A downhole electromagnetic telemetry unit for use with a tubing string (5) includes an insulated electrically conductive member (31) and a processing unit (15). The insulated electrically conductive member (31) is electrically coupled to the tubing string (5) at an upper measuring point (23) and a lower measuring point (25). The processing unit (15) is configured to process a voltage difference measured between the upper measuring point (23) and the lower measuring point (25) across the insulated electrically conductive member (31) and to derive therefrom a signal transmitted from a surface location (13).