Power line communications

DETAILED DESCRIPTION The present invention seeks to minimise the above problem.
According to a first aspect of the present invention there is provided an apparatus for coupling signals to a line, the apparatus comprising: an input for receiving a wanted signal; a first means for coupling the wanted signal onto the line at a first position; a cancelling means, coupled to the input, for deriving a cancelling signal from the wanted signal, the cancelling means being operable to phase-shift the wanted signal, a second means for coupling the cancelling signal onto the line at a second position, spaced from the first position; and wherein the apparatus is arranged so that the combination of the phase-shift and propagation delays experienced by the signals causes the wanted signal and the cancelling signal to destructively combine in a single direction of propagation along the line while enabling the wanted signal to propagate in the other direction along the line.
Preferably the cancelling means comprises a weight which is operable to scale the wanted signal in amplitude.
Preferably the spacing of the first and second couplers is substantially one quarter of a wavelength of the wanted signal.
This maximises signal power in the wanted direction of propagation.
Preferably the apparatus also has a monitor for sensing the combination of the wanted and cancelling signals at a position on the line and feeding the sensed signal to a calculating means which controls the cancelling means.
This allows a more effective cancellation.
The calculating means can perform an iterative technique in which perturbations are applied to the value of the weights and the sensed signal is monitored to determine the effect of the perturbations.
The calculating means may alternatively perform an iterative technique in which the sensed signal is correlated with a portion of the wanted signal to determine updated weight values.
Preferably the apparatus is used to couple telecommunications signals to a power line such as a distribution line of an electricity distribution network for serving a plurality of subscribers