EMC Awareness
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Radiocommunications Agency EMC Awareness |
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Issues with broadband and cable data networks
Cable TV interferes with aircraft navigation and communications
Possible interference with aircraft systems by ‘broadband Internet’
“Internet over the power lines” interferes with radio reception
| Cable TV interferes with aircraft navigation and communications |  |
Description
The German economics ministry is considering restricting cable TV networks because of mounting concerns about their possible impact on air traffic safety. Frank Krueger, an economics ministry spokesman, says that the possibility that interference from household cabling will interfere with aircraft navigation and ground communication systems has prompted the government to propose regulations.
“It is possible that, in individual cases, certain stations will have to be closed down after a review of the dangers posed by their frequencies.” said Krueger. According to the European Cable Communications Association, the discussion about cable broadcasting and air traffic safety is not confined to Germany.
EMC consultant Diethard Hansen has written: “In spite of using coaxial cables in the TV distribution systems there is a lot of shield leakage, based on technical imperfections and ageing. Catastrophic emissions in the aeronautical security bands are jamming Germany.”
Commentary
Cable TV networks use coaxial cables which spread for a great many miles. The frequency range of the video and data carried by the networks spreads over the entire HF spectrum. Even though the installation should use high-quality shielded cables and connectors the overall ‘leakage’ from such large networks as experienced at some height above the earth can be sufficient to cause interference problems with aircraft navigation systems and communications receivers.
References and links
Electronics Times, April 1999, page 1.
“Megabits per second on 50Hz power lines”, Diethard Hansen, IEEE EMC Society Newsletter, January 2001, http://www.ewh.ieee.org/soc/emcs.
Links to Mitigation Techniques
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| Shielding of cables |  |
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| Possible interference with aircraft systems by ‘broadband Internet’ | |
Description
Mass deployment of ADSL systems in Greater London has the potential to exceed the radio noise floor. In addition, the emission level is predicted to exceed the maximum co-channel interference level of an airborne ADF (Automatic Direction Finding) receiver by up to 15 dB over the centre of the city, reducing to 2 dB at the edge of the city.
Mass
deployment of VDSL systems has the potential to increase the noise floor by
up to 18 dB at 10MHz at a height over central London of 100m. At the centre
of London, the cumulative emissions level would exceed the ITU-specified noise
floor at all heights up to 20 km. At the edge of the city, an increase in the
noise floor of between 5 dB – 8 dB is anticipated at a height of between
5 km – 10 km.
Commentary
ADSL and VDSL are the technologies used for delivering ‘broadband Internet access’ over ordinary telephone wires, using digital signals with data rates of up to several MHz.
Telephone wires are unscreened twisted pairs, but were only designed to carry balanced signals at up to a few kHz. Consequently their ‘longitudinal conversion loss’ (or LCL – the rate at which they turn wanted balanced signals into unwanted common-mode noise emissions) is very high for the high frequencies needed to preserve the edges of the digital signals.
In other words, the telephone cables ‘leak’ the ADSL and VDSL signals into the ambient. At a height above the earth’s surface an aircraft navigation or communication receiver ‘sees’ the aggregate leakage from many tens or hundreds of miles of telephone cables at the same time. It is predicted that the effect will be to increase the background noise to levels which are higher than the aircraft equipment was designed to cope with.
References and links
“Prediction of interference due to telecommunication drop wires in the ADSL and VDSL bands”, A R Bullivant and A J Maddocks, IEE Seminar “EMC – It’s nearly all about the cabling”, Savoy Place, London, January 22nd 2003, sales@iee.org.uk, http://www.iee.org/shop or http://www.iee.org.uk/Library.
“Continuation of investigations into the possible effect of DSL related systems on Radio Services”, ERA Technology Ltd - Report 2001- 0333, RA Reference AY 3949, available from http://www.radio.gov.uk/topics/research/topics.htm#emc
“Cumulative effect of radiated emissions from metallic data distribution systems on radio based services”, York EMC Services Ltd, http://www.yorkemc.co.uk/Research/index.htm or from the Radiocommunication Agency’s index at http://www.radio.gov.uk/topics/research/topics.htm#emc
“Investigation of impedance and mode conversion of telecommunications cables for xDSL systems”, York EMC Services Ltd, http://www.yorkemc.co.uk/Research/index.htm or from the Radiocommunication Agency’s index at http://www.radio.gov.uk/topics/research/topics.htm#emc
“Broadband non-Radio Technology”, by the Radiocommunications Agency, has a number of links to articles, papers and resources describing what is being done in the U.K. to prevent interference from broadband Internet technologies. Go to http://www.radio.gov.uk/topics/broadband_non-radio/index.htm.
