An isolating Transmission Line Transformer (ITLT) for use in a data communications- system is disclosed. The ITLT comprises a core, a first port for connection to a data source, and a second port for connection to a transmission or data receiver line, said second port being on substantially the opposite side of the core to the first port. In addition, a transmission line is formed by first and second conductors, the first conductor being connected in series to the first port and the second conductor being connected in series to the second port to provide d.c. isolation between the ports. The conductors are arranged side- by-side though or around the core, and each executes a single turn or winding through or around the core.
