An Isolating Transmission Line Transformer (ITLT) for use in a data communications system is provided, the transformer comprising: a substantially planar substrate formed of electrically insulative material having opposed first and second surfaces; a first port formed of two separate terminals provided at one part of the substrate; a second port formed of two separate terminals provided at a second part of the substrate; a first conductor connected in series to the first port and arranged as a single loop; a second conductor which is electrically isolated from the first conductor and connected in series to the second port, the second conductor being arranged as a single loop in a substantially opposite orientation to the first conductor; wherein the first and second ports and at least part of the first and second conductors are provided on the substrate surface (s); and a core arranged between the first and second ports to cover the majority of the first and second conductors.