Glossary of Terms
The following words and terms are used when discussing radiation and are brought here for completeness of information presented in this site.
Average (temporal) power: The time-averaged rate of energy transfer.
Averaging time: The appropriate time period over which exposure is averaged for purposes of determining compliance with RF exposure limits.
Continuous exposure: Exposure for durations exceeding the corresponding averaging time.
Decibel (dB): Ten times the logarithm to the base ten of the ratio of two power levels.
Duty factor: The ratio of pulse duration to the pulse period of a periodic pulse train. Also, may be a measure of the temporal transmission characteristic of an intermittently transmitting RF source such as a paging antenna by dividing average transmission duration by the average period for transmissions. A duty factor of 1.0 corresponds to continuous operation.
Effective radiated power (ERP) (in a given direction): The product of the power supplied to the antenna and its gain relative to a half-wave dipole in a given direction.
Equivalent Isotropically Radiated Power (EIRP): The product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna.
Electric field strength (E): A field vector quantity that represents the force (F) on an infinitesimal unit positive test charge (q) at a point divided by that charge. Electric field strength is expressed in units of volts per meter (V/m).
Energy density (electromagnetic field): The electromagnetic energy contained in an infinitesimal volume divided by that volume.
Exposure: Exposure occurs whenever and wherever a person is subjected to electric, magnetic or electromagnetic fields other than those originating from physiological processes in the body and other natural phenomena.
Exposure, partial-body: Partial-body exposure results when RF fields are substantially non-uniform over the body. Fields which are non-uniform over volumes comparable to the human body may occur due to highly directional sources, standing-waves, re-radiating sources or in the near field. See RF "hot spot".
Far-field region: That region of the field of an antenna where the angular field distribution is essentially independent of the distance from the antenna. In this region (also called the free space region), the field has a predominantly plane-wave character, i.e., locally uniform distribution of electric field strength and magnetic field strength in planes transverse to the direction of propagation.
Gain (of an antenna): The ratio, usually expressed in decibels, of the power required at the input of a loss-free reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength or the same power density at the same distance. When not specified otherwise, the gain refers to the direction of maximum radiation.
Gain may be considered for a specified polarization. Gain may be referenced to an isotropic antenna (dBi) or a half-wave dipole (dBd).
General population/uncontrolled exposure: For FCC purposes, applies to human exposure to RF fields when the general public is exposed or in which persons who are exposed as a consequence of their employment may not be made fully aware of the potential for exposure or cannot exercise control over their exposure. Therefore, members of the general public always fall under this category when exposure is not employment-related.
Hertz (Hz): The unit for expressing frequency, (f). One hertz equals one cycle per second.
Magnetic field strength (H): A field vector that is equal to the magnetic flux density divided by the permeability of the medium. Magnetic field strength is expressed in units of amperes per meter (A/m).
Maximum permissible exposure (MPE): The rms and peak electric and magnetic field strength, their squares, or the plane-wave equivalent power densities associated with these fields to which a person may be exposed without harmful effect and with an acceptable safety factor.
Near-field region: A region generally in proximity to an antenna or other radiating structure, in which the electric and magnetic fields do not have a substantially plane-wave character, but vary considerably from point to point. The near-field region is further subdivided into the reactive near-field region, which is closest to the radiating structure and that contains most or nearly all of the stored energy, and the radiating near-field region where the radiation field predominates over the reactive field, but lacks substantial plane-wave character and is complicated in structure. For most antennas, the outer boundary of the reactive near field region is commonly taken to exist at a distance of one-half wavelength from the antenna surface.
Occupational/controlled exposure: For FCC purposes, applies to human exposure to RF fields when persons are exposed as a consequence of their employment and in which those persons who are exposed have been made fully aware of the potential for exposure and can exercise control over their exposure. Occupational/controlled exposure limits also apply where exposure is of a transient nature as a result of incidental passage through a location where exposure levels may be above general population/uncontrolled limits (see definition above), as long as the exposed person has been made fully aware of the potential for exposure and can exercise control over his or her exposure by leaving the area or by some other appropriate means.
Peak Envelope Power (PEP): The average power supplied to the antenna transmission line by a radio transmitter during one radiofrequency cycle at the crest of the modulation envelope taken under normal operating conditions.
Power density, average (temporal): The instantaneous power density integrated over a source repetition period.
Power density (S): Power per unit area normal to the direction of propagation, usually expressed in units of watts per square meter (W/m2) or, for convenience, units such as milliwatts per square centimeter (mW/cm2) or microwatts per square centimeter (μW/cm2). For plane waves, power density, electric field strength (E) and magnetic field strength (H) are related by the impedance of free space, i.e., 377 ohms, as discussed in Section 1 of this bulletin. Although many survey instruments indicate power density units ("far-field equivalent" power density), the actual quantities measured are E or E2 or H or H2.
Power density, peak: The maximum instantaneous power density occurring when power is transmitted.
Power density, plane-wave equivalent or far-field equivalent: A commonly-used terms associated with any electromagnetic wave, equal in magnitude to the power density of a plane wave having the same electric (E) or magnetic (H) field strength.
Radiofrequency (RF) spectrum: Although the RF spectrum is formally defined in terms of frequency as extending from 0 to 3000 GHz, for purposes of the FCC's exposure guidelines, the frequency range of interest in 300 kHz to 100 GHz.
Re-radiated field: An electromagnetic field resulting from currents induced in a secondary, predominantly conducting, object by electromagnetic waves incident on that object from one or more primary radiating structures or antennas. Re-radiated fields are sometimes called "reflected" or more correctly "scattered fields." The scattering object is sometimes called a "reradiator" or "secondary radiator".
RF "hot spot": A highly localized area of relatively more intense radio-frequency radiation that manifests itself in two principal ways:
(1) The presence of intense electric or magnetic fields immediately adjacent to conductive objects that are immersed in lower intensity ambient fields (often referred to as re-radiation), and
(2) Localized areas, not necessarily immediately close to conductive objects, in which there exists a concentration of RF fields caused by reflections and/or narrow beams produced by high-gain radiating antennas or other highly directional sources. In both cases, the fields are characterized by very rapid changes in field strength with distance.
RF hot spots are normally associated with very non-uniform exposure of the body (partial body exposure). This is not to be confused with an actual thermal hot spot within the absorbing body.
Root-mean-square (rms): The 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.
Scattered radiation: An electromagnetic field resulting from currents induced in a secondary, conducting or dielectric object by electromagnetic waves incident on that object from one or more primary sources.
Short-term exposure: Exposure for durations less than the corresponding averaging time.
Specific absorption rate (SAR): A measure of the rate of energy absorbed by (dissipated in) an incremental mass contained in a volume element of dielectric materials such as biological tissues.
SAR is usually expressed in terms of watts per kilogram (W/kg) or milliwatts per gram (mW/g).
Guidelines for human exposure to RF fields are based on SAR thresholds where adverse biological effects may occur. When the human body is exposed to an RF field, the SAR experienced is proportional to the squared value of the electric field strength induced in the body.
More information on SAR can be found here.
Wavelength (): The wavelength () of an electromagnetic wave is related to the frequency (f) and velocity (v) by the expression v=f. In free space the velocity of an electromagnetic wave is equal to the speed of light, i.e., approximately 3 x 108 m/s.