Editing Near and far field (section)

==Near-field characteristics==

The near field itself is further divided into the ''reactive'' near field and the ''radiative'' near field. The ''reactive'' and ''radiative'' near-field designations are also a function of wavelength (or distance). However, these boundary regions are a fraction of one wavelength within the near field. The outer boundary of the reactive near-field region is commonly considered to be a distance of <math display="inline">\frac{1}{2\pi}</math> times the wavelength (i.e., <math display="inline">\frac{\lambda}{2\pi}</math> or approximately {{math|0.159λ}}) from the antenna surface. The reactive near-field is also called the ''inductive'' near-field. The radiative near field (also called the ''Fresnel region'') covers the remainder of the near-field region, from <math display="inline">\frac{\lambda}{2\pi}</math> out to the Fraunhofer distance.<ref name=OSHA-EM-rad/>

===Reactive near field, or the nearest part of the near field===
In the reactive near field (very close to the antenna), the relationship between the strengths of the {{math|'''E'''}} and {{math|'''H'''}} fields is often too complicated to easily predict, and difficult to measure. Either field component ({{math|'''E'''}} or {{math|'''H'''}}) may dominate at one point, and the opposite relationship dominate at a point only a short distance away. This makes finding the true [[power density]] in this region problematic. This is because to calculate power, not only {{math|'''E'''}} and {{math|'''H'''}} both have to be measured but the [[phase (waves)|phase relationship]] between {{math|'''E'''}} and {{math|'''H'''}} as well as the angle between the two vectors must also be known in every point of space.<ref name=OSHA-EM-rad/>

In this reactive region, not only is an electromagnetic wave being radiated outward into far space but there is a reactive component to the electromagnetic field, meaning that the strength, direction, and phase of the electric and magnetic fields around the antenna are sensitive to EM absorption and re-emission in this region, and respond to it. In contrast, absorption far from the antenna has negligible effect on the fields near the antenna, and causes no back-reaction in the transmitter.

Very close to the antenna, in the reactive region, [[energy]] of a certain amount, if not absorbed by a receiver, is held back and is stored very near the antenna surface. This energy is carried back and forth from the antenna to the reactive near field by electromagnetic radiation of the type that slowly changes [[electrostatic]] and magnetostatic effects. For example, current flowing in the antenna creates a purely magnetic component in the near field, which then collapses as the antenna current begins to reverse, causing transfer of the field's magnetic energy back to electrons in the antenna as the changing magnetic field causes a self-inductive effect on the antenna that generated it. This returns energy to the antenna in a regenerative way, so that it is not lost. A similar process happens as electric charge builds up in one section of the antenna under the pressure of the signal voltage, and causes a local electric field around that section of antenna, due to the antenna's [[self-capacitance]]. When the signal reverses so that charge is allowed to flow away from this region again, the built-up electric field assists in pushing electrons back in the new direction of their flow, as with the discharge of any unipolar capacitor. This again transfers energy back to the antenna current.

Because of this energy storage and return effect, if either of the inductive or electrostatic effects in the reactive near field transfer any field energy to electrons in a different (nearby) conductor, then this energy is lost to the primary antenna. When this happens, an extra drain is seen on the transmitter, resulting from the reactive near-field energy that is not returned. This effect shows up as a different impedance in the antenna, as seen by the transmitter.

The reactive component of the near field can give ambiguous or undetermined results when attempting measurements in this region. In other regions, the power density is inversely proportional to the square of the distance from the antenna. In the vicinity very close to the antenna, however, the energy level can rise dramatically with only a small decrease in distance toward the antenna. This energy can adversely affect both humans and measurement equipment because of the high powers involved.<ref name=OSHA-EM-rad/>

===Radiative near field (Fresnel region), or farthest part of the near field===
The radiative near field (sometimes called the ''Fresnel region'') does not contain reactive field components from the source antenna, since it is far enough from the antenna that back-coupling of the fields becomes out of phase with the antenna signal, and thus cannot efficiently return inductive or capacitive energy from antenna currents or charges. The energy in the radiative near field is thus all [[radiant energy]], although its mixture of magnetic and electric components are still different from the far field. Further out into the radiative near field (one half wavelength to 1 wavelength from the source), the {{math|'''E'''}} and {{math|'''H'''}} field relationship is more predictable, but the {{math|'''E'''}} to {{math|'''H'''}} relationship is still complex. However, since the radiative near field is still part of the near field, there is potential for unanticipated (or adverse) conditions.

For example, metal objects such as steel beams can act as antennas by inductively receiving and then "re-radiating" some of the energy in the radiative near field, forming a new radiating surface to consider. Depending on antenna characteristics and frequencies, such coupling may be far more efficient than simple antenna reception in the yet-more-distant far field, so far more power may be transferred to the secondary "antenna" in this region than would be the case with a more distant antenna. When a secondary radiating antenna surface is thus activated, it then creates its own near-field regions, but the same conditions apply to them.<ref name="OSHA-EM-rad">
{{cite web |author=Occupational Safety and Health Administration, Cincinnati Technical Center |date=May 20, 1990 |title=Electromagnetic Radiation and How It Affects Your Instruments. Near field vs. Far field. |url=https://www.osha.gov/radiofrequency-and-microwave-radiation/electromagnetic-field-memo#section_6 |url-status=live |access-date=2025-04-25 |publisher=U.S. Dept of Labor}} Department of Labor – Public Domain content. Most of the content referenced by this work in this article is copied from a public domain document. In addition, this paper has provided [https://www.osha.gov/radiofrequency-and-microwave-radiation/electromagnetic-field-memo#section_10 references].</ref>

===Compared to the far field===
The near field is remarkable for reproducing classical [[electromagnetic induction]] and electric charge effects on the EM field, which effects "die-out" with increasing distance from the antenna: The magnetic field component that’s in phase quadrature to electric fields is proportional to the inverse-cube of the distance (<math>1/r^3</math>) and electric field strength proportional to inverse-square of distance (<math>1/r^2</math>). This fall-off is far more rapid than the classical radiated far-field ({{math|'''E'''}} and {{math|'''B'''}} fields, which are proportional to the simple inverse-distance (<math>1/r</math>). Typically near-field effects are not important farther away than a few wavelengths of the antenna.

More-distant near-field effects also involve energy transfer effects that couple directly to receivers near the antenna, affecting the power output of the transmitter if they do couple, but not otherwise. In a sense, the near field offers energy that is available to a receiver {{em|only}} if the energy is tapped, and this is sensed by the transmitter by means of responding to electromagnetic near fields emanating from the receiver. Again, this is the same principle that applies in [[electromagnetic induction|induction coupled]] devices, such as a [[transformer]], which draws more power at the primary circuit, if power is drawn from the secondary circuit. This is different with the far field, which constantly draws the same energy from the transmitter, whether it is immediately received, or not.

The amplitude of other components (non-radiative/non-dipole) of the electromagnetic field close to the antenna may be quite powerful, but, because of more rapid fall-off with distance than <math>1/r</math> behavior, they do not radiate energy to infinite distances. Instead, their energies remain trapped in the region near the antenna, not drawing power from the transmitter unless they excite a receiver in the area close to the antenna. Thus, the near fields only transfer energy to very nearby receivers, and, when they do, the result is felt as an extra power draw in the transmitter. As an example of such an effect, power is transferred across space in a common [[transformer]] or [[metal detector]] by means of near-field phenomena (in this case [[inductive coupling]]), in a strictly short-range effect (i.e., the range within one wavelength of the signal).