An EM antenna for location on a pipe (10) surrounded by a casing (12). The antenna has a power source (21) for injecting a current across a first insulated section (25) of the pipe (10) and an electrode (22) for conducting the current from the pipe (10) to the casing (12). There is also a second insulated section (23) of the pipe arranged to operate together with the electrode (22) and first insulated section (25) for directing a path flow of the current. Such antennas are described both in relation to repeater arrangements as well as land and sea applications.