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 a wide range of applications. RFH machines use high power radio waves in applications such as wood glue drying, plastic welding and sealing, textile dye fixing, textile drying and food processing.

Introduction

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 TV and radio communication 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.

The legal position

RFH machines are not wireless telegraphy apparatus as defined in the Wireless Telegraphy Act 2006 (WT Act 2006), so they are not subject to licensing. However, Section 54 of the WT Act 2006 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 after 28 October 1992 must also comply with the requirements of the Electromagnetic Compatibility Regulations 2005 (SI 2005/281) 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 location so far as nearby radio installations are concerned.

Limits for electromagnetic radiation from RFH machines are 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:

Table 1
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:

Table 2
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 term "Unrestricted" applies to the fundamental and all frequency components falling within the designated band.
The distance between the measuring aerial and the nearest point on the boundary of the premises where the RFH equipment is located shall be not more than 100 metres for frequencies 0.15 and 30 MHz and 30 metres for higher frequencies up to 1GHz.
Note that “Terminal Voltage” shall be measured on the electrical supply line terminals of the RFH equipment at any convenient point within the user's premises, but as close to the boundary as practicable.
Table 2 gives limits for radiation at different frequency ranges and for frequencies other than those specified in Table 1. It is current practice to give field strength in units of dBµ V/m. However the unit of microvolt per metre is given here as that is the unit used in the legislation.

What to do about interference

Interference can sometimes been cured by re-locating RFH machines, improving screening and earthing or, more often, by minor changes in the fundamental 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 another radio user.

If RFH equipment is causing interference, Ofcom may be called on to investigate and take measurements to confirm whether it is out of specification. If that is the case and the user does not take steps to suppress emissions to an acceptable level, Ofcom may have to initiate action that could lead to users having to cease using the equipment. If apparatus is found to be in breach of the 1971 Regulations, users should contact their service dealer or the manufacturer of the machine for a solution.

Penalties

Continued operation of RFH equipment in contravention of the 1971 Regulations can result in an enforcement notice being served by Ofcom under Section 55 of the WT Act 2006 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 WT Act 2006.

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.

For more information please contact:

Ofcom Contact Centre
Riverside House
2a Southwark Bridge Road
London
SE1 9HA
Tel: 0300 123 3333
Fax: 0300 123 0811
Email: contact@ofcom.org.uk