Interference and Mobile Communications Equipment

By Anton Domone C.Eng. MIEE, Azdec Limited

Introduction

For some period of time congestion within the electromagnetic spectrum has given rise to both susceptibility and interference problems associated with mobile communications equipment. In more recent times the significant use of mobile telephones has brought the health and safety issues of electromagnetic radiation to the fore. Concerns have been raised for emissions over the whole spectrum from the low frequencies used for electrical power transmission to the higher frequencies of mobile phones and microwave ovens.

Emissions from mobile radio

Most mobile communications equipment employs radio which uses frequencies from tens of megahertz to several gigahertz. Due to the requirements of ever increasing number of users and the bandwidth requirements, operating frequencies have increased over the years. In order to divide the available bandwidth and to improve the quality of communications even further, digital encoding is now being extensively used.

Although the average level of many of the mobile communications equipment is low the peak power radiated can cause significant problems when radio signals are induced into other electronic equipment. Many sensitive electronic circuits, although not damaged by the induced voltages and currents, can temporarily give extraneous output signals. These signals can cause subsequent damage in control loops or can cause the shutdown of process plants.

Many operators in industry have banned the use of radio equipment in computer rooms, near input and output interface panels, within control centres or near to shutdown systems. Aircraft operators have discovered that the plethora of mobile radio devices can cause breakthrough on internal voice circuits and disrupt navigation systems.

The author has had experience of control loops on process plants going completely unstable and in one example significant damage to large sluice gates was narrowly avoided when the strain gauge amplifiers gave extraneous outputs when radios were used.

Radio emissions from electrical equipment

The requirement for power electronics where large currents are switched at relatively high speeds, using power transistors or gate turn-off thyristors, can cause significant electromagnetic fields. Applications using these techniques can have sufficient fields to cause local heating of control panels or associated metalwork, the interference of radio communications and, in the instance of inductive loop communications and leaky feeders, render the communications system to complete failure. Many areas including industry use equipment which radiate electromagnetic fields these include: surface ships and submarines, oil production platforms and sites, steel manufacture, textile production, power generation, broadcast transmission and many more.

Several examples have been experienced where an accumulation of electrical drives in ships and on oil production platforms has completely degraded the use of radio and inductive loop voice communications.

Infra red as a communications medium

In the current trend of ever increasing carrier frequencies to reduce range which allows topographic isolation and permits the increase available bandwidths, infra red offers short range communications without any of the emission or susceptibility problems of radio transmissions. When infra red is employed with digital encoding techniques and incorporated with a distributed cellular approach, areas of thousands of square metres can be covered with multi-channel full duplex mobile communications.

Digitally encoded infra red communications which, because of the use of low level invisible light, have no ocular or health hazard, do not radiate radio waves and are totally unaffected by high levels of electromagnetic radiation from electronic/electrical equipment.

The quality of the transmitted signals even in close proximity to equipment which is radiating high levels of electromagnetic noise is outstanding. The infra red systems can be safely used adjacent to the most sensitive electronic equipment without any adverse effect.

Advantages of Infra Red

The following list defines the advantages of infra red as a communication medium:

Short range predictable area of coverage, allows total security and the segregation of systems by distance.
No potential health hazard.
Unaffected by other electromagnet radiation in the spectrum up to and including several GHz.
Little or no electromagnetic radiation below several GHz.
No licence requirements.
Exceptionally clear communications.

Previously digital infra red systems have only been available to the military but now commercial version are also available.

Commercial Applications

The are a large number of applications for theses infra red systems where either there is a significant level of electrical interference or where the emission of radio waves is not permitted.

Locations where high levels of electromagnetic radiation exist are:

Public broadcast transmitters.
Industry using variable speed drives.
Ships with electrical propulsion.
Power generation.
Radar installations.

Locations where the use of radio emissions is inadvisable:

In the close proximity of medical or life support equipment.
The handling of explosives.
Adjacent to sensitive industrial control systems.
Control rooms and control towers.
Security conscious areas, where information is to be contained within the venue.
Aircraft.

Anton Domone. C.Eng., MIEE.
January, 2000

Archive Index

EMC Journal Home

© Nutwood UK Ltd 2001	Eddystone Court - De Lank Lane St Breward - BODMIN - PL30 4NQ Tel: +44 (0)1208 851530 - Fax: +44 (0)1208 850871 nutwooduk@nutwood.eu.com