divided into three zones:

1. Reactive Near Field Zone. This is the volume of space immediately surrounding the antenna or leakage source where the reactive (non-radiating) components predominate and energy is stored in the field. The reactive near field extends to a distance of approximately one wavelength from the antenna, except for the case of electrically large antennas (whose physical size is greater, in any dimension, than several wavelengths).
2. Radiating Near Field Zone (Fresnel Zone). In this zone, which starts at a distance from the antenna where the reactive field has diminished to an insignificant amount, the antenna gain and the angular distribution of the radiated field vary proportionally with distance from the antenna. This is because the phase and amplitude relationships of the various waves arriving at the observation point from different areas of the antenna change with distance. For reflector type antennas, such as parabolic dishes, the radiation is somewhat more complex in its distribution pattern.
3. Far field Zone (Fraunhofer Zone). This is sufficiently far from the source that the phase and amplitude relationships of the waves arriving from different areas of the antenna do not change appreciably with distance. The antenna gain and angular pattern are essentially independent of distance, and the power density is inversely proportional to the square of the distance from the source. Although the transition from the non-radiating near field is a gradual one, the far field region is commonly assumed to begin at a distance of about 2a²/λ for antennas with equiphase excitation and extends to infinity ("a" being the largest linear aperture dimension and "λ" the wavelength at the frequency of interest).

This criterion is not adequate for all types of antennas and should not be applied indiscriminately.

To compute an approximate value for the maximum power density "W" in the Fresnel and far field regions of an antenna, use the equation (2.8) of Section 2.2.3.

For commonly used horn and reflector antennas, the maximum power density W_m expected in the radiating near field can be estimated by equation (2.7) of Section 2.2.3.

The values predicted by Eq. (2.7) will be within ± 3 dB of the correct value (in the absence of reflections) for square apertures with uniform, cosine, and cosine square amplitude tapers, and for circular apertures with tapers ranging from uniform up to (1-q²)³ (see Ref. [9]). (The taper or aperture field distribution of circular apertures can be represented by the function (1-q²)^p, where q=r/a, in which "a" is the outside radius of the circular aperture and "r" is a radius within the aperture. When the exponent "p" increases, the field distribution becomes more highly tapered (i.e. it becomes more concentrated at the aperture centre). When p decreases and approaches zero, the aperture field distribution approaches uniform illumination.)

If a computation indicates that the approximate power density is substantially less than the exposure limit recommended in Safety Code 6, then there is usually no need for further calculation since Eq. (2.7) provides the maximum power density that can exist on the axis of the beam of an antenna that is focused at infinity, in the absence of reflections. (An antenna focused at a lesser distance could produce a higher power density in the region of its focal point, but this condition is unusual.)

If the computation from Eq. (2.7) reveals a power density value that is equal to or greater than the recommended exposure limit, then it must be assumed that this value may exist at any point in the radiating near field region and attention should be directed to the exposure fields in the far field regions.

Equations (2.7) and (2.8) do not include the effect of ground reflections. Values of power density that exceed the free-space value by a factor of four times can result when the main beam is directed toward a planar ground or reflecting surface. If the shape of the reflecting surface is such that it produces focusing effects, even greater values may result. After considering the sources of error cited above one may calculate the distance to the boundary of the potentially hazardous zone (in the presence of reflections) as follows:

Antennas - Off Axis

It is more difficult to calculate the power density off the axis of the main beam, and requires the solution of complex mathematical equations. One approach reveals that the collimated beam in the radiating near field falls off with increasing distance approximately 12 dB per unit of antenna radius. Many antennas do not have simple shapes or illumination tapers. In such cases, the approximate formula above will not apply directly, and a more complex analysis is indicated. However, a high order of precision is not warranted when computing the expected power density because of the many physical parameters in the environment that create significant variations in the values predicted by idealized computations.

Scanning Correction

In the case of scanning antennas, the average power density at a fixed point will be reduced by the value of the effective antenna-pattern beamwidth divided by the scanning angle (the number of degrees of antenna rotation during a scan). This assumes that a constant rotational velocity is used, and that the antenna rotates in one direction, rather than stopping after a scan, and reversing direction. Accordingly, the potentially hazardous distance is decreased by at least the square root of this ratio (if the period of rotation is less than the averaging time specified in Safety Code 6). The antenna's effective beamwidth in the far field will, in general, be somewhat different from the 3 dB beamwidth. The exact value depends upon the form factor of radiation pattern and associated sidelobes.

