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Interference From Radio Frequency Heating (RFH) Machines |
Radio Frequency Heating (RFH) machines belong to a class of apparatus known as Industrial, Scientific and Medical (ISM) equipment which is used in an wide range of applications. For example medical ISM equipment is used by medical practitioners to induce heat into injured parts of the body prior to therapy. The machines with which this information sheet is concerned use high power radio waves in applications such as wood glue drying, plastic welding and sealing, textile dye fixing, textile drying and food processing.
While RFH machines are very effective in doing the work for which they are designed, they must be carefully installed and maintained if they are not to cause interference to other radio users. The machines themselves should operate on frequencies set aside for them. These are set out in Column 1 of Table 1. However interference to radiocommunication systems can be caused by harmonics or multiples of the fundamental frequency. These fall in frequency bands that are assigned to a variety of radio users, including emergency services and aircraft communication systems. Limits are therefore set for the amount of radiation permitted outside of the designated RFH bands. These are shown in Table 2.
RFH machines are not wireless telegraphy apparatus as defined in the Wireless Telegraphy Act 1949 (1949 Act). While we cannot licence such apparatus, Section 10 of the 1949 Act provides for regulation of non-wireless telegraphy apparatus which causes undue interference to authorised radio services.
The limits and measurement methods for RFH machines are given in the Wireless Telegraphy (Control of Interference from Radio-Frequency Heating Apparatus) Regulations 1971 (SI 1971/1675) (the 1971 Regulations) which specifies limits within which RFH equipment shall operate. Table 1 also gives the levels for protected services in the UK as set out in Schedule 1 of the 1971 Regulations.
While all RFH machines in use have had to comply with the 1971 Regulations since their inception and need to continue to do so, machines first supplied or taken into service in the European Community must now conform to the requirements of the Electromagnetic Compatibility Regulations 1992 (SI 1992/2372) as amended - the EMC Regulations. If a manufacturer chooses to comply through the Standards route, the appropriate standard is BS EN 55011, but a manufacturer is free to choose the Technical Construction file route if they so wish. There are differences between the 1971 Regulations (based on BS4809 and BS EN/55011), but these are marginal and relate essentially to the measurement method. The manufacturer or supplier of a RFH machine should ensure that it is supplied and taken into service in accordance with the EMC Regulations and other requirements and should have regard to its siting so far as nearby radio installations are concerned. However, interference problems may nevertheless still occur.
Limits for electromagnetic radiation from RFH machines as set out in Schedule 1 to the Wireless Telegraphy (Control of interference from Radio Frequency Heating Apparatus) Regulations, 1971 (SI 1971 No. 1675).The limits of field strength and terminal voltage shall be as follows:
| Col. 1 | Col. 2 | Col. 3 | Col. 4 | Col. 5 | |
|---|---|---|---|---|---|
| Frequency Range in MHz | Maximum radiated field strength in microvolts per metre | Maximum terminal voltage in microvolts | For protection of safety of life service | ||
| Exceeding | Not Exceeding | Maximum radiated field strength in microvolts per metre | Maximum terminal voltage in microvolts | ||
| 13.533 | 13.553 | 300,000 | 5,000,000 | ||
| 13.553 | 13.567 | unlimited | unlimited | ||
| 13.567 | 13.587 | 300,000 | 5,000,000 | ||
| 26.957 | 27.283 | unlimited | unlimited | ||
| 83.996 | 84.004 | 3,000,000 | |||
| 167.996 | 168.008 | 3,000,000 | |||
| 886 | 906 | 1,000,000 | |||
For all other frequencies in the ranges specified below, the limits of field strength and terminal voltage shall be as follows:
| Col. 1 | Col. 2 | Col. 3 | Col. 4 | Col. 5 | |
|---|---|---|---|---|---|
| 0.15 | 0.2 | 50 | 3,000 | 15 | 1,000 |
| 0.2 | 0.285 | 50 | 2,000 | 15 | 650 |
| 0.285 | 0.49 | 250 | 2,000 | 80 | 650 |
| 0.49 | 0.5 | 50 | 2,000 | 15 | 650 |
| 0.5 | 1.605 | 50 | 1,000 | 15 | 350 |
| 1.605 | 3.95 | 250 | 1,000 | 80 | 350 |
| 3.95 | 30.00 | 50 | 1,000 | 15 | 350 |
| 30.00 | 470.00 | 30 | 10 | ||
| 470.00 | 1,000.00 | 100 | 35 | ||
The National Radiological Protection Board (NRPB) has issued guidelines on restricting human exposure to electromagnetic fields and radiation (Documents of the NRPB, Volume 4, No. 5 1993) which include the fields emitted by ISM machines. For advice on the Health and Safety issues, the NRPB can be contacted on Tel: 01235 831600.
Interference has sometimes been cured by resiting RFH machines, improving screening and earthing or, more often, by minor changes in the fundamental frequency that have resulted in the interference-causing harmonic being moved away from the victim frequency. However, as the spectrum has become more crowded, retuning has become less and less viable: shifting the fundamental frequency and thus the harmonic may simply transfer the interference to some other user. The Radiocommunications Agency (Agency) has undertaken a research project to assist in the identification of the most cost-effective solution for any problem machine. The results are on the website.
Where RFH equipment is causing interference, those responsible who do not ensure that the emissions are suppressed to an acceptable level may find that they have to cease using the equipment. Agency staff may advise you that they believe your machine is out of specification or they may take measurements to show that this is the case. Initially where apparatus is found to be operated in breach of the 1971 Regulations, you should contact your service dealer or the manufacturer of the machine who may have come across your problems before and who may offer a solution. If they are unable to help, the British National Committee for Electroheat (BNCE) (020 7963 5700, 30 Millbank, London SW1P 4RD) may also be of assistance, as may the Federation of High Frequency Welders (FHFW) (01364 653057, 66 East Street, Ashburton, Devon TQ13 7AX). If after you have consulted with the manufacturer and the BNCE or FHFW you are still unable to identify a solution, the Agency's mobile EMC team can be consulted on a commercial basis to take supporting measurements. The RA may however be unable to carry out measurements on a commercial basis due to other priorities. Further information on this service can be obtained by contacting the team on: 01462 428543 or 428500.
Continued operation of the RFH equipment in contravention of the 1971 Regulations can result in an enforcement notice being served by the Agency under Section 11 of the 1949 Act requiring that use of the equipment ceases or is modified in its operation within 28 days from the date of notice. Where the equipment is causing interference to a safety of life service, for example the police, fire, ambulance, coastguard and air traffic control, the notice has immediate effect. Failure to comply with an enforcement notice constitutes a summary offence under the 1949 Act. An appeal may be made against such enforcement notice to a tribunal established under the 1949 Act.
The maximum penalty on conviction, where there is a risk of safety of life, is a fine of £5,000 and/or three months' imprisonment. If the interference is to a non-safety of life service the maximum penalty is a £1,000 fine.
Further assistance and information on this information sheet can be obtained by telephoning the Agency on 020 7211 0461.
The Radiocommunications Agency has produced a range of Information Sheets, General Publications, Licence Application Forms and Guidance Notes, concerning the use of the radio spectrum. These publications, unless otherwise stated, are available free of charge on a single copy basis and may be obtained from:
The Library and Information Service |
Tel: 020 7211 0502/0505
Fax: 020 7211 0507
e-mail: library@ra.gsi.gov.uk
RA 340 (Rev 2)
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