PLT, or power-line telecommunications, refers to a method of networking digital equipment within the home by means of the mains power distribution system. Power-line adaptors are commercially available to plug into the 13 amp wall sockets. At the ‘transmit’ end, an adaptor converts Ethernet data frames into a high frequency RF signal using a wideband modulation technique. The RF signal travels along the mains wiring to a second ‘receive’ adaptor, which converts the signal back into Ethernet data frames.

The mains power infrastructure has obvious attractions for data distribution — it is readily accessible and does not involve any expense other than that of the adaptors. However, power lines were not designed with such use in mind, and some of the RF is radiated into the environment. The resulting interference is clearly of concern to broadcasters and listeners alike. As some adaptors are capable of working at up to 300 MHz, even the VHF/FM and DAB bands are at risk.

A previous White Paper looked at radiated interference levels in typical environments, but no attempt was made to relate the interference to the signal power on the mains. This White Paper partly fills the gap by describing a ‘Box’ that allows the signal power to be measured. Although such a function might seem mundane, there are a couple of challenges to be met: the signal needs to be separated (safely) from the mains voltage and the measurement must remain accurate at 300 MHz. The second of these is particularly difficult, as the signal passes through a 13 A plug and socket — designed for 50 Hz rather than 300 MHz!

With the Box constructed, some measurements have been made with PLT adaptors running at various data-rates. The signal spectra are illustrated here.

There is further work to be done, and it is hoped eventually to develop a model to relate the above measurements to the radiated interference levels.