When telecoms operator Mercury won the first licence to compete head-on with BT, the company had to look for a new way to get its signals round the country without digging up roads and pavements.

Plans to lay optical fibres through the sewers were abandoned when trials showed that rats will gnaw through anything, even metal armour plating, hoping there is food inside. So Mercury bought space alongside the UK's railway lines.

Other telecoms operators have slung cables from mains distribution pylons and bought the right to dig up the tow paths alongside Britain's canal network. Microwave links can send data hopping round the country. Mercury set up local dish links for large offices.

But the 'last mile' into the home or small office is always the big problem. That's the copper wire connection from the local exchange to the living room. Most homes in the UK already have twisted pair copper wires laid many years ago by the Post Office or BT when they had their cosy monopoly.

These wires can now carry several Mbps using ADSL technology, but BT still controls them. Either you buy your broadband connection from BT, or you pay a telecoms company that is paying BT for the right to use BT's last mile.

So are there any other options? In this feature we'll look at the pros and cons of one controversial way of bringing broadband to the masses - by transmitting data over mains electricity cables.

Previous attempts to use alternative ways of getting data into people's homes haven't been too successful. Ionica tried piping microwave signals direct into homes, for instance, but that failed because it cost too much.

Two-way satellite links were too expensive as well. One-way satellite links still need a phone line for the return path. Mobile and third-generation (3G) connections cost far more than a fixed phone line and run at slower data rates - despite what the adverts may claim.

Only the cable TV companies, which spent a fortune on digging up roads and pavements, have private lines running direct from the street into homes. So the only affordable way to get broadband into the home is currently through a cable TV company or - one way or another - via BT lines.

There is, however, one other future option. Almost every home has a mains supply cable running in from the street. Theoretically it is possible to piggyback high-speed data on top of the 50Hz mains supply. Anyone who can offer a safe and reliable service for delivering two-way broadband data down mains cables into the house and then around the house, is onto a cash cow winner.

It is in the interests of the power companies to push power line broadband systems into homes as fast as possible, because satellite operators such as Astra are now offering cheaper broadband satellite services into the home.

It isn't easy though. The mains is a very hostile environment, with spiky interference caused by electrical switches and radio pick-up because the cables act as an efficient receiver aerial.

A warning from history
In 1998 Canadian electronics company Nortel and British electricity provider United Utilities announced a joint venture called Nor.Web, to sell their Digital Power Line system around the world.

Nor.Web claimed to have received more than 1,000 enquiries, which whittled down to 40 serious approaches. Ten companies signed agreements to use the technology.

"The technology is available today - and will revolutionise mass communications on the internet," said Nor.Web chairman John Beckitt. "It's the most elegant solution; the most significant communication tool since the mobile phone."

Steve Pusey, CEO of Nor.Web, was even better with the soundbite hyperbole: "We have a mission to lead the world. There is a light bulb everywhere. We see a massive opportunity in speeding access to the world wide wait. We are working on the principle that if you build a motorway, people will come."

Nor.Web's Digital Power Line system provided a 1Mbps signal into and out of the street. Transformer substations were connected to the internet, with combiners to add a spread spectrum carrier to the mains. Interference on the mains at one frequency left most of the other carriers untouched.

The first tests were at a school in Manchester and Nor.Web promised to have trials in 2,000 homes in the UK by the end of 1998, with "mass connection at low cost".

At the London press launch we asked about interference from Nor.Web's high-frequency carriers leaking out of the mains. Our concern was glibly brushed aside. But within a few months Nor.Web had found that streetlamps near the Manchester test site were just the right vertical length to act as transmitter aerials for radio signals in the 2-10MHz band.

The BBC's World Service, the Civil Aviation Authority and even GCHQ, the government's electronic snoop centre, were affected. So were amateur radio buffs. The project was killed off and Nor.Web shut down at the end of 1999.

