What Do We All Know?
We all know that distance is the enemy of RF energy. The farther you are from an antenna, the less fidelity you can expect in terms of the original audio for phone QSOs. We can therefore identify “fields” of RF energy. The field of most interest is, of course, the far field. This is the field that potentially reaches to the four corners of the globe.
Something Else of Interest
There is another RF field that is also of interest. This is the near-field. This field is generally of much less interest to the amateur radio station operator. However, when it is of interest, it is a HOT TOPIC. And the subject of greatest interest regarding the near-field is how far it extends. While the far field extends to the four corners of the globe, the near field distance is frequency dependent and typically only extends to a few feet.
The near-field is distinguished form the far-field in that the near-field stores energy and does not spend it. That is, it does not spend it UNLESS…unless its reactive region is disturbed by something such as your hand. In such a case you could find yourself looking for a box of bandaids because it will spend its stored energy on your hand. What is more, because of phase differences within the near-field, the peak power density can be four times the average power density.
As the illustration above depicts, the near field is subjugated into two regions–reactive and radiating. The distance of the reactive near-field is considered to have the range following. The equation on the left represents distance for small antennae (less than one wavelength) and the one on the right for very long antennae.
wavelength/(2*pi) > Near-Fieldreactive < D2/(wavelength*2)
What Does this Work Out To Be?
Assume an antenna D of 19″ (small), which is 48 cm. For 2-meter operating frequencies, this translates to 13 inches. For an antenna of 57″, it would be 20 inches distance. But it is important to remember that the density is falling off as a reciprocal of the distance cubed. The near-field reactive region does not simply fall off the edge at the prescribed distance.
At lower VHF frequencies (roughly 117 MHz), the two equations are nearly equal to each other for the named conditions showing up at 16 inches.
For a more detailed treatise on the subject, see section 18.104.22.168 of Basic Physics of Radiofrequency.