EMC Question of the Week: May 15, 2023

Illustration of near-field and far-field regions of a dipole antenna

A linear dipole antenna is driven by a 100-MHz sinusoidal source. At all points in free space, the electric and magnetic fields generated by the source must  

  1. be in phase
  2. be 90o out-of-phase
  3. be perpendicular
  4. have a ratio of 377 Ω 


The best answer is “c.” Faraday's Law ( × E = j ω μ 0 H ) tells us that E and H must always be perpendicular in linear materials at frequencies other than 0 Hz. This is true in both the near-field and the far-field of an antenna. In fact, it must be true for any number of antennas no matter how they are oriented relative to one another.

In the far field of an antenna, E and H are approximately in phase and have a ratio of 377 Ω, but these relationships do not hold in the near field. 

In a lossless resonant cavity, E and H are always 90o out-of-phase. They can also be approximately 90o out-of-phase in the reactive near field of an antenna.

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