In the Fresnel Zone, the effective angle of the beamwidth will vary with distance. Here the average power density "W" of the scanning antenna is given approximately by the following relationship:

where:

θ = the scanned angle, in degrees,

P = the average power transmitted,

A = the effective area of the antenna

a = antenna diameter or width

r = distance from the antenna

If the source and propagation information on which the choice of measurement procedures is based has been deemed adequate, then the surveyor, after making estimates of expected field strengths and selecting an instrument, may proceed with the survey. The surveyor should use a high-power probe with the range switch set on the most sensitive scale. The high-intensity field areas (e.g. the main beam of a directional antenna) should be approached from a distance to avoid probe burnout. The surveyor then gradually proceeds to move progressively closer to the regions of higher field strength. Extreme care must be exercised to avoid overexposure of the surveyor and survey instrument.

On the other hand, if the information is not well defined (for example, reports of strong, intermittent interference), then it may be difficult to make a hazard survey without first conducting an empirical hazard assessment. A survey for potentially hazardous fields of unknown frequency, modulation, distribution within an area, etc. may require use of several instruments. Examples of such instruments are spectrum analyzers or field strength meters that display frequency-domain information with a means to analyze amplitude modulation characteristics, and which have a wide dynamic range (e.g. 60 dB in power). After this preliminary procedure is performed, it may be possible to continue a more meaningful survey with isotropic survey instruments.

4.2.2 Far Field Measurements: Single Source

The measurement of a linearly polarized plane-wave field whose source location, frequency, and polarization are known may be performed with a tunable field strength meter of acceptable accuracy, which covers the frequency range of interest. This instrument is used with a calibrated conventional antenna such as a standard-gain horn or dipole. Alternatively, an isotropic hazard probe may be used.

Multipath reflections may create highly non-uniform field distributions, particularly at frequencies in excess of 300 MHz. The spatial average of the field within that area should be considered as the appropriate level for comparison with whatever exposure limit is being employed as a criterion. Measurements near metallic objects should be made with the edge of the probe at least 3 "probe lengths" (e.g. 20 cm, from the object).

While mounting or holding the measuring antenna or probe, care must be taken to avoid reflections or perturbations of the field by support structures or by the operator's body. Where required, to avoid field perturbation, metallic portions of the measuring device, or support structure, should be covered with absorbing material of appropriate quality. Where possible, probe interconnect cables should be oriented normal to the electric field. When that is not practical, or where several multipath effects produce fields originating from multiple directions, metallic cables should be covered with absorber unless tests demonstrate that the cable position does not affect the measurement. Dielectric fixtures should be as small as possible (minimum reflection cross section) and should be of low dielectric-constant material, or be less than one-quarter wavelength in effective thickness T_E. The effective thickness is given by:

where, "T" is the physical thickness, and "Ε_r" is the relative permittivity. Even dielectric slabs (Ε_r> 2) can significantly alter plane wave fields if the effective thickness is greater than 0.1 wavelengths. For highest accuracy, sources of error can be accounted for, so that the true field strengths may be ascertained with less than ± 2 dB of uncertainty. To obtain this level of accuracy at frequencies above approximately 300 MHz, a scanned measurement or many point measurements per wavelength must be performed in order to obtain information on the variations in field strength in that area due to multipath and other reflections.

4.2.3 Far Field Measurements: Complex Source

When measuring the fields from multiple, relatively distant sources of unknown frequency, polarization, or direction of propagation, a broadband isotropic probe is required. Since standing wave effects and multiple-source field interactions must be accounted for, it is necessary to scan a volume of space in the zone of interest. The area should be divided into a grid of one metre squares and measurements should be taken at each grid intersection. Scans should also be made in the vertical plane at grid intersection points.

In the case of multiple sources of unknown polarizations, a single axis probe (linear dipole) cannot be used to provide accurate data in a reasonable length of time, since measurements with three orthogonal orientations of the probe must be performed to ensure that all components of the field are accounted for. If a single axis probe or linearly polarized antenna must be used, one must ensure that the field being measured is time invariant. Even if an isotropic probe is used, it must be relatively free from sources of measurement errors caused by reflections from the probe, cables, readout case, and the surveyor. The use of long (many metres) high resistance or fibre optic probe interconnect cables will minimize the reflection problems mentioned above.

