Electromagnetic fields propagate as waves and travel at the speed of light (c). The wavelength is proportional to the frequency.

If the distance to the field source is less than one wavelength, then we are usually in the near field (almost always the case in the low frequency range up to 30 kHz). If the distance is more than three wavelengths, then far field conditions usually hold.

This distinction between near and far fields is important when it comes to measurements. In the near field, the ratio of electric field strength (E [V/m]) and magnetic field strength (H [A/m]) is not constant, so we have to measure each separately.

In the far field, however, it is enough to just measure one field quantity since we can compute the other, their ratio being a known constant.

An electric field is easily shielded, e.g. using a thin, grounded metal foil. However, a magnetic field will penetrate almost all known materials.