How many categories do FM transmitters have

FM transmitters generally fall into three categories, broadcast FM stations servicing a very wide physical area, low power FM stations covering smaller areas, and unlicensed transmitters.

Broadcast FM transmitters service very large areas. These antennas are accordingly mounted on tall masts with high transmit powers (e.g. 100kW ERP on a 450m mast). These higher powers lead to a considerable exclusion zone around the antennas which mandate that such sites need to be powered down when working near the actual antennas. However in many of these instances if there is a mast sharing agreement in place the mobile telecommunications antennas will be mounted much lower down on the mast as the cell size is naturally smaller. This allows an RF worker to work on or near such antennas without entering into the exclusion zones of the FM transmitter.

Smaller community radio stations or LPFM as referred to by the FCC for instance service smaller areas such as towns and as such use much less power. Here the antenna needs to be placed at 30m above the average terrain and an ERP of 100 W is permitted. A simulation of such a mast and antenna configuration (see picture below) shows the size of the exclusion zones in terms of the ICNIRP reference levels [red for occupational and yellow for general public][0]. Here it can be seen that the Occupational exclusion zone typically only encloses the antenna itself, and any nearby mounted antennas would most likely be outside of this area.

ICNIRP exclusion zones around a 30m mast mounted, 87MHz folded dipole antenna, with 100W EIRP

Unlicensed transmitters radiate very low power levels which make them inherently safe, and each country will control the permissible power level based on the exposure guidelines established. The European limits laid out by CEPT are 50nW ERP.

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