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POWER LINE TECHNOLOGY THREAT TO HF SPECTRUM

POWER LINE TECHNOLOGY THREAT TO HF SPECTRUM
Prepared by C. Szabo – EUROCONTROL

21 Dec 04. BATTLESPACE understands that the push by the government to maximise the use of broadband in the UK using Power Line Technology (PLT) could threaten the HF frequency in particular. Broadband use is expected to rise to bring the UK in line with Germany and France and the quickest and cheapest way is to take it down power lines and not through fibre optic cable. This open-wire system radiates RF energy from the lines, the amount of which is largely dependant upon the radio spectrum used by the transmission system.

In Korea the government is investing $4bn in laying fibre optics into homes which provides better speed of delivery and bandwidth. Initial objections by the MoD and CESG have been met with caveats from the government and the people concerned warned off attending the meetings arranged by amateur radio enthusiasts in particular on the subject.

PLT delivers data services, such as the Internet, to homes and businesses over the electrical distribution system. A typical electrical supply sub-station provides electrical power to some 250 to 300 homes within a 250 meter radius of the electrical distribution sub-station.

By injecting a signal at the sub station, modulated with data information, most of the homes connected to the sub-station can be provided with the data service. A modem at the customer end, operating on the same frequency as the sub-station and time division multiplex, is used to inject radio signals back into the electrical distribution cables to provide the return path for the data system.

The data distribution from the Switch (data hub) out to the electrical distribution sub-station is suing fibre optics. However when fibre is not available, there will be a requirement to link electrical sub-stations with microwave links to get the data from the hub to the sub-station.

The frequency band used by PLT is from 1 MHz to 30MHz so covers the MF and HF bands. Short waves have the unique long range propagation characteristics, resulting from the refracting proprieties of the ionosphere. Considerable efforts are made to protect short waves from manmade harmful interference. The electrical distribution system was not designed to carry RF signals and as a consequence it radiates them into free space. These unwanted emissions have the potential to interfere with legitimate users of the radio spectrum; therefore some control over their level will need to be put in place to protect them. Although such systems rely on radio frequencies, they are not Wireless telegraphy as defined in Section 19 of the WT Act 1949.

In Western Europe access to Internet services is far from saturation. The analogue telephone lines can provide only low speed access. ISDN, XDSL and Coaxial Cable Networks or Satellite links can offer much higher access speed solutions. But another potential telecommunication support is the PLT. Existing power-line networks are currently used for energy transport only; they have potential as carriers for telecommunications purposes.

Users of the HF band threatened with this technology include civil aviation, the Armed Forces, Radio Broadcast, Radio Amateurs, News Agencies and Embassies. The CAA has raised concern that the PLT radiation has the potential to adversely affect, or even deny HF communications.

Electrical distribution cables were never intended to carry radio signals and they act as very poor transmission lines, consequently energy at radio frequencies is not contained within the cable and escapes into free space. Results of measurement on a test PLT installation have shown that the resulting unwanted emission in the narrow band below 10 MHz can be as high as 66 dB measured in a 9 kHz bandwidth, in the band 1-10 MHz. The unwanted emission has the potential to disrupt radio communications within a radius of 100 to 200 metres of any electrical distribution network.

Results of measurement

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