An apparatus for electrical line communication includes a transmitter, a receiver, a modem, and a coupler at each of two or more locations along an electrical line. The couplers have capacitive circuits serially connected with an air-core transformer. The capacitive circuits resonate with the air-core...http://www.google.ca/patents/US6104707?utm_source=gb-gplus-sharePatent US6104707 - Transformer coupler for communication over various lines
