EMC Question of the Week: February 17, 2020

The far-field radiated emissions from a vertically polarized 30-MHz source located one-meter above a ground plane have a maximum value 

  1. at the ground plane
  2. 1 meter above the ground plane
  3. between 1 and 4 meters above the ground plane
  4. below the ground plane


The correct answer is "a". The radiation pattern of a vertically polarized source above a ground plane always peaks at the ground plane. The image of a vertically polarized source is in phase with the source. Points on the ground plane are the same distance from the source and its image, so the emissions from both are in phase.

Radiated Emissions from Vertically Polarized Sources Add at the Ground Plane

Emissions from a 30 MHz source have a 10-meter wavelength. A 30-MHz source that is only 1 meter above the plane will have a pattern with only one maximum (the one at the ground plane).

Horizontally polarized sources, on the other hand, will have a minimum value at the ground plane. The source and its image are 180o out of phase, so the field at points equidistant from both will cancel each other.

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