White Paper 195 : VHF emissions from PLT devices - First investigation of potential interference to broadcast reception

Mark Waddell and Jonathan Stott

March 2011

Abstract

Power-Line Transmission (PLT) of data using HF signals injected on to mains wiring has previously been demonstrated to cause interference to reception of HF broadcasting. PowerLine Adaptors (PLAs) have recently been introduced that use VHF spectrum for home networking. This paper describes experiments on a pair of such PLAs, conducted in the laboratory and in two homes, to study their potential to cause interference to reception of FM and DAB radio broadcasts, which occupy part of the same spectral range.

It was confirmed that the PLAs examined use spectrum from 50 to 300 MHz to achieve greater throughput than is possible at HF, although it was also found that in the real-home environment the PLAs did not always achieve VHF networking anyway, several factors being found that appeared to disturb their operation.

Operation of the PLAs caused interference to indoor-portable reception of both FM and DAB broadcasts, in varying degrees from no effect to total disruption. The ‘digital cliff’ of DAB reception means that when interference occurs the impact is extreme. The PLAs were also found to disturb reception of FM using an external antenna at one of the homes.

It is difficult to extrapolate with precision from these results in just the two homes visited. Since interference was shown to occur in conditions that were not equivalent to edge of coverage (they had a substantial margin above that), the number of homes whose reception of FM and DAB broadcasts would be affected if such PLAs were widely used would clearly be appreciable. However, conditions were also found in which no significant interference was noted, so we can also say that not all homes would be so affected.

A possible prediction exercise to try to refine this estimate is proposed, together with other possible future experiments.

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