Near field from cell phone antennas?

[shoestring]

I don't know much about antennas, but I've seen descriptions of the field around dipole antennas that make a difference between the "near field" and the "far field", where the far field is more or less a pure radiation field of photons traveling away from the source, and the near field more like an dipole field (which is not as easily characterized as a stream of photons). There's of course no clear boundary between the two, but a rough estimate of where the radiation field begins to dominate is a wavelength away from the source.

Because radiation from cell phones have wavelengths in the cm - dm range (though I'm not sure exactly what wavelengths are used) I wonder if there's a near field (that isn't a pure radiation field) around cell phones that perhaps should be taken into account when possible effects on humans are discussed.

I'm aware that possible health effects from cell phones is a controversial topic, but here I'm interested in the technical part of the issue, if there can be a noticeable near field, somewhat different in character from the radiation field, right outside a cell phone.

Thanks :-)

 

[skeptic2]

My understanding of near fields is that they are the part of the field that normally is not radiated. For an incomplete example, think of an inductor with an alternating current passing through it. Around the inductor there is a magnetic field some of which radiates and some of which returns to the inductor and contributes to the inductance.

If a closed circuit with an inductor in the circuit is brought nearby, some of the alternating field that normally returns to the inductor now couples into the nearby inductor and transfers some of the energy from the first inductor to the second. In so doing, the inductance of the first inductor diminishes
slightly.

At cellphone frequencies this effect still exists but a similar effect with capacitive coupling also becomes noticeable. What we call the near field effect is a combination of those two effects.

And yes there is radiative coupling into the person's head while he is using a cellphone and the effect is to cause a slight increase in temperature in the brain. The temperature increase is likely less than the temperature increase resulting from walking around in the sun without a hat. The limits of electromagnetic exposure to human tissue are very conservative and the frequency of the electromagnetic radiation is far below what is needed to cause ionization, which is believed to be necessary to cause cancer.

 

[sophiecentaur]

It's interesting to note that the E and H field variations in the far field are in phase but E and H right next to the transmitter are more or less in phase quadrature.