Cellular, Mobile Phone Towers, Antennas and Sites & RF Radiation
These notes provide a brief explanation of the technology used by mobile phones and sites. It is intended to provide the reader with a high-level understanding of the issues associated with non-ionizing radiation in order that they may appreciate the levels of risk associated with the use of mobile phones and proximity to mobile sites.
How do mobile phones work?
Mobile phones work by connecting to a base station over a low power radio link whenever a call is made or received.
What is a base station?
Base stations are the interface between the mobile phone and the rest of the telephone network. They contain multi-channel two-way radios to communicate with mobile phones and a cable or radio link to communicate with the telephone network. Whenever you make or receive a phone call using a mobile phone, the phone is connected to a base station. All mobile phone towers have an associated base station which contains the radio equipment.
Are mobile phones dangerous?
Interested parties have carried out numerous studies to find out the effects of mobile phones on people and while some recommend limiting the total amount of time spent on calls no reputable body has said that people should stop using them.
People have heard that phones emit radio frequency (RF) radiation and believe this to be dangerous. RF radiation is just another name for radio waves.
Isn't radiation dangerous?
There are two types of radiation we can be exposed to: ionizing and non-ionizing radiation. RF radiation is non-ionizing radiation (NIR). We are constantly exposed to non-ionizing radiation from natural as well as man-made sources.
Non-ionizing radiation does not have sufficient energy to cause ionization in biological tissues. A sufficiently high exposure causes tissue heating. A microwave oven works by using very high levels of non-ionizing radiation to heat food. The level of non-ionizing radiation emitted by mobile phone antennas is thousands of times less than that emitted in a microwave oven and is generated over a larger area.
Ionizing radiation on the other hand causes electrons to be stripped from atoms and molecules. Exposure to it can cause damage to biological tissues. X-rays are an example of ionizing radiation, which is why exposure to them is strictly controlled in hospitals and in industry. Mobile phones and base-stations do not emit any ionizing radiation.
Both forms of radiation can occur naturally. A natural source of ionizing radiation is the radioactive decay of substances such as radon gas. Natural sources of non-ionizing radiation are the earths magnetic field, the sun and lightning. Visible light is a form of non-ionizing radiation.
How much non-ionizing radiation is emitted from a mobile phone?
When a mobile phone is connected to a base station, the communication takes place using a specific frequency. The frequencies are similar to broadcast frequencies, e.g. 98FM or FM104. The numbers 98 and 104 correspond to the frequency in megahertz that that station broadcasts on. The frequencies used for GSM mobile phones are either 900 or 1800 megahertz.
The maximum output power from a mobile phone is two watts. However, in reality the level used during normal operation is much lower for two reasons.
First by using sophisticated speech compression techniques, each frequency is shared by eight other mobile phones so each phone only transmits for 1/8 of the time while a call is active. Therefore the maximum average output when the phone is at full output power is only 0.25 watts. Similar compression techniques are used by digital television to increase the number of television channels available.
The second reason is that the phone reduces its output power if it is close to a base station, as less power is required to communicate over shorter distances. This reduces the drain on the phones battery and increases the time between charges. As the number of base stations increase the mobile phones operate at a reduced output power more frequently.
Examples of output power of other common RF transmitters to which people are exposed include 0.1 watt for radio control models and 0.5 watt for the "walkie-talkies" used in construction sites.
Are base station towers and masts dangerous?
Mobile phone base stations transmit at a higher power level than mobile phones. The level depends on the number of calls being handled by the base station. However to ensure good communications, the antennas are installed on rooftops or on towers, well away from people.
Guidelines have been drawn up to limit the amount of RF radiation emitted from base stations to which both the public and RF workers (e.g. telecommunications engineers) can be exposed. Standing a few meters directly in front of a base station's antennas may cause RF exposure to exceed the general public guidelines. However sites are designed so that members of the general public would never be this close to the antennas.
Where there is a possibility that exposure could exceed the guidelines, exclusion zones are defined around antennae. Depending on the type of base station, the exclusion zones can vary from a few centimeters to a few meters. The exclusion zone would never extend to ground level.
Unlike mobile phone antennae most of the antennae used at base stations are directional, i.e. the RF is radiated from the antenna in a similar manner to the way light is radiated from a torch. Most of the radiation is emitted in one direction with very little radiation emitted above, below or behind the antenna. The tower or mast to which an antenna is fixed does not itself radiate any RF.
