The horizontal distance between DUT and Antenna for automotive EMC compliance is 1 meter. For other non-automotive regulatory and standard-based measurements these distances are 3m, 10m, or 30m.
One-meter DUT-to-antenna distance measurements are carried out based on MIL-STD 461 (military), RTCA DO-160 & EUROCAE ED-14 (commercial aircraft), and CISPR 25 (automotive).
CISPR 25 specifies a one-meter antenna distance to be used for radiated emissions from Components/Modules in an Absorber Lined Shielded Enclosure (ALSE).
The near field and far field are regions of the electromagnetic field (EM) around an object.
Far-field E (electric) and B (magnetic) field strength decreases as the distance from the source increases, resulting in an inverse-square law for the radiated power intensity of electromagnetic radiation.
Near-field E (electric) and B (magnetic) field strength decrease more rapidly with distance:
- the radiative field decreases by the inverse-distance squared, resulting in a diminished power in the parts of the electric field by an inverse fourth-power
- the reactive field by an inverse cubed law, resulting in a diminished power in the parts of the electric field by an inverse sixth-power
The rapid drop in power contained in the near-field ensures that effects due to the near-field essentially vanish a few wavelengths away from the radiating part of the antenna.
dF = (2* D^2)/λ
D= largest dimension of the radiator or diameter of antenna
λ = wavelength of the radio wave
dF = 2*(D/λ)^2
λ = 2* (D/λ)*D
dF >> D
dF >> λ
Near-field and far-field regions for an antenna (diameter or length D) larger than the wavelength of the radiation it emits, so that D⁄λ ≫ 1:
- Near Field
R = near field antenna to radiating filed distance
R = 0.62 * (D^3/λ)^1/2
- Far Field
Ro = far field antenna to radiating filed distance
Ro = 2*(D^2/ λ)