Also visit: http://www.radio.gov.uk/topics/interference/documents/dslplt.htm.
Links to Mitigation Techniques
| Installation | Design & Development | Resources | |
| Filtering with CM cable-mounted chokes | |
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| Balanced interconnections and twisted pair | |
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| “Internet over the power lines” interferes with radio reception | |
Description
Early field trials of power line telecommunications (PLT) in the UK, Germany and Switzerland showed radiated emissions of up to 40dB more than the NB30 RegTP limits, which are themselves about 20dB more relaxed (in the short wave bands) than the U.K. Radiocommunications Agency’s April 2000 version of MPT 1570.
As a result, broadcast, military, commercial as well as licensed amateur radio services have started seriously objecting to a nationwide implementation of PLT. The RA has been at the forefront of research to try to anticipate the potential problems of PLT so that a sensible compromise can be reached which both protects the radio spectrum and does not hinder the implementation of PLT. Meanwhile, the BBC and the RSGB (representing broadcasters and radio amateurs) have both put forward arguments which show that severe limitations should be placed on PLT emissions, while the PLT companies themselves have been arguing for significantly relaxed limits in order to allow the most economical implementation.
Since the UK government is actively committed to the rapid implementation of broadband delivery services, including PLT, this argument is rapidly acquiring a political edge.
Commentary
PLT (sometimes called PLC) is a method of using the low voltage mains distribution networks to carry high-rate digital telecommunications signals. It is typically marketed to national governments as a means of providing Internet access that does not depend on the telephone network, providing more competition in this area.
Unfortunately, since the mains networks in Europe were only designed to carry frequencies of 50Hz, they tend to act as transmitting antennas for PLT signals (i.e. to ‘leak’), possibly causing the background noise floor to rise to levels that could make some existing broadcast and radiocommunication services suffer unacceptable levels of interference, especially in the frequency bands below 30MHz.
When the cables leave the ground, for example to power street lights, houses, apartment blocks, offices, etc., they emit significant noise. A possible solution is to fit filters (and the internet modem connections) at every point where the cables leave the ground, but this adds cost to what is intended to be a low-cost system.
References and links
“Update on Power Line Telecommunication (PLT) Activities in Europe” by Diethard Hansen of Euro EMC Services (EES) www.euro-emc-service.co.de, chairman of ATRT WG PLC, RegTP, Germany, presented at the IEEE’s International EMC Symposium held in Minneapolis, Minnesota, August 19-23 2002, and published in the Symposium Record on pages 17 - 22, http://www.ewh.ieee.org/soc/emcs.
“Megabits per second on 50Hz power lines”, Diethard Hansen, IEEE EMC Society Newsletter, January 2001, http://www.ewh.ieee.org/soc/emcs.
The ARRL (the U.S.A’s association for Amateur Radio) has 24 pages of references and links on PLC (PLT), including many recordings of the sounds of this interference, at: http://www.arrl.org/tis/info/HTML/plc.
Do EMC limits protect broadcasting as intended?, J H Stott, BBC R&D White Paper WHP055, from http://www.bbc.co.uk/rd/pubs/whp/whp055.html
The RSGB (Radio Society of Great Britain): go to http://www.rsgb.org and click on “Powerline Telecommunications”. They have some recordings of the sounds of PLT interference with radio reception.
“Cumulative effect of radiated emissions from metallic data distribution systems on radio based services”, York EMC Services Ltd, http://www.yorkemc.co.uk/Research/index.htm or from the Radiocommunication Agency’s index at http://www.radio.gov.uk/topics/research/topics.htm#emc
“Broadband non-Radio Technology”, by the Radiocommunications Agency, has a number of links to articles, papers and resources describing what is being done in the U.K. to prevent interference from PLT and other broadband Internet technologies. Go to http://www.radio.gov.uk/topics/broadband_non-radio/index.htm
Also visit: http://www.radio.gov.uk/topics/interference/documents/dslplt.htm.
Links to Mitigation Techniques
| Installation | Design & Development | Resources | |
| Balanced interconnections and twisted pair | |
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| Filtering with CM cable-mounted chokes | |
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| Filtering | |
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