Cities In Motion 2 1.6.3
Antennae broadcasting in the long- and medium-wave bands tend tobe constructed as tall metal towers, with cables linking thetowers to each other and to the ground. Often, a singlelow-frequency (LF) or medium-frequency (MF) radiating structuremay involve several closely located towers that are fed in sucha way that a directional beam pattern is formed. Some towers areenergized and insulated from the ground, while others aregrounded and act as reflectors. Transmitters designed to providelocal radio services, e.g. around cities, use powers in therange of 100 W to 10 kW, while a small number of transmittersthat provide national services over large distances radiate upto a few hundred kilowatts (ICNIRP, 2009a).
Cities in Motion 2 1.6.3
Tracking radar is used in military systems to lock-on to andfollow targets such as aircraft and missiles. The antennae canrotate, execute a nodding motion, point in a fixed direction, orfollow a target. Targets are not expected to assist with beingtracked and may even be designed with stealth in mind and tosuppress the extent to which they reflect radar pulses. Hence,tracking radar systems generally involve higher powers thannavigation systems and use peak powers of up to severalmegawatts. Systems mostly operate between 2 and 8 GHz. Certaintracking radar systems can produce mean power densities > 100W/m2 at distances in excess of a kilometre, evenafter duty-cycle correction (ICNIRP, 2009a).
The most recognized effect of RF radiation in biological systems istissue heating. The absorption of RF-EMF energy by biological systemsgenerates an oscillating current that is transferred into molecularmotion of charged particles and water molecules, which are stronglydipolar and are the major component of biological tissues. Polarmolecules move to align themselves with the EMF to minimize thepotential energy of the dipoles. Absorption and resonant oscillations inpolar subgroups of macromolecules (e.g. proteins, DNA) are largelydamped by collisions with surrounding water molecules. Damping orfriction slows the motion of the oscillator. These collisions dispersethe energy of the RF signal into random molecular motion. Tissue heatingoccurs because the rotational motion of molecular dipoles is hindered bythe viscosity of water and interactions with other molecules, i.e. therotational energy is transferred to the surrounding aqueous environmentas heat. The magnitude of motion that results from the interaction ofpolar substances with electric fields is dependent on the strength andfrequency of the field. In addition, the actual increase in temperatureis dependent on the ability of the organism to thermoregulate. At highfrequencies where the orientation of dipoles cannot keep up with theoscillations of the field, the system behaves like a non-polar substance(Stuchly, 1979).
Using a mobile phone in areas of good reception (such as incities where mobile phone-base stations are close together) alsodecreases exposure as it allows the phone to transmit at reducedpower. Conversely, people using a phone in rural areas wheremobile-phone reception is poorer may receive higher RF exposure.This was one factor examined in a study of 512 subjects in 12countries who were asked to use GSM software-modified phones;the study, monitored date, time and duration of each call,frequency band and power output for a month (Vrijheid etal., 2009a). The main predictors ofpower output were the study location, the network, and theduration of the call, with shorter calls being associated withhigher power output. The measured power levels in GSM networkswere substantially higher than the average levels theoreticallyachievable, which has important implications for estimatingexposure in epidemiological studies. Rural location was only amajor factor in Sweden, where subjects were living in verysparsely populated areas; these results are consistent withthose of an earlier paper from Lönn et al. (2004)in Sweden, who reported that the highest power level was usedabout 50% of the time in the rural areas, but only about 25% ofthe time in urban areas. This highlights the problem ofidentifying genuinely sparsely populated rural areas where majordifferences in power output can be found. Another paper from theInterphone study reported an investigation of the effects ofparameters that were thought to influence the level of RF SAR inthe brain. Total cumulative specific energy was estimated, basedon data collected during the Interphone study, to assess therelative importance of the different factors and these resultswere used to develop an algorithm, which was tested on studysubjects in five countries (Cardis et al., 2011b). This studyfound that the type of phone with the highest mean totalspecific cumulative energy (TSCE) was AMPS800 (5165 J/kg),followed by D-AMPS800 (3946 J/kg), GSM800/900 (2452 J/kg),GSM1800 (4675 J/kg), CDMA1900 (1855 J/kg), and CDMA800 (164J/kg). The main determinants were communication system,frequency band, and number and duration of mobile-phone calls.The study also identified several uncertainties in relation toSAR estimation, including those related to spatial SARdistribution for each phone class, error in recall of phone use,and laterality and uncertainties about the most biologicallyrelevant dose metric. 041b061a72