Knowra Impedance of free space Impedance of free space The ratio of electric-field to magnetic-field amplitudes in a plane electromagnetic wave traveling through vacuum. Its value is approximately 376.73 ohms.
Maxwell's equations : Four equations describing how electric and magnetic fields arise, vary, and interact with matter and currents. In vacuum, these equations relate the electric and magnetic amplitudes of a traveling wave.
Electric field : A vector field that describes the electric force per unit charge at each point in space. It forms the numerator of the vacuum wave’s field-amplitude ratio.
Poynting vector : A vector describing the directional energy flux of an electromagnetic field, given by the cross product of its electric and magnetic fields. The vacuum impedance converts between field amplitudes when calculating a wave’s energy flux.
Characteristic impedance : The ratio of voltage to current for a traveling wave on a transmission line. It is a traveling-wave ratio like vacuum impedance, but belongs to a guided circuit structure.
Electromagnetic wave equation : A differential equation governing the propagation of electromagnetic fields through space and media. Its vacuum solutions propagate at light speed, linking impedance to the medium’s electromagnetic constants.
Magnetic field : A vector field describing magnetic effects on moving charges, currents, and magnetic materials. Its wave amplitude forms the denominator of the ratio.
Free-space path loss : The decrease in received electromagnetic power as a wave spreads over distance in unobstructed space. Power calculations for radiated waves use the relation between field strength and impedance.
Intrinsic impedance : The ratio of electric to magnetic field amplitudes for a plane wave in a specified medium. Vacuum impedance is the special case of intrinsic impedance when the medium is vacuum.
Speed of light : The speed at which light and other electromagnetic radiation propagate in vacuum, exactly 299,792,458 meters per second. Together with the vacuum permeability, this speed determines the impedance’s value.
Ohm : The SI unit of electrical resistance, equal to one volt per ampere. Electric field divided by magnetic field has units equivalent to ohms.
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