FAQ: Frequently asked questions and answers
The fieldSENSE personal RF monitors were designed to specifically focus on the needs of RF workers working mainly on mobile telecommunications base transmitter stations, and hence the operating frequency range of 380 MHz to 2.7 GHz.
Often policy makers mandate the use of a "DC-daylight" safety monitor without giving any thought to the practicality of this. Firstly there is NO monitor able to detect all sources of radiation across the spectrum, even high end spectrum analysers are limited, not to mention cumbersome. In focusing on a specific part of the spectrum one is able to produce an affordable, robust and easily operated hand held device, which is then within reach of those who need it.
An exclusion zone emanating from a transmitting antenna is the physical area either partially or completely enclosing the antenna where the power density [W/m2] exceeds the relevant guidelines [ICNIRP, NCRP- FCC, IEEE etc.]. The shape of these zones is determined by the type of antenna [omnidirectional, sector, parabolic, folded dipole array etc.] The size of the zone is determined by the amount of power being transmitted by the antenna. Most modern communications systems such as GSM and WCDMA have varying power levels depending on the amount of traffic on the site. This means that the exclusion zones are actually changing size during operation, and may for instance be negligible in off peak hours.
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]. 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.
Microwave links are commonly used as the communications backbone of a network and are not designed to service a given area. As such they utilise high gain parabolic reflector or panel antennas which are pointed directly at each other over a given distance. Due to the gain of such antennas ranging from 20 to 60 dBi often very low power is needed to be able to establish a communication link. Common powers ranging from 21-30dBm (0.125 to 1W) are used. Exclusion zones for parabolic antennas are calculated according to the ETSI TR 102 457 formulas and in general there are no occupational exclusion zones for the 21-24dBm cases. The physically smaller dishes exhibit exclusion zones at lower power levels due to the smaller projected area resulting in a higher power density, S [W/m2].
In terms of safety these systems are seldom if ever installed where any RF worker or member of the public would need access. The reason for this is that the link would be interrupted if any person stands in front of the dish. In all cases the exclusion zones are only in front of the antenna with no concern over back or side lobes.
A typical 18GHz example is shown below with both a 1ft and 4ft parabolic dish with transmit power ranging from 21 dBm to 30 dBm. ICNIRP Occupational exclusion zones shown in red, public in yellow
Body worn RF monitors suffer from large inaccuracies due to the effect of the body in loading the probe itself, and from the user shielding the device from the RF source. The loading can be accounted for, but shielding cannot. Errors in the order of 6dB are easily encountered which can place the user under a misconception of being safe when actual exposure levels exceed the guidelines. To ensure that the readings are accurate and neither influenced by loading or shielding the fieldSENSE was designed to be held away from the body as shown below.
Due to the rapid fall off of the field intensity around transmitting antennas it is often only when in close proximity to such antennas that a risk of overexposure exists, and due to this the fieldSENSE is placed in Measure mode to assess an actual area where work is to be performed. Following this it is placed in Monitor mode and mounted in the area where work is being performed where it continually monitors the radiation level, giving an audible warning should the levels approach overexposure.
Data logging is often a feature required for research purposes where one tries to ascertain the levels of radiation an individual is exposed to over a specific period of time. An RF worker's exposure will range from almost 100% of the relevant standard down to the typical levels measured in public (0.1% and lower), a device for research purposes needs to have a tremendous dynamic range.
The fieldSENSE focuses primarily on the safety aspects and for this reason will only begin reporting measured field values via the LED display as one approaches a transmitting antenna. Logging is not included as this feature would have made the device considerably more expensive, and the audio visual cues were seen as the most important aspects for the RF workers safety.