GL-01 — Guidelines for the Measurement of Radio Frequency Fields at Frequencies from 3 KHz to 300 GHz
The use and application of electrical devices has steadily increased over the past decades resulting in a corresponding increase in electromagnetic (EM) fields (also termed non-ionizing radiation) in the environment. Public concern over the exposure to these fields has prompted many scientists and researchers to investigate possible effects and risks to human health.
Throughout the world, considerable research effort has been devoted to determine the effects of non-ionizing radiation exposure on human and public authorities responded to this concern by issuing exposure limits and safety guidelines for exposure to radio frequency fields.
Health Canada has issued a document entitled Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3 kHz to 300 GHz - Safety Code 6 (SC6) which outlines recommended limits of exposure to radio frequency electromagnetic (RF) fields from 3 kHz-300 GHzFootnote 1. In conjunction with this guideline, Industry Canada requires an assessment of non-ionizing radiation as part of the environmental review process for all telecommunication service licence applications. In order to assist the applicants on the subject, the Department has developed software programs to predict levels of RF energy near transmitter sites. However, field measurement may still be required in certain cases to evaluate the actual levels of radio frequency fields.
This document outlines some principles and background information for the measurement of radio frequency electromagnetic fields. It also provides a number of recommended measurement procedures for the different types of telecommunication service. The techniques for both the near field and far field measurements are based on the instrumentation that is currently available. The recommended procedures are not considered to be appropriate for the measurement of electromagnetic fields in the reactive near field region.
The document is to be used by people who work in the RF discipline and with the assumption that the user has a basic knowledge of electromagnetic field theory and practice.
The recommended procedures do not extend to measurements in the very low frequency band (below 1 kHz).
The procedures presented in this document may be used for the following:
- Measurement of radiating EM fields;
- Measurement of leakage and re-radiated EM fields;
- Measurement of induced EM fields in the body.
Sources of emission in the present context refer to the different types of radio frequency transmitters employed in the different telecommunication services. These transmitters may exhibit very different spectral, spatial and temporal characteristics due to the nature and the requirements of each type of service. The recommended procedures take into consideration the normal features and the circumstances of each type of service and the characteristics of the transmitters and radiation patterns.
Services in this frequency range include maritime navigational communications, areonautical radionavigation and radiocommunication, analog AM radio broadcasting, shortwave broadcasting, land mobile communication and fixed services, VHF radio (FM) and television broadcasting and amateur radio communication.
Measurement procedures and techniques over this frequency range vary according to the frequency and the type of service. In general, for services below 300 MHz, measurements of both the electric (E) fields and the magnetic (H) fields may be required. In addition, in cases of some high-power transmissions (e.g. AM radio service) measurements of induced current and contact current may also be required.
Services in this frequency range include UHF television and digital radio broadcasting, fixed, landmobile/PCS and satellite systems. Over this frequency range, the wavelengths of the electromagnetic fields and the dimensions of the antenna are relatively short, measurement locations are usually situated in the far field region, and in general, only electric (E) field measurements are required. In the far field region, the magnetic (H) field and the electric (E) field are related by a constant. In this case, measuring only the |E|2 component can approximate the power density.
Radio frequency electromagnetic sources radiate energy into space through antennas installed on towers and buildings. These sources have widely different characteristics and thus require proper selection of instrumentation in hazard determination. Below are the pertinent characteristics of these sources.
Transmitted electromagnetic waves may have various forms. The most fundamental form is a continuous wave (CW) or un-modulated carrier in which the wave oscillates at a single frequency. Such carriers may be modulated by another signal or message. When a CW wave is modulated by pulsing, or by varying its amplitude, frequency or phase, the wave is called a pulse, an amplitude, a frequency, or a phase-modulated wave, respectively.
The space around a radiating antenna can be divided essentially into two regions, the near field and the far field region. For an antenna with a maximum overall dimension that is small compared to the wavelength, the near field region is mostly reactive and the electric and magnetic field components store energy while producing little radiation. This stored energy is transferred periodically between the antenna and the near field. The reactive near field region extends from the antenna up to a distance "R".
where "λ" is the wavelength.
There is no general formula for estimation of the field strength in the near field for small antennas. Exact calculations can be made only for well-defined sources such as dipoles and monopoles.
