Interference from Mobile Radio Equipment
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 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 authors have 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 form
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 contained within the
venue.
Aircraft.
Anton Domone. C.Eng., MIEE,
January, 2000.
Azdec Limited,
32 Gladstone Road,
Southampton,
SO19 8GT,
UK.
Telephone +44 (0)23 8044 4393
Fax +44 (0)23 8043 2071
Azdec Limited 07/01/00
Infra Red Communications
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