Directional antennas located at base stations are usually either rectangular or circular and are usually installed at least 10 meters above ground level. The rectangular antennas, called sector antennas, communicate with mobile phones and the circular antennas are used to link base stations to the rest of the telephone network.
The circular antennas, commonly called "dishes", are highly directional point-to-point links with a very tightly focused radiation pattern pointing at a remote dish. They are always positioned so that people cannot inadvertently stand in or otherwise break the beam, see fig.1.
Fig.1 Radiation Patterns of Link Dishes and Sector Antennas
Note dangerous exposures in penthouses, rooftops and upper levels of high building.
The sector antennas are usually tilted slightly downwards (at times inadvertly or due to shifting directed into habitable areas) , both to increase the signal quality at ground level and to prevent interference between base stations. If a sector antenna is mounted on a rooftop or tower 10 meters above the ground and is tilted 8 degrees downwards, the center of the beam reaches ground level around 70 meters from the rooftop or tower. At this distance RF radiation from the base station does not present a health hazard. In fact the exposure should be less than 1% of the level permitted by the guidelines
What are the effects of excessive exposure to RF radiation?
Excessive exposure to RF radiation causes an increase in body temperature. We all have a natural thermoregulatory system that keeps our core body temperature within very tight limits. When our core temperature rises, more blood flows near the skin surface and we perspire to reduce our temperature. When our core temperature drops blood flow near the surface is reduced to conserve heat and we shiver to generate heat.
Small increases in temperature resulting from overexposure to RF radiation can be controlled by the body's thermoregulatory system. However, very large exposures can overwhelm it. Exposure levels capable of overwhelming the thermoregulatory system should never be encountered as a result of radiation from mobile phones or base stations.
Can mobile phones cause cancer?
Many bodies have conducted studies to examine the possibility of a link between RF radiation and cancer and none have shown a conclusive link. Studies of radar workers in the aircraft industry and in the US military since the Second World War have found no evidence of increased mortality from any cause. The consensus of scientific opinion is that exposure to RF below the accepted guidelines does not pose a health risk. Mobile phones expose you to a fraction of the level defined in the accepted guidelines.
Who decides what the exposure limits are?
An independent scientific organization - the International Commission on Non-Ionizing Radiation (ICNIRP) - was founded in 1992. ICNIRP is the formally recognized non-governmental organization in non-ionizing radiation protection for the World Health Organization (WHO), the International Labour Organization (ILO), and the European Union (EU). They have published stringent guidelines for limiting RF radiation exposure that will provide protection against known health effects. Their guidelines cover the entire frequency spectrum and not just mobile phone frequencies.
All broadcasters and mobile phone service providers in Ireland have an obligation as part of their license agreement to operate within the current ICNIRP guidelines.
How do ICNIRP know what is safe?
For the past few decades' scientists have been researching the biological effects of RF radiation on animals and on humans. The ICNIRP guidelines have been formulated using the knowledge gained as a result of this research. The limits for general public exposure set in their guidelines are between 20 and 50 times lower than the threshold where effects begin to be noticed.
What are the conclusions of other organizations that have studied the RF radiation associated with mobile phones?
"The balance of evidence to date suggests that exposures to RF radiation below ICNIRP guidelines do not cause adverse health effects to the general population."
'We conclude that the balance of evidence indicates that there is no general risk to the health of people living near to base stations on the basis that exposures are expected to be small fractions of guidelines."
The United Kingdom Independent Expert Group Report (also known as the Stewart Report) 2000
"None of the recent reviews have concluded that exposure to the RF fields from mobile phones or their base stations causes any adverse health consequence."
The World Health Organization (WHO) 2000
"Despite the rather limited epidemiological and experimental data available, NRPB concludes that the totality of the evidence available does not suggest that the use of mobile phones have any detrimental effect on human health."
U.K. National Radiological Protection Board (NRPB) 1999
"...the available science does not allow us to conclude that mobile phones are absolutely safe, or that they are unsafe. However, the available scientific evidence does not demonstrate any adverse health effects associated with the use of mobile phones."
U.S. Food and Drug Administration (FDA) 2001
Where can I find more information?
The International Commission on Non-Ionizing Radiation Protection (ICNIRP)
The National Radiological Protection Board
The World Health Organization