# Near-field energy transfer

If you absorb energy in the near field of the antenna, it will produce a loading effect on the source. Whereas in the far field it will not.

Is there an intuitive explanation why this is true for one type of the field and not the other?

Can the common electrical transformer be thought of as two antennas in each others near field?

• Are you looking for a theoretical-physics type of explanation? It must be because fields fall like $1/r^2$ in the far field and then influences back onto the first antenna will fall like $1/r^4$. – Kostas Dec 5 '19 at 13:37
• You say that absorption of the far field does not produce a loading effect, but I think it should. I expect a reflection with an out of phase component. Where did you get this information? – my2cts Dec 9 '19 at 21:50

## 2 Answers

You can think of two antennas in each other's near field as two halves of an air-core transformer. As such, they will load each other in ways that don't happen in the far field.

This principle can be used to couple RF power to an antenna in a manner that prevents induced currents from other nearby antennas from propagating in the antenna leads. Instead of antenna leads that travel directly from the transmitter to the antenna, the antenna leads are broken and a pair of dipole antennas are inserted there, less than a wavelength apart. The near-field coupling is almost as good as the unbroken case, and the presence of a physical break in the antenna line interrupts the induced currents.

The far field is a traveling wave, once generated it radiates away from the source off into free space. The near field is a standing wave, which stores reactive energy. Your question as to why coupled near fields load each other and coupled far fields do not is not exactly correct; they both load each other in different ways.

For example, the transformer you described is coupled by a mutual inductance, a reactive coupling. An antenna has a radiation resistance, typically describing its ability to radiate into free space. This is a resistance and not a reactance because the power is lost. It radiates into space unlike the near field, which is stored. If radiating antennas are very close, they will load each other both reactively and resistively since they will be in each other's near and far field. The Yagi-Uda antenna is an example of such a case where mutual impedance must be taken into account for proper antenna design.

For practical communication purposes, however, antennas are usually too far to load each other. Which makes sense, because the power received is many orders of magnitude below the power transmitted.