4.2.4 Near Field Measurements

Since large field gradients exist in the near field of an active radiator or passive re-radiator, their measurement requires the use of a probe with an electrically small array of three orthogonal dipoles, and for frequencies below approximately 300 MHz, an array of three electrically small orthogonal loops, in order to provide satisfactory performance for the resolution of these spatial gradients. Otherwise, a large probe will measure the spatially averaged value (one with an effective area greater than one-quarter wavelength in cross section). In addition, a small antenna array produces minimal perturbation of the field and the radiation characteristics of the source are not modified (alteration of reactive near fields). Since the polarization of the fields in near field situations is usually unknown, under most circumstances an isotropic probe must be used. If the frequency and polarization are known, a broadband instrument is not required. Instead, a narrowband probe with uniform response in a single plane (similar to some commercial, microwave oven survey instruments with two orthogonal dipoles) may be used.

4.2.5 Specific Absorption Rate (SAR) Measurement

A very careful and well-documented assessment of SAR has to be performed for conformity with the requirements in Safety Code 6. 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 SAR for near field exposures of humans is difficult and can be done only on simulated models of the human body under laboratory conditions. To be valid, they have to be reliable and reasonably accurate. Examples of numerical methods for SAR calculations are the impedance method, the method of moments and the finite-difference-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. This numerical tool allows a detailed modeling 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 SAR 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. 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:

where "ΔΤ" is the temperature rise (in °C) within the time interval "Δτ" (in seconds), and "c" is the tissue (or phantom material) specific heat capacity, in J/kg°C. Calculations of SAR 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 heat transfer within and out of the body does not influence temperature rise. 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 "ρ" 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. This instrumentation has recently become commercially available and has been used to test portable transmitters for compliance evaluation.

4.2.6 Induced Current and Contact Current Measurement

Safety Code 6 requires that access to high field strength areas be restricted so as to limit the induced current and contact current experienced by a person so exposed, and acceptable limits of induced current and contact current are prescribed. Induced current is RF current induced in a human body through exposure to RF fields. The induced current can be measured by means of a special clamp-on current probe. Contact current is RF current that flows through a human body that is in contact with ungrounded or poorly grounded conductive objects in which RF potentials have been induced due to exposure to RF fields. The contact current flowing in the body is determined by the frequency and strength of the RF field, the size and shape of the body, and the body's impedance, which in turn depends on several factors, such as height, weight, body composition (i.e. fat vs. lean tissue), and the nature and degree of contact. Contact current is determined with an electric circuit simulating the impedance of a human body grasping an insulated, conductive object energised by an RF field. Further discussion concerning measurement of induced current and contact current may be found in Safety Code 6.

References

1. Health Canada, Health Protection Branch, Limits of Human Exposure to Radiofrequency Fields in the Frequency Range from 3 kHz-300 GHz Safety Code 6, 99-EHD-237, 1999.
2. National Health and Welfare, Health Aspects of Radiofrequency and Microwave Radiation Exposure, Part 1, Environmental Health Criteria Document, Report No. 77-EHD-13, 1977.
3. IEEE C95.3-1991, Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields - RF and Microwave, IEEE Inc., New York 1991.
4. R. Cleveland, Evaluating Compliance with FCC Specified Guidelines for Human Exposure to Radiofrequency Radiation, FCC OST Bulletin No. 65, Federal Communication Commission, Washington, DC, Oct. 1985.
5. M.A. Stuchly and S.S. Stuchly, Measurements of Electromagnetic Fields in Biomedical Applications, CRC Critical Review in Biomedical Engineering, vol. 14, Issue 3, pp. 241-288, 1987.
6. R.A. Tell, RF hot spot fields: the problem of determining compliance with the ansi radiofrequency protection guide, 1990 NAB Engineering Conference Proceedings, pp. 419-431.
7. H.I. Bassen and T.M. Babij, "Experimental techniques and instrumentation," Biological Effects and Medical Applications of Electromagnetic Energy, O.P. Gandhi, ed., pp. 141-173. Englewood Cliffs, NJ: Prentice Hall, 1990.
8. R.W.P. King, Electromagnetic Engineering, vol. I, McGraw-Hill, New York, 1945.
9. W .W. Mumford, Some Technical Aspects of Microwave Radiation Hazards, IRE Proceedings, no. 49, Feb. 1961, pp. 427-447.
10. Richard A. Tell, Recommended Practice for Measuring Radiofrequency Fields Associated with Land Mobile, Cellular and PCS Base Stations for Compliance with Safety Code 6, June 1999.
11. Industry Canada RSS-123, Issue 1, Low Power Licensed Radiocommunication Devices, 1999.
12. Industry Canada RSS-102, Issue 2, Radio Frequency Exposure Compliance of Radiocommunication Apparatus (All Frequency Bands).
13. National Physical Laboratory, UK, Measurement Good Practices Guides.