- Radiated emissions from high frequency powerline telecommunication (PLT) systems - IainSummers (first published OMEGA Spring 2012)
- Sit Back to Sit Forward - Broadcasting and The OU - Jan Henderson (first published OMEGA Summer 2011)
Radiated emissions from high frequency powerline telecommunication (PLT) systems - IainSummers (first published OMEGA Spring 2012)
Most houses in the UK have internet access delivered through dedicated cable networks or telephone lines. Cable networks are available to less than 50% of the UK population and reliable access to broadband through telephone lines requires the user’s premises to be within 1 mile of the telephone exchange. This has resulted in poor broadband performance in rural communities within the UK and is exacerbated in other countries that have a geographically dispersed population such as the USA and China.
An alternative technology is Power Line Telecommunications (PLT) where data is passed along the mains network to a user’s premises (access PLT) and distributed around the house to any power socket (in-house PLT). See Figure 1.
Figure 1 – PLT architecture
As every house is connected to the mains network, the infrastructure already exists and can provide an alternative revenue stream for electricity distribution companies. It has been used by utility companies at low data rates (a few kilobits per second) for a number of years to provide status and control information for electrical distribution circuitry. More recently “Smart Meters” have been deployed and PLT has been used to allow the remote reading of gas and electricity consumption. Higher data rates comparable with mainstream broadband providers requires data rates of tens of megabits per second using operating frequencies in the range of 1.6MHz to 30MHz - and potentially up to 300Mhz. At these frequencies mains wiring does not act as an ideal transmission system, resulting in electromagnetic radiation which causes interference on collocated radio receivers. This interference is normally controlled by national and European Electromagnetic Compatibility (EMC) compliance standards. In the case of PLT the method of measurement is poorly defined and the radiated acceptance limits have not been agreed. The purpose of my research was to investigate the errors in EMC measurement of PLT and come to a conclusion on the test methodology and recommend an acceptable limit.
There are two EMC tests that would traditionally be required for the operating frequencies of PLT, namely:
Conducted Emissions which measure unintentional voltages present on the mains cabling of the appliance at frequencies below 30MHz.
Radiated Emissions which measure unintentional radiated fields through space from the appliance at frequencies above 30MHz.
These are typically undertaken in an anechoic chamber which allows the unit under test to be shielded from the external Radio Frequency (RF) environment caused by both intentional and unintentional ambient radiated emissions, eg TV and radio broadcasts (Figure 2).
Figure 2 – Typical anechoic chamber for EMC measurement
In the case of PLT this is not possible because unlike normal conducted emission measurements, PLT signals are intentionally placed on the mains cable and therefore would exceed normal acceptable limits. Furthermore a radiated emission takes place below 30MHz which is not normally measured in EMC testing due to the difficulty in making the measurement at these frequencies. Additionally, because PLT consists of the mains network of a house it cannot be placed in an anechoic chamber and therefore the ambient RF signals make the measurement prone to errors. There are also technical factors that make the measurement of radiated fields below 30MHz particularly difficult. The measured field should always be taken in the “far-field” of an electromagnetic wave which is where it starts to become a predictable standing wave. Using the analogy of a child splashing in a paddling pool; close to the child the waves are messy but as they move further away they start to become wavelike. The distance at which the electromagnetic far-field begins is a function of the frequency of the wave and the dimensions of the radiating structure and in the 1.6MHz – 30MHz band can be as far as 100m from the house. At these distances the ambient RF signal is much larger than the signal attempting to be measured. To attempt to overcome this, some researchers have postulated that the measurement can be taken in the “near-field” and extrapolated to the “far-field” using a theoretically predicted attenuation factor of 1/distance. Other researchers have postulated that it may be possible to measure the conducted measurement along the mains cabling and predict the radiated emission using a mathematical factor called the “k-factor” but again this has not been repeatedly measured in practice.
The main objectives of this research were to measure the attenuation factor and k-factor at the limited PLT test sites that exist and comment on their usefulness and their associated errors. In addition, a number of researchers had undertaken radiated emission measurements of PLT at single locations using different techniques but they had never been correlated to determine effects on location (urban, rural, semi-rural), country (UK, Europe, USA, Asia, Australasia) or different types of mains wiring. My research also presented findings in this area. Finally I presented a novel measurement technique derived from military EMC testing that not only measures the radiated field but subjectively assesses the effect on victim receivers by assessing the annoyance of radiated emissions from PLT.
Four main conclusions were reached. Firstly, with respect to attenuation of the electric field with distance, the theoretical prediction of 1/distance was not consistent and in fact the attenuation ranged from 1/distance2 to 1/√distance. Furthermore at many frequencies there were standing waves caused by a number of system architecture factors. Figure 3 demonstrates the range of attenuation of the electric field with distance referenced to 1/distance (shown as 1/r) for differing frequencies and antenna orientations.
Figure 3 – Attenuation of electric field with distance graphs
Secondly, k Factor was demonstrated to have no utility in the prediction of radiated emissions due to the amount of systematic errors inherent in the measurement technique. Thirdly, by analysing worldwide PLT radiated emission measurements of other researchers, a typical averaged value for PLT was calculated as well as a determination of the errors inherent in the measurement. Finally, the novel new technique demonstrated the actual effects on PLT systems by using a subjective assessment of the annoyance factor to a number of overseas radio broadcasts and noting the radiated emission levels when the annoyance became unacceptable. This allowed an “acceptable limit” to be presented as a starting point for EMC compliance measurements. This limit was stated along with a process of mediation if complaints were received due to PLT radiation.
