RF ANTENNAS!

11.11.2009

ANNEX -

The largest mobile phone base-stations transmit at approximately 100 W (rms

mounted on masts. Lower power levels are used in the antennas that are mounted on roof tops/ building

walls (typically 1-20 W). The main transmission RF beams are narrow in the vertical crosssection

and targeted towards the horizon or some degrees below it. Hence, they are not transmitting

directly towards people living underneath or in the vicinity of the source (as in Fig. 1). Contractors

are instructed to install base-stations in places inaccessible from e.g., ventilation windows or balconies.

Moreover, high power RF antennas should not be pointed directly towards any neighbouring

house, even though the safety distance in the main beam is only a few meters. If these precaution

measures are followed, the exposure to the general public caused by such base-stations is very low.

Measurements have been conducted in upper floor apartments with base-station antenna either directly

above or on the roof of a neighbouring house: typical power densities have been 0.1–10

mW/m

power densities at street level have been lower, typically well below 0.1 mW/m

given in ICNIRP recommendation is 4 500–10 000 mW/m

The TV- and radio broadcasts as well as GSM base-station networks cover most areas of the Nordic

countries. According to measurement campaigns, these technologies produce the highest background

RF fields in the environment. However, measured values are still typically only 1/10000–1/1000 of

the reference levels for power density given in the ICNIRP exposure limits. Many other technologies,

such as TETRA base-stations might cause power densities that are in order of magnitude of 1/10000

of the reference levels, but only present in the proximity of such source. As the exposure level decreases

rapidly with increasing distance from the source, the exposure caused by small transmitters,

such as WLAN base stations, falls below the GSM base-station level if the distance is approximately

more than a meter, such that peoples overall exposure does not increase.

The rapid decrease in exposure from RF with increasing distance from the source also leads to the

fact that exposure from multiple sources seldom increases the overall exposure when compared to

exposure from one or two nearby sources. Moreover, old technologies such as AM-radio or analogous

TV broadcasts have been replaced by technologies capable of significantly more efficient use of

the radio spectrum. Therefore, the fast development of technologies utilising radio waves has not

lead to a similar fast increase of the exposure of the general public; indeed, according to a Swedish

measurement campaign the “background” RF field level has remained the same in the period 2001-

2007.

In addition to the radio transmitters mentioned here, there exist a large number of short range devices,

such as RFID (radio frequency identification) electronic article surveillance gates in e.g. libraries,

car keys, wireless thermometers and alarm systems in shops. The power output of these devices

is small and hence the additional exposure is expected to be very low. Various high power applications

such as radars and satellite links also exist, though according to measurement campaigns and

risk assessment studies (e.g. Mulige helseeffekter av yrkesmessig strålingseksponering fra radar,

2007; Riks-Radiumhospitalet HF 05.07)) these do not cause significant exposure to the general public

under normal conditions.

More information about background RF fields and exposure of the general public is available on the

internet sites of the Nordic radiation safety authorities. The data presented in this document is mainly

based on the results of a Swedish background field measurement campaign (reported in SSI rapport

2008:13) and a Finnish study (reported in STUK-TR 5 2008 in Finnish). Both reports are available

on the web.4*) and they are also2 with measured maximums in individual cases up to 50–250 mW/m2. Measured outdoor2. The reference value2, depending on the frequency.