For antennas large in terms of wavelength, the near field region consists of the reactive field extending to the distance given by (2.1), followed by a radiating region. In the radiating near field, the field strength does not necessarily decrease steadily with distance away from the antenna, but may exhibit an oscillatory character.
The criterion commonly used to define the distance from the source where the far field begins is that the phase of the fields from all points on the radiating antenna does not differ more than λ/16. The distance from the antenna corresponding to this criterion is:
Where "a" is the greatest dimension of the antenna.
For a paraboloidal circular-cross-section antenna, a realistic estimate for "R", which provides close agreement with experimental results, can be obtained using the following relationship:
where "a" denotes the antenna diameter.
In the radiating near field, the electric field strength (E) and magnetic field strength (H) are interrelated with each other as:
The power density S is:
where "η" is the intrinsic impedance.
The value of "η" may vary with the distance in the near field region. In the far field region, the field has a predominantly plane wave character (i.e. the electric field vector is perpendicular to the magnetic field vector, and they are both transverse to the direction of propagation). The ratio of the electric field strength to the magnetic field strength is constant at any location and in free space it is equal to:
Section 4.2 provides a detailed description of reactive near field, radiating near field and far field regions.
Radiated power is frequently expressed in decibels above 1 mW (dBm) or 1 W (dBW) reference power levels. Depending upon the type of service and source, the range of typical power radiated by transmitting antennas is from under 1 W or 0 dBW (e.g. portable transmitters) to over 100 kW or 50 dBW or higher (e.g. radars, VLF transmitters). For safety and efficiency, it is important to have the information on the radiated power, prior to taking measurements.
For antennas with reflectors, such as parabolic dishes, the maximum power density (within the antenna beam) in the radiating near field region can be conservatively estimated as:
where "Pa" is the power into the antenna, and "A" is the physical aperture area.
In the far field region, the power density on the antenna axis can be calculated from the expression:
where "r" is the distance from the antenna and "G" is the antenna directive gain.
The directive gain of an antenna in a given direction is 4π times the ratio of the radiation intensity in that direction to the total power radiated by the antenna.
The antenna gain is related to the antenna dimensions by the following equation:
where "Ae" is the effective area of the antenna, Ae = pA, "A" is the physical surface area on the antenna, "p" is the antenna efficiency and "λ" is the wavelength. It should be noted that the effective area of some antennas (e.g. linear arrays), has to be derived by other means since the physical area may not be easily determined.
The free-space electric field strength (rms value) at a distance "r", from a source with effective radiating power "Pe" (the source average output power multiplied by the antenna gain), on the antenna axis is equal to:
and "E" is expressed in volts per meter (V/m).
Electromagnetic waves are radiated into space by means of antennas. The radiation pattern of an antenna determines the spatial distribution of the radiated energy. A pattern taken in the plane containing the electric field vector is referred to as an E-plane pattern. A pattern taken in a plane perpendicular to an E-plane is called an H-plane pattern. The directional pattern of an antenna describes how much it concentrates energy in one direction in preference to radiation in other directions.
In the near field, the radiation pattern of an antenna changes with distance from the source, whereas in the far field no significant change with distance occurs.
The orientation of an electric field vector in the plane orthogonal to the direction of propagation is called polarization. If the electric field vector is always oriented in a given direction, the wave is linearly polarized. If the electric field vector rotates around the direction of propagation, maintaining a constant magnitude, the wave is circularly polarized. If the extremity of the electric field vector traces an ellipse, the wave is elliptically polarized. The rotation of the electric field vector occurs in one of two directions, either clockwise or counter-clockwise.
It is difficult to predict the orientation of the electric field in the near field region, as the transmitting antenna cannot be considered as a point source in this region. In the far field region, the antenna becomes a point source, the electric and magnetic components of the field become orthogonal to the direction of propagation and their polarization characteristics do not vary with distance.
At a measurement survey site, there may be only a single source or several sources of electromagnetic fields. A single source may have strong harmonic content that can produce electromagnetic fields at multiple frequencies. In addition, several types of RF sources such as AM, FM, TV, land-mobile and fixed transmitters may commonly be installed on an antenna farm or multiple-use tower and can produce a complex electromagnetic environment. In these situations, it is difficult to estimate the maximum expected field levels. Both broadband and narrowband instrumentation should be employed to fully characterize the electromagnetic environment.