The research presented a purely factual and technical assessment of the radiated emissions from PLT but it also discussed other non-engineering factors that will determine whether PLT will become a prevalent technology. One of the key factors is the political imperative behind the global roll out of high speed broadband and also the use of the “Smart Grid” to reduce greenhouse emissions. The Smart Grid in particular will use PLT to communicate between each user appliance and the energy provider to regulate the use of electricity and gas at peak and off peak times to reduce costs and improve security of supply.
The overall conclusion is that there is no straightforward way of accurately and repeatedly measuring the radiated emissions from PLT systems and therefore the determination and compliance with EMC limits is error prone. For this reason it is unlikely that power line companies will invest in broadband technologies without support from governments to accommodate EMC compliance. With the rollout of fast broadband and the imperative to comply with greenhouse gas emissions, that political support is beginning to look more likely than at any time in the previous ten years.
Iain Summers – Winner of 2011AOUG Baroness Lee of Asheridge Award
Sit Back to Sit Forward - Broadcasting and The OU - Jan Henderson (first published OMEGA Summer 2011)
The OU and the BBC have recently signed the ‘6th Agreement’. This is the most recent of a series of agreements unpinning their unique 41 year partnership. In a world where teaching and learning material are now delivered to students online (or via DVD) and where the OU can reach new audiences via YouTube and iTunesU, what does broadcast have to offer the Open University? From the moment of receiving its Royal Charter in 1969, The OU has been known as the ‘University of the Air’, signalling in part its most enduring public partnership with the BBC. The University needed the BBC for two key reasons. First, it provided the only way of delivering certain unique teaching and learning services to students around the country. Second, as stated in its Charter, there was, (and still is), a requirement that the OU engages the whole UK community in learning through means ‘such as broadcasting’.
From the start rich, audio visual media were considered essential to the teaching and learning experience and played a unique role. While some uses of rich media can help compensate for the necessary constraints of distance education, other features provide unique and powerful complements to more traditional text based, or face to face, forms of delivery (in principle providing OU students with advantages over students of more ‘ordinary’ universities):
- Unique Access: Learners can be given access to worlds which are difficult to access even in traditional teaching contexts (arctic ice formations, deep sea life),
- Impossible Access: Learners can be shown simulations or animations of events or activities that are out of reach of human experience (inside the core of a working nuclear reactor, the unfolding process of the big bang, or the complexities of sub atomic interactions).
- Archive: Much of the history of the 20th century is now captured on film in documentary and news archives. These archives provide students with unique access to new ways of understanding politics and history.
- The Human element: Rich media have enabled learners to listen to, or watch, interviews to help not just understand what key people have to say, but how they say it. It might be possible to provide a summary account or transcript of a poetry reading or an interview with a victim of a crime. However, a first person recording can have both a powerful impact and provide potentially important or essential information missing within a transcript (intonation, emotion, pace etc).
- The Emotive element: Rich media combinations of carefully composed picture sequences, words and music can ensure not just that information is conveyed but that it is enhanced with emotional or visceral impact as well.
- Time based media and processes: Revealing or explaining the structure and process of a complex system that changes over time can be significantly enhanced through time based media, for example animations of the working human heart or the workings of a combustion engine. Time lapse can also provide unique understanding of processes that could never be perceived or properly understood in real time.
With the rise of VHS, DVD and now internet, broadcast is not the only means of delivering these rich media benefits. Away from the constraints of broadcast schedules or the fashion of the latest TV format, these key teaching and learning features could be tuned and enhanced further and focussed entirely on the student and learner experience.
Since the 1990s the OU/BBC partnership agreements have focussed much more on the Charter requirement to engage the public in learning. This meant we had to exploit the broadcast platform for what it could deliver most effectively; namely reach and the catalysis of interest. The OU now only commission peak-time series from the BBC channels. Each year there are around twenty TV, or radio, series bearing The Open University logo: Coast, Child of our Time, Chinese School, Fossil Detectives, James May’s Big Ideas, History of Scotland, Olympic Dreams, More of Less, The Money Programme, Timewatch, Bang Goes the Theory, Life. These are all Open University programmes and ones which have often been taken up by other broadcasters around the world.
These programmes attract larger and more diverse audiences than the old course related programmes could hope to. There were over 350 million views of OU/BBC programmes last year (and many of these are re-broadcast on international channels - so the total global number is probably in the region on 1 billion views). Some have questioned whether the public realises these are OU programmes. However, we know from independent research that around 38% of the audience recognise these as OU programmes (that’s considered a quite high level of ‘brand recognition’ given the context). More importantly, every year around 2.5 million people followed up their interest inspired by the BBC/OU programmes. These viewers sit forward and go online with the OU (previously at Open2.net, now at open.ac.uk/openlearn) and start learning journeys with us. Some take that journey just for a few hours and out of passing interest, but thousands more make a life changing transition from sit back viewer to sit forward OU student. That journey would not start without the reach and inspirational output of the BBC.
Andrew Law - Director of the Open Media Unit, Open University