Threat to short wave radio
But all this is already a distant memory. New companies round the world are pretending it never happened or promising new magic fixes to stop it happening again. They assure us that the newer Power Line Telecoms (PLT) systems work at lower powers than Nor.Web's Digital Power Line, so should cause less trouble.

It would be wonderful if this were all true because if the new ventures follow Nor.Web down the drain, the IT industry will lose a golden opportunity to deliver broadband in real competition with encumbents like BT.

But broadcast engineers are warning that because of interference from PLT the world may soon have to choose between short wave radio and mains-borne internet access. Does it matter, though? Who needs short wave radio, except for a few radio hams behaving like Tony Hancock?

Short wave broadcasts travel remarkably long distances, by hugging the Earth's curvature or reflecting off the upper atmosphere. They provide news from home and bypass local broadcast censors.

World broadcasters have recently agreed the standard for a completely new short wave radio system called Digital Radio Mondiale. DRM uses compression to squeeze clear digital sound into the narrow radio channels that currently carry crackly analogue signals. So short wave radio could now sound far better than ever before, but interference can destroy DRM reception.

The longest aerial in the UK
Both ADSL and PLT work by putting high-speed data pulses on high-frequency analogue carriers. The modem is a radio transmitter. A transmitter is no use without an aerial, but phone lines and power lines behave as aerials. This is because they were never designed for high-frequency use.

They are not shielded to prevent them radiating radio frequencies. In fact they work like the 'leaky feeders' used to extend mobile phone cover and radio reception down through tunnels by 'leaking' the radio signals which they carry.

ADSL works (like digital television and radio in Europe) by splitting the data signal into many small channels so that each is carrying only a relatively low bit rate. Each channel has its frequency carrier, of slightly different frequency to the others and there are around 256 carriers ranging up to 1MHz.

The broadcasters and telcos have co-operated over the past 10 years to set carrier power standards for ADSL in order to limit interference. This is possible because the ADSL signals are running alongside weak telephone signals, so the ADSL carriers can be at low power.

Unfortunately PLT signals must fight to stay above the interference hash which pollutes the mains supply. (Hi-fi buffs pay a fortune for mains cleaning equipment.) So the carriers must be more powerful. The trick is to stop them radiating interference that spoils radio reception in the vicinity.

The power industry is mainly privatised and fragmented. Different operators are free to use different proprietary power line technologies. The proprietary PLT systems now being tested in the US and Europe vary widely, with details kept secret.

But all the competing systems have one thing in common: they radiate radio waves in the band below 30MHz. And this is just where Long, Medium and Short Wave broadcasts sit.

A need for standards
The BBC's warning that this will change the face of broadcasting is particularly potent because the BBC website is one of the most successful in the world, and the BBC has every reason to want better broadband services. What is clearly needed is an agreed standard for measuring interference field strength and drawing a line between acceptable and unacceptable levels.

Field strength is expressed as a dB comparison with a reference level of one microvolt per metre (a measure of the signal which radio waves induce in an aerial). This varies with the frequency but at 6MHz the German standard, called NB30, is 33dB greater than one microvolt/metre.

Broadcasters want a standard that is nearly 20dB or a hundred times tighter, at around 15dB. Powerline companies want a standard which is over 20dB or a hundred times slacker than the Germans' NB30, at around 55dB. The PLT companies' limit would allow their interference to be stronger than the broadcasters' signals!

There is also dispute over the distance at which the measurements should be made. The German standard says three metres from the power line; broadcasters say it should be at one metre, because every radio in a home will be around one metre from at least one mains power cable. Measuring at three metres instead of one reduces the apparent interference level by 10dB or 10 times.

The BBC warns that some of the spectrum analysers used by power company engineers have a worse noise floor than a transistor radio - so they do not accurately reveal the real interference field strength.

"At the German acceptable level of 30dB you are talking about a noise-to-signal ratio rather than signal-to-noise', said Andrew Oliphant, head of the BBC's Transmission Systems Group at the research labs near Kingswood Warren in Surrey.

