Electromagnetic compatibility is the ability of a piece of equipment to function properly in its own electromagnetic environment without disturbing other equipment nearby. We use increasing numbers of electronic products – including phones, microwave ovens, TVs, laptops, car alarms and toys – so the radio spectrum is becoming more crowded and radio frequencies allocated to one type of product are getting closer to adjacent frequencies used by others. Nearly all electronic equipment is susceptible to interference; for example TV reception can be affected by microwave ovens, passing aircraft, hairdryers, or automatic garage doors.
Laboratory and clinical tests have found that digital wireless phones might interfere under certain conditions with some pacemakers and hearing aids. Often, there are steps users can take to minimise or prevent interference, such as keeping an operating phone six inches (15 cm) from an implanted pacemaker or adopting other measures to accommodate the use of hearing aids. Users should follow the advice provided by the manufacturers of medical equipment.
Since 2000 a European Directive known as the RTTE has required equipment to be constructed so it does not generate a disturbance of a magnitude which would affect equipment working at adjacent frequencies, and that it is itself protected from interference. The CE mark on equipment certifies that it complies with the directive. Regulations require the transmitting characteristics of electromagnetic equipment to be confined to specified ranges or ‘masks’ within the spectrum. Outside these ranges power levels must be kept to a minimum. All TETRA equipment complies fully and operates squarely within its allocated frequency bands.
However, equipment that pre-dates the directive, or does not comply with it – for example older car alarms or TV sets or radio receivers which are not required to comply – may cause or be susceptible to interference. An example would be when a car alarm is triggered by the operation of some nearby equipment which is operating properly within its frequency band. Once a potential problem is identified it can usually be remedied or alleviated; some equipment, not including radio transmitters, can have suppression circuits fitted.
TV and Radio Interference
When planning TETRA services considerable care is taken in siting transmitters to avoid interference. Occasionally, TV interference occurs in areas of poor reception or with older sets which pre-date the regulations when boosters or filters may need to be fitted. Regulators, like the Radio & Television Investigation Service in the UK, advise householders and aerial installers on the steps they can take to ensure that domestic TV and radio receivers perform to an adequate standard with minimal interference.
Many hospitals insist that all equipment which transmits radio signals should be switched off inside clinical areas in case there is interference with sensitive medical equipment. In this case TETRA handsets are no different from mobile phones, although users benefit from the transmit-inhibit feature which allows handsets to receive communications but prevents transmission. This can be a useful feature for use in medical environments. Regulatory agencies like the UK’s Medicines and Healthcare Products Regulatory Agency (MHRA) accept that communications equipment can be essential in hospitals but acknowledge the risk of interference. The MHRA does not recommend a blanket ban on the use of mobile phones in hospitals, however, under certain circumstances, the electromagnetic interference from a mobile can affect the performance of some medical devices. See the relevant page of the MHRA web site for further information – click here (opens in a new window). Users should respect any local guidelines and should switch off or use transmit-inhibit mode in any areas where critical care or life support equipment may be in use.
Laboratory and clinical tests have found that digital wireless phones, which operate in a similar manner to TETRA, might interfere under certain conditions with some pacemakers and hearing aids. Users can take steps to minimise or prevent interference, such as keeping an operating handset the recommended minimum distance from an implanted pacemaker or adopting other measures to accommodate the use of hearing aids. Following US research in the mid-1990s, a minimum distance of six inches (15 cm) was recommended; however, more recent research in Austria, carried out during the period 2009-2011 with the aim of ensuring that citizens with pacemakers were not put at risk during any encounters with the emergency services whose personnel were using TETRA handsets, recommends a minimum distance of 30cm between an implanted pacemaker and a TETRA handset. The Austrian study’s findings are summarised in a short brochure which may be viewed here (opens in a new window). In general. users should always follow the advice provided by the manufacturers of medical equipment.