EMC Question of the Week: May 15, 2023

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

Answer

The best answer is “c.” Faraday's Law ( $\nabla \times \overrightarrow{E}=j\omega {\mu }_0\overrightarrow{H}$ ) 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 90^o out-of-phase. They can also be approximately 90^o out-of-phase in the reactive near field of an antenna.

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