absorption (electromagnetic): A process characterized by the net transfer of energy from an electromagnetic field to matter (e.g. tissue) involving the eventual irresponsibility conversion of the electromagnetic energy into other energy forms, ultimate ly to heat.

antenna: a structure that is designed to radiate or receive EM fields efficiently. Any simple wire can be configured to behave as an antenna. The body also has antenna-like capabilities.

decibel (db): ten times the logarithm to the base ten of the ratio of two power levels. The quantity dbm refers to one mW (milliwatt).

diathermy: general term to describe tissue heating

dielectric constant: see relative permittivity. Electric dipole: two equal charges of opposite sign separated by a small distance.

effective area (antenna): in a given direction, the ratio of the power available at the terminals of an antenna, to the power per unit area (power density) of a plane wave incident on the antenna from that direction, polarized coincident with the polar ization that the antenna would radiate, expressed in units of square meters.

electric field (strength): a vector force field that is used to represent the forces between electric charges. Electric-field strength is a unit defined as the force per unit charge on an infinitesimally small charge at any given point in space, and it is usually represented by the simple E. The unit of electric-field strength is volt per meter (V m-1).

electromagnetic radiation: the propagation of energy in the form of electromagnetic waves through space.

electromagnetic wave: a wave characterized by variations of electric and magnetic fields.

emission: fields generated at a given distance from an RF source. This should not be confused with exposure, that is, emission does not depend on the presence of a person.

energy density (volume): EM energy in a given volume of space divided by that volume. Expressed in units of Joules per cubic meter (J m-3).

energy density (surface): EM energy incident on a surface per unit surface area. Expressed in units of Joules per square meter (J m-3). exposure: external fields incident on occupied areas. Exposure depends on the duration and the strength of the field, which in turn depends on the distance from the source.

far field: the EM field at a point far enough away from the RF source such that the fields are approximately plane-wave in nature. Frequency: the time rate at which a quantity, such as an electric field, oscillates.

gain (antenna): the ratio of the power per unit solid angle radiated in a given direction and at a given frequency to the power per unit solid angle radiated by a hypothetical, isotropic, lossless radiator for the same input power to both antennas. Po wer game is a function of direction, frequency, distance, and polarization.

ionizing radiation: radiation having a sufficiently large energy to directly ionize atomic or molecular systems.

Isotropic antenna: a hypothetical antenna radiate or receiving equally in all directions, representing a convenient reference for expressing directional properties of actual antennas.

lossy: a material characteristic which specifies attenuation or dissipation of electrical energy.

magnetic-field strength: a vector field, designated by H, that is equal to the magnetic-flux density B divided by the permeability of the medium. Magnetic-field strength is the component of the magnetic field that is measured. It is expressed in units of amperes per meter (A m-1).

near field: the EM field close enough to the RF Force such that the field is not plane-wave in nature. Spatial variation of the strength of the EM wave is usually more rapid in the near field than in the far field.

Non-thermal effect: a change in a medium or system that is not directly associated with heat production when electromagnetic energy is absorbed.

permeability: a property of a material that indicates how much magnitization occurs when a magnetic field is applied to it. The unit is henry per meter (h m-1).

permittivity: a property of material that indicates how much polarization occurs when an electric field is applied to it. The unit is farad per meter (F m-1).

plane wave: a wave in which the wave fronts are planar, E and H have constant values in the planes of the wave fronts, and E, H, and the direction of propagation are all mutually perpendicular.

polarization of EM wave: orientation of the incident electric- and magnetic-field vectors with respect to the absorbing object. When associated with an antenna, polarization generally referred to the direction of the electric-field vector.

power: the rate of change of energy with time, expressed in units of watt (W). Typically used to describe the rate of energy flow across a surface, or the rate of change of energy from one form to another.

power density (S): the power incident on a surface per unit surface area.

Poynting vector: a vector equal to the vector product of E and H. It is usually designated as S and has the unit watt per square meter (W m-2).

radiation: the propagation of EM energy in the form of waves.

relative permittivity: the permittivity of a material divided by the permittivity of free space.

scattering: the process that causes waves incident on boundaries in media to be changed in direction, frequency, phase or polarization.

Specific Absorption Rate (SAR): the time derivative of the incremental energy (dW) absorbed by, or dissipated in an incremental mass (dm) contained in a volume element (dV) of a given density (r) (A, p. 296) Average SAR in a body is the time rate of the total energy absorbed divided by the total mass of the body the unit is watt per kg (W kg-1).

spherical wave: a wave in which E and H are uniform on a surface of a sphere. E, H and the direction of propagation are all mutually perpendicular. An idealized point source radiate spherical waves.

thermal effect: a change in a medium or system that is directly associated with heat production when electromagnetic energy is absorbed.

vector: a quantity having both a magnitude and a direction. A common example is velocity.

wavelength: the distance between two adjacent crafts of a wave, expressed in meters (m).