Sealed modular downhole antenna

A sealed, modular downhole antenna 200 for measuring electrical characteristics of wellbore fluids and formations, includes a thin-walled curved metal tube 202, which houses coils of a length of conductive wire 208. The wire may be insulated by an insulating coating, as well as by a filler in the tube. The tube includes a coupling gap 204, to facilitate coupling electromagnetic energy through the tube, to and from the conductive wire. The tube also has a wall opening 206, for ends 208a, 208b of the wire to attach to an adapter-connector 210. The tube is coated with a sealant layer, to hermetically seal the tube, coupling gap, and junction between the adapter-connector and the tube. In alternative embodiments more than one tube may be provided to define a plurality of coupling gaps or the tube, adaptor-connector and sealant layer may be replaced by a moulded jacket around the coils.