A potential area of concern with SPEAR is the level of electromagnetic field to which people are exposed during operation. Field strength values are compared with the most up-to-date recommendations below. This indicates that the field strength values during operation are all well below the recommended safe limits for exposure.
The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has published guidelines  for the maximum levels of electromagnetic fields to which people should be exposed. In line with all recent work two levels of exposure are identified. The occupational limit applies to staff working in environments where exposure may occur whilst lower limits are appropriate to members of the general public in uncontrolled environment.
The UK  and other EU member counties are committed to adopting these standards by 2008 and other European countries are likely to adopt the ICNIRP recommendations in a similar time frame. The recommendations as they apply to the frequencies used by SPEAR are indicated in the table below for both occupational and general public exposure. For comparison the current recommendations published by the FCC  in the US are also included.Note that throughout this document the field strength values are the 'Equivelant plane wave power density, Seq', expressed in the SI unit Watts per metre squared (W/m2) and NOT the mW/cm2 unit commonly used in the US. ( 10 W/m2 = 1 mW/cm2 )
|Equivalent plane wave power density Seq (W/m2)|
|Frequency||ICNIPR||US FCC||ICNIPR||US FCC|
The higher recommended values adopted in the US reflect the fact that these are current recommendations in the US whilst those from the ICNIPR are to be adopted by 2008. It is clear that the ICNIPR recommendations are the more appropriate when considering potential hazards in a European context.
The measurements in the table below were made on the 6th of October 2004 when all 48 transmitters and antennas were operating at 2kW with the beam direction aligned to the magnetic field. The operating frequency was 4.45 MHz and measurements are averages over 4 minute periods during which the system was transmitting continuously i.e. CW operation. The measurements were made with a calibrated total field strength meter with an isotropic sensor mounted 1.5 metres above the ground. The corrected readings below are the measured values times the appropriate correction factor for the frequency range 4 to 6 MHz.
|Location||Corrected reading||Safety Factor|
|(see diagram)||W/m2||Occupational||General Public|
Locations 1 to 5 are positions within the antenna field
Location 6 is the field immediately outside the control containers and is relevant for operating staff
Location 7 is a 'hot spot' within the antenna field where reflection from containers cause a localised concentration of field strength
Measurements made at locations further from the antenna array were too small to be resolved by the meter
The safety factors in the table above indicate how many times the actual field strengths are below the recommended safe limits for both occupational and general public exposure.
The distributed nature of the SPEAR design spreads the transmitter power over multiple antennas instead of concentrating it in a single antenna. This produces a lower field on the ground without affecting the desired field overhead. The horizontal dipole antennas radiate energy vertically and the individual elements are phased to form a vertical beam with little energy being emitted in the horizontal direction.
The electric and magnetic near field components of the full 6x6 SPEAR antenna array, driven with a total power of 288 kW (72 x 4kW) have been calculated with the aid of a mathematical model. The results have been compared with the recommended values for public exposure to electromagnetic radiation as established by the Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines for limiting exposure in time-varying electric, magnetic, and electromagnetic fields  and the ANSI/IEEE C95.1-1992 recommendations. These recommendations are endorsed by the United Nations (UN) World Health Organisation (WHO) and form the basis of most national legislation including that in EU countries. Account has also been taken of the ICNIRP Statement "Use of the ICNIRP emf guidelines"  and the safety factors recommended therein.
The field strength in the area immediately below the array, up to a height of 4 metres (2m of snow + 2m person). The modelled RF field strength in the area directly below the array is at least 10 times below the maximum level recommended by ANSI/IEEE C95.1-1992 for exposure of the general population in an uncontrolled environment.
The area extending out to 20 metres beyond the array edge has also been modelled. At a distance of 20 metres from the edge of the array the RF field strength has reduced to at least 20 times below the level recommended by ANSI/IEEE C95.1-1992.
These results provide a strong indication that field strength in the proximity of the antenna array will be well within accepted safe limits. After construction, and at regular intervals thereafter, measurements will be carried out around the site to ensure that the level of electromagnetic radiation encountered in practice agree with the mathematically modelled values and are within the recommended limits.
The EISCAT Tromsø heater, which is similar to the SPEAR configuration but generates ten (10) times more power, has operated for many years with no safety problems. Since the field produced by SPEAR will be only 10% of that at Tromsø, we can be confident that it will present no risk to human safety.
More information on Aircraft Safety, and Potential interference from SPEAR
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