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How base stations work When a user of a wireless phone makes a call, it is transmitted to the nearest base station, which receives and transmits radio signals in its area - thus acting as a two-way radio. The area covered by a base station is called a cell, which is why wireless phones are sometimes called cell or cellular phones. The largest cells are known as macrocells; smaller cells, particularly in urban areas, are called microcells or picocells. The number of cells varies in different areas, depending on the volume of use. Areas with a high volume of mobile phone use will have more cells. As the phone user moves around, the radio signal can be switched from one cell to another, maintaining a good connection. The call is connected with the local phone network and delivered by phone lines, using a ground based antenna, or when the call is going to another wireless phone, by radio signals. Each base station covers a limited radius. Beyond that, the power density (strength) is too small for a mobile phone to work. When this happens, the phone is automatically switched to a closer base station.
As you travel the signal is automatically switched to a closer base station (with a higher power density) The
power level generated by a cellular phone to maintain a good connection
depends on the distance from the base station; the greater the distance,
the more power is needed. Cellular phones automatically step down to
the lowest power level that maintains communication with the base station.
Since there may be interference from neighbouring channels in the cell,
or from physical obstacles, the amount of power needed may vary within
a single telephone call. Base stations produce radiofrequency radiation (RFR), which is non-ionizing. This means that it does not break chemical bonds and is very different from x-rays. For more information on this, see the FAQ # 5. Antennas from base stations transmit radio waves that travel as fast as the speed of light. A good analogy is the beam of light coming from a flashlight. As with the beam of the flashlight, antenna beam width can be limited in both the vertical and horizontal planes. The beam is tilted a few degrees in the vertical direction, so that it intersects the ground a considerable distance away from the antenna tower - usually between 50 and 200 metres from the foot of the mast. Base stations typically transmit using beam widths roughly seven degrees in the vertical plane in elevation, and about 120 degrees or one third of a circle, in the horizontal plane. Outside this small beam the radiation is typically 100 to 1000 times less. Usually towers have several antennas transmitting signals in different directions.
Base stations are typically placed close together and transmit around 60 watts of power The total power output of a typical mobile phone base station depends on the type of antenna, the number of analog and digital channels that operate at a given time and at their maximum strength, and the antenna gain which gives the signals direction and strength. The power output is less when the base station is close to the phone user. Hence, base stations are being placed closer together, to make it simpler for a phone and a base station to communicate reliably. The power output is measured in watts, and is about 1,000 times less than that produced by a television transmitter. Maximum powers radiated from the antenna at the top of the mast are in the region of 60 watts. However, the situation is more complicated when there are multiple antennas at the same site. It is even more complicated when base stations are located at the same site as television or radio transmitters. In summary, factors that influence how much RFR an individual may be exposed to include:
It
is obvious from the above that the amount of exposure that an individual
experiences from a base station cannot be estimated simply by the distance
from the transmitter.RF field exposures, when measured on the ground
near a base station, have always been very low. In Canada a survey of
five Vancouver schools, conducted in response to parental concerns about
safety, showed levels of radiofrequency radiation many times below established
safety limits. The maximum level at one school with a PCS antenna across
the street was 1620 µW/m², whereas the Canadian safety limit is 10 million
µW/m². At a second school with an analogue base station on the roof,
the maximum level was 25,600 µW/m². At a third, also with an analogue
station nearby, the maximum level was 2,250 µW/m². The safety limit
for these two situations is 5.9 million µW/m². Other studies in Ontario,
Canada, in response to concerns about analogue base stations had even
lower measurements - 10 µW/m² at one site, and 0.2 µW/m² at another.
Regulation of base stations Guidelines for maximum exposure of the public to RFR have been formed by various agencies. In Canada, the guidelines are called Safety Code 6, and they use principles similar to those adopted in other countries. They are based on the minimum level of RFR that has been shown to produce biological effects. A 50-fold safety factor is then applied. Hence, the maximum RFR allowed is 2% of the level associated with biological effects. Each country has its own regulatory system. In Canada, Industry Canada requires that all operators of radiofrequency transmitters ensure that electromagnetic fields produced by installations do not exceed maximum levels contained in Safety Code 6. In the USA the Federal Communications Commission (FCC) performs this function, and in the UK it is the National Radiological Protection Board (NRPB). ). For further information about the EMF regulations in your country, see the web site of the World Health Organization International EMF Project. This is found at www.who.int/peh-emf . Follow the links to the EMF Standards Worldwide Database. Independent reviews of the risks from base stations. Independent expert groups around the world have stated that there is no risk to the general public from base stations. The World Health Organization, for example, stated: "None of the recent reviews have concluded that exposure to the radiofrequency fields from mobile phones or their base stations causes any adverse health consequence". An Expert Panel of the Royal Society of Canada said "it appears that exposure of the public to radiofrequency fields emitted from wireless telecommunication base station transmitters is of sufficiently low intensity that biological or adverse health effects are not anticipated". The Independent Expert Group on Mobile Phones in the UK said
Current
Research References:
Schuz J, Mann S (2000): A discussion of potential exposure metrics for
use in epidemiological studies on human exposure to radiowaves from
mobile phone base stations. Journal of Exposure Analysis and Environmental
Epidemiology 10:600-605. |
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