How Do Mobile Phones Work?
from Electromagnetic Fields of Influence MAE-WAN HO / The Institute of Science in Society (ISIS) 15dec04
Mobile telephony is based on radio communication between a portable handset and the nearest base-station. Every base-station serves a ‘cell’, varying in radius from hundreds of metres in densely populated areas to kilometres in rural areas, and is connected both to the conventional landline telephone network, and by tightly focused microwave links to neighbouring stations. As the mobile-phone user moves from cell to cell, the call is transferred from one base-station to the next without interruption.
The radio communication depends on microwaves at 900 or 1800 megahertz (MHz) (a million cycles per second) to carry voice information via small modulations of the wave’s frequency. A base-station antenna typically radiates 60W and a handset between 1 and 2 W (peak). The antenna of a handset radiates equally in all directions, but a base-station produces a beam that is much more directional. In addition, the stations have subsidiary beams called side-lobes, into which a small fraction of the emitted power is channelled. Unlike the main beam, the side-lobes are located in the immediate vicinity of the mast, and, despite their low power, the power density can be comparable with that of the main beam much further away from the mast. At 150 to 200m, the power density in the main beam near the ground level is typically tenths of microWatt/cm2.
A handset in operation also has a low-frequency magnetic field associated, not with the emitted microwaves, but with surges of electric current from the battery that’s necessary to implement ‘time division multiple access’, the system used to increase the number of people who can simultaneously communicate with the base-station. Every communication channel has 8 time slots (thus the average power of a handset is 1/8 of the peak values, ie, beween 0.125 and 0.25W), which are transmitted as 576 microsecond bursts. Together, the 8 slots define a frame, the repetition of which is 217 Hz. The frames transmitted by both handsets and base-stations are groups into ‘multi-frames’ of 25 by the absence of every 26th frame. This results in an additional low frequency pulsing of the signal at 8.34Hz, which, unlike that at 217 Hz, is unaffected by call density, and is thus a permanent feature of the emission. With handsets that have an energy-saving discontinuous transmission mode (DTX), there is an even lower frequency pulsing at 2 Hz, which occurs when the user is listening but not speaking.
Thus, the fields to which users are exposed can be quite complex.