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How do antennas behave as they are moved to near-field distances? Can they still receive a radio signal over a distance closer than a wavelength?

Background:
In this comment it is said, that the device “has a probably ferrite loaded loop antenna (a coil with a core) and there is a direct magnetic coupling (just as in a magnetic transformer) between that and the transmitter”.
I suppose, that an electric coupling between two antenna rods is also possible.

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    $\begingroup$ Please make your question more specific. "What are your experimental experiences" is extremely broad and also invites personal stories rather than objective answers. $\endgroup$ – Kevin Reid AG6YO Dec 29 '18 at 21:23
  • $\begingroup$ @KevinReidAG6YO Better now? $\endgroup$ – HolgerFiedler Dec 29 '18 at 21:36
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    $\begingroup$ The question is still asking for personal experience. Ask about facts, not experiences. It would also be good to give more detail on exactly what you're asking for — NFC and wireless power transfer both demonstrate that this is possible and practical on an everyday basis, so are you looking for an explanation of the principles of inductive/capacitive coupling, or how antennas behave as they are moved to near-field distances, or something else? $\endgroup$ – Kevin Reid AG6YO Dec 29 '18 at 21:45
  • $\begingroup$ @KevinReidAG6YO, how are antennas behave as they are moved to near-field distances, that is it. Will edit it. $\endgroup$ – HolgerFiedler Dec 29 '18 at 21:50
  • $\begingroup$ That'll do. Reopened. $\endgroup$ – Kevin Reid AG6YO Dec 29 '18 at 23:08
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How do antennas behave as they are moved to near-field distances?

The near field zone of an antenna consists of highly reactive fields that can easily couple to nearby conductive objects. As a result, antennas that are in the near field of another can easily transfer power between them. Whether or not communications is possible under this closely coupled condition is a factor of transmit power, receiver sensitivity and any system related losses.

System losses can arise due to the mutual impedance which is defined as the negative ratio of the voltage induced at the open terminals of one antenna to the current in other antenna. This mutual impedance alters the self impedance of the feedpoint which typically results in mismatch losses.

Another source of system loss relates to the dominant form of the reactive waves in the antenna near field. Close coupling of a near field inductive dominant antenna (such as a small loop) with a near field electrostatic dominant antenna (such as a short dipole) antenna can give rise to system losses.

It is reasonable to generalize that if two antennas enable communications in their far field that bringing them within the near field of the other will still allow communications to occur.

Can they still receive a radio signal over a distance closer than a wavelength?

If the antennas permit communications at a distance of greater than one meter, then they will also permit communication at a distance of less than one wavelength. With all other things being equal, the power received by the receiving antenna in its far field is inversely related to the distance squared. So for example, moving from 2 wavelengths to 1 wavelength of separation results in a 4 times power increase.

Antennas that are spaced less than one wavelength apart may still be in their far fields. A rough approximation of the near field boundary is given by:

$$r\approx 0.62\sqrt{\frac{D^3}{\lambda}} \tag 1$$

where D is the maximum antenna dimension in meters and $\lambda$ is the wavelength in meters.

Applying formula 1 to a 30 cm antenna operated at 11 meters (27 MHz), the near field region would start at approximately 30 mm from the antenna. Although this is a very rough approximation, it is clearly far less than the 11 meter wavelength involved.

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    $\begingroup$ @HolgerFiedler I updated my answer. $\endgroup$ – Glenn W9IQ Dec 30 '18 at 11:41
  • $\begingroup$ Glenn, perfect. Especially that you mention the small loop and the short dipole. And... $\endgroup$ – HolgerFiedler Dec 30 '18 at 13:46
  • $\begingroup$ I cited you on PhysicsStackExchange $\endgroup$ – HolgerFiedler Dec 30 '18 at 15:16

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