Safety considerations
Contributed by Ramjee Prasad and Luis
M. Correia
Edited by Jean-Paul Linnartz
Up to not too many years ago, the analysis of possible harmful effects of
electromagnetic radiation on people was devoted mainly to power lines and radars,
due to the huge power levels involved in those systems; even when mobile phone
systems appeared, there was no major concern, since the antennas were installed
on the roofs of the cars. With the development of personal communication systems,
in which users carry the mobile phone inside their pocket, and the antenna radiates
a few centimeters from the head, the safety problem gained a great importance
and a new perspective; a lot of work can be found in the literature not only
on the absorption of power inside the head, but also on the influence of the
head on the antenna's radiation pattern and input impedance. However, these
works have addressed only the frequency bands at use in today's systems, i.e.,
up to 2 GHz (mainly on the 900 and 1800 MHz bands), and only very few references
are done to systems working at higher frequencies, as it is the case of WBMCSs.
Infra-red
The problems associated with infra-red are different from those posed by microwave
and millimetre waves. Eye safety, rather than power absorption inside the head,
is the issue in this case, since the eye acts as a filter to the electromagnetic
radiation, allowing only light and near frequency radiation to enter into it,
and the amount of power absorption inside the human body is negligible. Exposure
of the eye to high levels of infra-red radiation may cause cataract-like diseases,
and the maximum allowed transmitter power seems to limit the range to a few metres.
If this is the case, safety restrictions will pose severe limitations to the use
of infra-red in WBMCSs, as far as general applications are concerned. The question
in this case is not that there are always problems during system operation (as
in mobile phones for example), but the damages that may be caused if someone looks
to the transmitter during operation.
Micro and millimetre waves
Microwaves and millimetre waves have no special effect on eyes, but power absorption
can be really the problem. In the case of WLANs, antennas will not radiate very
near (1 or 2 cm) the user as in the mobile phone case, thus enabling power limitations
to be less restrictive; also if mobile multimedia terminals are to be used like
PDAs, the case will be similar to the previous one. But if terminals are to be
used in the same form as mobile phones, then maximum transmitter powers have to
be established, as it happens for the current personal communication systems.
The standards for safety levels have already been set in the USA and in Europe,
since the ones used for UHF extend up to 300 GHz (IEEE/ANSI and CENELEC recommendations
are the references), thus it is only left to researchers in this area to extend
their work to higher frequencies, by evaluating the SAR (the amount of power dissipated
by unit of mass) levels inside the head (or other part of the human body very
near the radiating system), from which maximum transmitter powers will be established.
However, this may not be as straightforward as it seems, since the calculation
of SAR is usually done by solving integral or differential equations via numerical
methods (method of moments or finite difference), which require models of the
head made of small elements (cubes for example) with dimensions of the order of
a tenth of the wavelength; this requires already powerful computer resources (in
memory and CPU time) for frequencies in the high UHF band, and may limit the possibility
of analyzing frequencies that are much higher than UHF. On the other hand, the
higher the frequency, the smaller the penetration of radio waves into the human
body, hence making possible to have models of only some deep. This is really an
area for further research.
centimeter