"OFDM notching with a smart system that does not use frequencies that cause interference might be an answer," said the BBC's principal R&D engineer Jonathan Stott. "We need to get together outside the standard committees and work together." But so far there is no sign of this happening.

The bigger picture
In Singapore, trials of home networking systems run by the government were slated to use power line technology from electricity company Singapore Power (which had previously signed up to use the ill-fated Nor.Web system).

But when we visited one of the test rigs a few months ago, the power line connections had been abandoned and ADSL by phone line used instead. We never did get a straight answer when we asked why this had happened.

Elsewhere, though, PLT is on a roll, with 100 sites now working across Europe. In Germany there are 5,000 homes online in Mannheim, with commercial rollouts in Hameln, Dresden and Linz (which straddles the German border).

Three experimental sites are up and running in Spain, in Madrid, Zaragoza and Seville, with several thousand subscribers. In Switzerland an installation in Fribourg is working and, according to early tests, producing high interference levels. There are also trials in France and Finland. In the US there are trials in Potomac, St.Louis, Pennsylvania and Manassas City, Virginia.

Broadcast interference
Scottish and Southern Energy is making all the running in the UK. The power company has been testing various PLT systems in Crieff, Campbeltown and Stonehaven in Scotland before starting trials south of the border in Winchester. Around 500 homes in each town get a 1Mbps service for £30 a month.

The company's publicity material says a lot about data speed, convenience and cost, but nothing about interference risks.

However, Scottish and Southern let the BBC run tests in Crieff. The corporation found high levels of interference inside homes with modems, and next door to them.

This interference was mainly caused by internal wiring, especially the 'stubs' which rise vertically up the walls to switches and sockets and behave like mast antennae. Even when the modem is idle, the radio makes irritating clicks; when someone is using a PC to stream internet data, short wave radio reception is completely destroyed by continuous clicking.

"Interference from different homes can add up and perhaps interfere with aircraft which are using the short wave band," said the BBC's Stott.

When we asked about interference, we were unsure whether the Scottish and Southern spokesman was genuinely ignorant of the issue or just hoping we were. He assured us he knew of the Nortel and Nor.Web problems but felt confident in the new system because "we are able to exclude frequencies used by the emergency services".

Effects on amateur radio
Colin Wooff of the UK government's Radio Communications Agency (which became part of Ofcom in December) published a paper on PLT, which warns that "leakage emissions are inherent and unavoidable under normal operating conditions".

The new technology is "likely to be incompatible with 1.6 to 30MHz reception within the domestic environment" and, "although short wave reception is generally considered a minority interest enjoyed by the technically aware, this may change with the introduction of broadcasts".

The Radio Society of Great Britain (RSGB) complains that the British government is out of touch in thinking that "few people listen to high-frequency broadcasts" and "there have not been any interference complaints about the recent [Scottish] trials".

This is hardly surprising, said the RSGB, as only one amateur radio enthusiast lives in the area where the trials took place, and he does not work on the frequencies used in the trial.

The RSGB's magazine recently reported tests run by the Open University and University of Duisburg-Essen. These showed that living next door to someone who uses a power line modem would be worse for radio reception than living next door to someone who has a network of 200 PCs running continuously, but not quite as bad as having electric trams running close to the house 24 hours a day.

Fix it or forget it
The BBC's Research Labs have now responded with clinical precision, writing: "To the best of our knowledge there are no relevant 'British Regulatory Standards'. We have published the (Crieff) recordings along with the White Paper.

These recordings should be required listening for the accountants in any power company that is planning to launch a commercial PLT service. They can then ask two simple questions. Does our system cause similar levels of interference? And, if it does, are we willing to gamble shareholders' money on building a telecoms system which the government may shut down because it interferes with radio reception and emergency service communication?

This is one IT problem that will not be solved by the IT industry's usual PR hype and flannel. If PLT is flawed the only solution is to fix it or junk it. As the BBC's Oliphant reminded us: "The radio spectrum is already packed and unfortunately they've stopped making any more of it."