Near vs Far Field EM

[6E5]

Can someone explain to me exactly the difference between near field EM, such as in wireless charging, and far field EM, such as in radio?

[coppice]

Can someone explain to me exactly the difference between near field EM, such as in wireless charging, and far field EM, such as in radio?

One way to look at it is if the receiver is near enough to the transmitter to influence it, its in the near field. If the transmitter is unperturbed by the presence or absence of the receiver its in the far field.

[w2aew]

Can someone explain to me exactly the difference between near field EM, such as in wireless charging, and far field EM, such as in radio?

s

Another difference is in the "impedance" of the radiated wave.  If the source is a large varying current, then the near field radiation will be largely an H-field (magnetic).  If the source is a varying voltage, then the near field will be largely an E-field (electric field).  The ratio of the E to H field will be dominated by the nature of the source of the emitted energy (signal and structure).  In the far field, the ratio of the E and H field component magnitudes will have adjusted to the "impedance" of air, about 377 ohms.

[coppice]

In the far field, the ratio of the E and H field component magnitudes will have adjusted to the "impedance" of air, about 377 ohms.

If you are not familiar with that strange number, its 120*pi. It falls out of the maths of propagation in free space.

[T3sl4co1l]

Near field, the impedance can be very different (at DC, the impedance of a static magnetic field is zero and the impedance of a static electric field is infinite, but it also doesn't propagate).  Veeeery roughly, the ratio of impedances is the ratio of attenuation (power in versus power radiated), or the Q*, or ratio of dimensions to the wavelength, or stuff like that.

*If you can actually calculate or measure the radiation resistance independently from other losses, this is actually a less rough, and more useful, matter.

Analogously, the impedance of a section of transmission line (whether a wide pad, a thin trace, or a component lead, or...) varies from characteristic impedance below the cutoff frequency (say under 1/4 wave or so) to inductive or capacitive a proportional amount away.  The only reason we approximate e.g. traces as inductive (for low system impedances) or capacitive (for high system impedances) is because the line impedance is sufficiently different not to mind the precise difference; but if you're looking at it over a very wide bandwidth, even the transmission line stuff becomes relevant, and it's important to remember its origin.

Tim

[IanB]

Can someone explain to me exactly the difference between near field EM, such as in wireless charging, and far field EM, such as in radio?

I think that when near field is applied to wireless charging it is, in grossly simplified terms, a transformer where the primary and secondary parts can be  separated and brought back together again. The charging coil is the primary winding, the receiving coil is the secondary winding, and when you put the device to be charged on the charging station you assemble an air cored transformer.

The tight coupling between primary and secondary obtainable in a transformer allows a reasonably large amount of power to be transferred.

With far field, such as with a wireless transmitter, the power is radiated out into space from the antenna. Even if you shape the radiation into a beam such as with a microwave antenna, the beam will still spread out over distance and any given receiver will only be able to pick up a fraction of the transmitted power.

As a result, near field is most useful for transmitting power while far field is most useful for transmitting information.

[G0HZU]

Can someone explain to me exactly the difference between near field EM, such as in wireless charging, and far field EM, such as in radio?

The near field can be considered as stored energy (very close to the antenna) and the far field is radiated energy that propagates away from the antenna.
When the transmitter is turned off the near field will collapse but the energy in the far field keeps radiating out.

Think of the energy 'stored' in the near field as being a bit like storing energy in a reactive component like a capacitor or an inductor. This energy is stored in the system without any radiation. The near field is often referred to as the reactive field.

You can exploit or tap into this energy storage in the (reactive) near field with EM induction using a suitable circuit placed in the near field. eg your wireless charger or an RFID tag etc.