EMC Question of the Week: June 6, 2022

Plot of impedance vs. distance from electric dipole and magnetic dipole sources

The graph in this figure plots impedance as a function of distance from electric and magnetic dipole sources. Plots similar to this appear in countless EMC texts and articles. What type of impedance is this?

  1. intrinsic
  2. characteristic
  3. wave
  4. input


The correct answer is “c.” The plot shows the ratio of the electric field magnitude to the magnetic field magnitude, which is (by definition) the wave impedance. Intrinsic impedance is a property of materials and is equal to the square root of the ratio of the permeability to the permittivity. Characteristic impedance is a property of transmission lines and is the ratio of the voltage to the current in a forward traveling wave. Input impedance is the ratio of the voltage to the current at an input (e.g., an antenna input).

Of the four choices provided, wave impedance is the only one that is a function of the distance from the source. For electrically small sources, at distances greater than about a wavelength, the wave impedance is independent of the type of source and is approximately equal to the intrinsic impedance of the medium. In free space, that is about 377 Ω as indicated in the figure.

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