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 E-field intensity, or strength.
electric-field intensity: Another term for E-field strength.
electric-field strength: A vector-force field used to represent the forces between electric charges. E-field strength is defined as the vector force per unit charge on an infinitesimal charge at a given place in space.
electric-flux 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 E-field 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 E-field 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/m3).
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 magnetic-flux density, also commonly used to mean the same as magnetic-field intensity. The term has no clear definition or pattern of usage.
magnetic-field intensity: A vector field equal to the magnetic-flux density divided by the permeability. H is a useful designation because it is independent of the magnetization current in materials.
magnetic-flux density: A vector-force field used to describe the force on a moving charged particle, and perpendicular to the velocity of the particle. Magnetic-flux 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 magnetic-flux 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 H-field 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 energy-flux (power) density, and
has units of watts per square meter
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 reflected-wave magnitude to incident-wave 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.
standing-wave ratio: Ratio of Emax to Emin where E max is the maximum value, and Emin the minimum, of the magnitude of the E-field 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).
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 right-handed 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 78235-5301