Electromagnetic radiation consists of waves, the majority of which are invisible. Only one small part of this type of radiation can be identified by the human eye, and that is the visible light which produces the various colors of the rainbow.

Diagram 1 - Electromagnetic Spectrum

Electromagnetic fields are generated across a wide spectrum of frequencies (known as the electromagnetic spectrum) which is divided into specific frequency bands or zones (Diagram 1). The frequency spectrum includes ionizing and non-ionizing radiation. Ionizing radiation has a frequency higher than visible light. It has a smaller wavelength and carries high quantities of energy.

Ionizing radiation includes cosmic radiation, X-rays and radioactive α-decay, β-decay and γ-rays. It is called "ionizing" because it causes matter to ionize, in other words -to use scientific terminology- the photon of the radiation carries so much energy that it can remove an electron from an atom of matter. This radiation can cause immediate harm to biological matter, particularly cell DNA.

The sources of electromagnetic fields, which we are daily exposed to (radio waves, microwaves, electricity) have high wavelengths and low frequencies. They cannot cause ionization, because they carry low quantities of energy. They cannot break the chemical bonds in cell molecules.

Electromagnetic fields generated by electricity cables and household appliances have exceptionally low frequencies reaching only 300 Hz max. Radio frequencies are located between 10 MHz and 300 GHz.

Electromagnetic fields are to be found everywhere in the environment and come from either natural or artificial sources. The Earth's electromagnetic field, sunlight, lightning, heartbeats, and the human central nervous system are all natural sources of electromagnetic fields. Artificial sources include household appliances (vacuum cleaners, microwaves, refrigerators, televisions, etc.), electricity cables, television and radio stations, mobile telephony base stations, radars and so on (Diagram 2).

Diagram 2 - Sources of Electromagnetic Radiation

It should be noted that the existence of electromagnetic fields causes electric current. Electromagnetic fields can be of high or low strength, continuous or short-term.

Electromagnetic fields are generated due to the difference in electric potential. The higher the potential difference, the stronger the electric field generated. The measurement unit of electric field strength is the Volt per meter (V/m).

Magnetic fields are generated wherever current flows. The higher the current intensity, the greater the strength of the magnetic field. When the current is turned off, the magnetic field drops away. An appliance, like a hairdryer for instance, produces a magnetic field only when plugged in and turned on. When the current is turned off, the magnetic field drops away. The measurement unit of magnetic field strength is the Ampere per meter (A/m).

The first practical application of electromagnetic waves in telecommunications dates back to 1895 and Marconi, while in the 1980s and 1990s they began to be used in mobile and satellite communications respectively.

In 1987 as part of the Single European Policy for Telecommunications the Global System for Mobile Communications (GSM) was set up and today operates in more than 210 countries worldwide.

The basic philosophy behind the GSM network is radio coverage of an area so that at any given moment it is possible to connect mobile phones to base stations. Each base station generates radio coverage cells across small geographical areas. For this reason GSM is also known as a cellular system (Diagram 1).

Diagram 3 - Example of cellular telecom system

The size of a cell depends on the expected number of mobile phone users in the area. In sparsely populated areas (rural areas for example) cells are larger with a diameter that may even exceed 35 km. On the contrary, in densely populated areas (such as large cities), cells are small and do not exceed a few hundred meters in diameter. This is because the use of mobile phones in large towns and cities is particularly common and widespread and so more base stations, each one covering a small area, are required to ensure coverage in any specific area. In such cases the operating power level for each base station is very low because otherwise the station would interfere with the operation of other nearby stations and lead to poor quality in communications. Consequently, the more base stations installed in an area the lower the operating power of each one.

Each time you dial a number, the telecom signal is transmitted via electromagnetic waves to the closest base station and from there to the telephone exchange of the recipient's mobile or land-based phone company before finally reaching the dialed number (through telephone lines or again electromagnetic waves, depending on whether it corresponds to a fixed or mobile phone).