At many transmitting sites, there may be unexpected radiation leakage emanating from electronic equipment (e.g. power amplifiers), a crack in the shielding cabinet or conduit, a joint between transmission cables or sections of waveguide. These leakages can result in localized hot spots with the electromagnetic fields in excess of the exposure limits. The nature of the leakage fields is similar to that of the near field around an antenna. Therefore, any type of polarizations may exist in the vicinity of the leak location. There is no reliable method to predict the extent of the leakage radiation, or the type of the field produced (reactive or radiating). In general, the location of the leak is not known and may only be detected by trial and error. Although many types of instruments are available for field measurements, those that have isotropic characteristics are generally better suited to probe radiation leakage.
RF electromagnetic energy from an active radiator induces electric charges or currents on ungrounded or poorly grounded conducting objects such as metal flag poles, sign posts, window frames, fences and walls of metallic buildings. The amount of the induced current depends on the physical characteristics of the object (size, shape, orientation with respect to the source) and the frequency of the incident field. This current produces its own electric and magnetic fields in close proximity to the object. The produced fields, which are generally reactive, interact with the incident field and may result in so-called "hot spots" or the enhanced E and/or H fields close to the object surface. Since the conducting objects act as secondary radiators when exposed to ambient RF fields, they are sometimes referred to as passive or parasitic re-radiators. The enhanced fields generally diminish to the ambient levels in the surrounding areas within very short distances of the secondary source. Field strength reduction is generally exponential with highest strengths on the surface of the re-radiating object. The enhanced fields are highly non-uniform in their spatial distribution on the re-radiating object and are generally difficult to predict by theoretical methods. Hot spots are best evaluated by measurements.
An RF field induces an alternating electric potential on ungrounded or poorly grounded conducting objects. When a person touches such objects, RF current flows through the person's body to the ground. This type of current is known as contact current. Even though a person may not be touching a metallic object, RF current that is induced in the body by RF fields may also flow through the body to the ground. This type of current is referred to as induced body current. Modest levels of these RF currents may cause perception, while higher values may result in shock or burns. The 1999 version of Safety Code 6 includes recommended limits for both contact and induced currents in the frequency range from 3 kHz to 110 MHz, with the intention to reduce the potential for shock or burns. Under certain exposure conditions, the contact and induced currents shall be evaluated as they may exceed the limits, even though the field strength limits are not exceeded. These conditions may occur when the electric field strength is as low as 20-25% of the exposure limit.
Specific absorption rate (SAR) is the rate of RF energy absorption per unit mass in the body. SAR has units of joules per second per kilogram or watts per kilogram (W/kg). This parameter is used as a primary indicator of RF energy absorbed in the body when quantifying the biological effects and thus defining the basic exposure limits. At frequencies between 100 kHz and 10 GHz, SAR limits take precedence over field strength and power density limits and shall not be exceeded. When carrying out compliance evaluation, the SAR should be determined for cases where exposures take place at a distance of 0.2 metres or less from the source. For conditions where SAR determination is impractical, field strength or power density measurement shall be carried out.
The following factors affect the measurement accuracy of radio frequency electromagnetic fields:
- If the probe is very close to an active radiator, coupling may occur between the probe's antennas and the radiator.
- The ground as well as nearby objects such as a metallic wall can cause partial or total reflection or scattering of the incident waves. These reflections or scattering combine with the energy received directly from the source, and create interference patterns or multipath effects that may enhance or reduce the field strength at the measurement location.
- Fields may have several frequencies because of multiple sources of emission or a single source with strong harmonic content or both.
- Certain types of modulation may affect the reliability of measured results.
- Spurious responses of the probes may be a factor, for example, the H field probe may be sensitive to electric fields, and vice versa.
- Temperature and humidity may affect the accuracy of the measuring probe. Be sure that these are within the working range of the probe. Under very low and high temperature and humidity conditions, correction factors would have to be applied to the measurement data.
- Errors may result from uncalibrated or mis-calibrated measuring instruments.
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