I recently did some EM field measurements in the vicinity of a GSM base
station. It is a typical urban base station, with sector antenna (gain
16-17dBi) and four 50W transmitters (minus some loses in the cables and
splitters). Urban transmitters are generally limited to 1kW ERP, so the
situation I measured is probably the worst case situation.
But:
The transmitted power is always less than the nominal power. Base station
transmitted power cannot be determined in a straightforward manner. It
depends on:
a) the number of active connections - the transmitted power rises with the
number of active connections.
b) the quality of the radio channel for each connection - transmitted power
per connection (channel) is adjusted to the minimum needed for wanted
quality - poor quality means more power transmitted.
Everything together is multiplied by the number of transmitters employed on
the base station. The one I measured had 3 transmitters (channels) plus BCCH
(control channel). The worst case is a fully loaded capacity, during the
working hours.
Even then, I measured:
1. antenna level, perpendicular to the main lobe direction, 1m from the
antenna: 10V/m
2. antenna level, direction straight back from the antenna, 1m from the
antenna: 4.5V/m
3. antenna level, any point on the terrace, up to 5m from the antenna: <3V/m
4. one floor under the antenna, directly under the antenna: <2V/m
(undetectable using this measuring equipment)
ICNIRP guidelines (900MHz):
PEL (Permitted Exposure Level) for general public: 41.25 V/m
PEL for workers: 90 V/m
The conclusion is that there is no danger, according to present standards or
guidelines. The question is whether those PELs will get lower in a few
years. Present limits are based on proved health effects, while lots of
people fear from health risks that are not proven or even considered in the
present guidelines (like carcinogenic effects).
I can mail more info in the form of article to those that are interested.
Best regards to the group,
Antonio
Antonio Sarolic
Faculty of Electrical Engineering and Computing
Dept. of Radiocommunications and Microwave Engineering
Unska 3, HR-10000 Zagreb
CROATIA
tel. +385 1 61 29 789, fax. +385 1 61 29 717
E-mail: antonio.saro...@fer.hr
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