Radiofrequency Radiation

antenna: A structure that is designed to radiate or pick up electromagnetic fields efficiently. Individual antennas are often used in combinations called antenna arrays.
dielectric constant: Another name for relative permittivity.
electric dipole: Two equal charges of opposite sign separated by an infinitesimally small distance.
electric field: A term often used to mean the same as Efield intensity, or strength.
electricfield intensity: Another term for Efield strength.
electricfield strength: A vectorforce field used to represent the forces between electric charges. Efield strength is defined as the vector force per unit charge on an infinitesimal charge at a given place in space.
electricflux density (displacement): The electric flux passing through a surface, divided by the area of the surface. The total electric flux passing through a closed surface is equal to the total charge enclosed inside the surface, also equal to the Efield intensity times the permittivity.
electric polarization: Separation of charges in a material to form electric dipoles or alignment of existing electric dipoles in a material when an Efield is applied. Usually designated P, the units of polarization are dipole moments per cubic meter.
energy density: Electromagnetic energy in a given volume of space divided by the volume. The units are joules per cubic meter (J/m^{3}).
far fields: Electromagnetic fields far enough away from the source producing them that the fields are approximately planewave in nature.
field: A correspondence between a set of points and a set of values. That is, a value is assigned to each of the points. If the value is a scalar, the field is a scalar field; if the value is a vector, the field is a vector field. The temperature at all points in a room is an example of a scalar field. The velocity of the air at all points in a room is an example of a vector field.
field point: A point at which the electric or magnetic field is being evaluated.
frequency: The time rate at which a quantity, such as electric field, oscillates. Frequency is equal to the number of cycles through which the quantity changes per second.
impedance, wave: The ratio of the electric field to magnetic field in a wave. For a planewave in free space, the wave impedance is 377 ohms. For a planewave in a material, the wave impedance is equal to 377 times the square root of the permeability divided by the square root of the permittivity.
magnetic field: A term often used to mean the same as magneticflux density, also commonly used to mean the same as magneticfield intensity. The term has no clear definition or pattern of usage.
magneticfield intensity: A vector field equal to the magneticflux density divided by the permeability. H is a useful designation because it is independent of the magnetization current in materials.
magneticflux density: A vectorforce field used to describe the force on a moving charged particle, and perpendicular to the velocity of the particle. Magneticflux density is defined as the force per unit charge on an infinitesimal charge at a given point in space: F/q = v x B, where F is the vector force acting on the particle, q is the particle's charge, v is its velocity, and B is the magneticflux density.
near fields: Electromagnetic fields close enough to a source that the fields are not planewave in nature. Near fields usually vary more rapidly with space than far fields do.
nodes: Positions at which the amplitude is always zero in a standing wave.
permeability: A property of material that indicates how much magnetization occurs when a magnetic field is applied.
permittivity: A property of material that indicates how much polarization occurs when an electric field is applied. Complex permittivity is a property that describes both polarization and absorption of energy. The real part is related to polarization; the imaginary part, to energy absorption.
planewave: A wave in which the wave fronts are planar. The E and H vectors are uniform in the planes of the wave fronts; and E, H, and the direction of propagation (k) are all mutually perpendicular.
polarization: Orientation of the incident E and Hfield vectors with respect to the absorbing object.
Poynting vector: A vector equal to the cross
product of E and H. The Poynting vector
represents the instantaneous power transmitted through a
surface per unit surface area. it is usually designated as
S, is also known as energyflux (power) density, and
has units of watts per square meter
(W/m^{2} ).
propagation constant: A quantity that describes the propagation of a wave. Usually designated k, it is equal to the radian frequency divided by the phase velocity, and has units of per meter (m^{1} ). A complex propagation constant describes both propagation and attenuation. The real part describes attenuation; the imaginary part, propagation.
radian frequency: Number of radians per second at which a quantity is oscillating. The radian frequency is equal to 2f, where f is the frequency.
radiation: Electromagnetic fields emitted by a source.
reflection coefficient: Ratio of reflectedwave magnitude to incidentwave magnitude.
relative permittivity: Permittivity of a material divided by the permittivity of free space.
scalar field: See field.
specific absorption rate (SAR): Time rate of energy absorbed in an incremental mass, divided by that mass. Average SAR in a body is the time rate of the total energy absorbed divided by the total mass of the body. The units are watts per kilogram (W/kg).
spherical wave: A wave in which the wave fronts are spheres. An idealized point source radiates spherical waves.
standing wave: The wave pattern that results from two waves of the same frequency and amplitude propagating in opposite directions. Destructive interference produces nodes at regularly spaced positions.
standingwave ratio: Ratio of E_{max} to E_{min} where E _{max} is the maximum value, and E_{min} the minimum, of the magnitude of the Efield intensity anywhere along the path of the wave. A similar definition holds for other quantities that have wave properties.
vector: A quantity having both a magnitude and a direction. Velocity is an example of a vector: Direction of motion is the direction of the velocity vector, and speed is its magnitude.
vector field: See field.
velocity of propagation: Velocity at which a wave propagates. Units are meters per second (m/s). It is equal to how far one point on the wave, such as the crest or trough, travels in 1s.
wave impedance: (See impedance, wave).
wave length: The distance between two crests of the wave (or between two troughs or other corresponding points). Units are meters (m).
Quantity  Unit  Symbol  
Length  meter  m  
Mass  kilogram  kg  
Time  second  s  
Electric current  ampere  A  
Temperature  Kelvin  K  
Luminous intensity  candela  cd 
A + B = C (Equation 3.1)
where C is the vector along the parallelogram shown in Figure 3.2. The negative of a vector A is defined as a vector having the same magnitude as A but opposite direction. Subtraction of any two vectors A and B is defined as
A  B = A + (B) (Equation 3.2)
where B is the negative of B.
A · B = A B cos (Equation 3.3)
where is the angle between A and B, as shown in Figure 3.3. The dot product of two vectors is a scalar. As indicated in Figure 3.3, A ·B is also equal to the projection of A on B, times B. This interpretation is often very useful. When two vectors are perpendicular, their dot product is zero because the cosine of 90° is zero (the projection of one along the other is zero).
A x B = C (Equation 3.4)
where C is a vector whose direction is perpendicular to both A and B and whose magnitude is given by
C = A B sin (Equation 3.5)
As shown in Figure 3.4, the direction of C is the direction a righthanded screw would travel if turned in the direction of A turned into B. The cross product of two parallel vectors is always zero because the sine of zero is zero.
Go to Chapter 3.2
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Last modified: November 12, 1996
© October 1986, USAF School of Aerospace Medicine, Aerospace Medical Division (AFSC), Brooks Air Force Base, TX 782355301