Downhole telemetry system

An antenna for operating downhole on a drill pipe that is separated from a surrounding formation by a casing, the antenna comprising:

• a power source capable of injecting a current across a first insulated section of the drill pipe;
• an electrode for conducting the current from the drill pipe to the casing and formation;
• and
• a second insulated section of the drill pipe, arranged to operate together with the electrode and first insulated section, for shaping a path flow for the current.

A method of lengthening an electromagnetic dipole on a downhole antenna, the antenna is located on a pipe having an annular spacing from a casing lining a formation, the method comprising the steps of:

• injecting current across a first insulated section of the drill string;
• conducting current through an electrode from the pipe to the casing;
• and
• directing a path for the current to flow using a second insulated section configured to operate together with the first insulated section and the electrode for lengthening the electromagnetic dipole.

An antenna locatable downhole on a pipe surrounded by a casing, the antenna comprising:

• a power source capable of injecting a current across a first insulated section of the pipe, the first insulated section providing an electrical discontinuity in the pipe;
• a first electrode for conducting the current from a first location of the pipe to the casing;
• a second electrode for conducting the current from the casing to a second location of the pipe;
• and
• a second insulated section insulating a selected length of the pipe between the power source and the first insulated section.

An antenna locatable downhole on a pipe surrounded by a casing, the pipe having a plurality of segments joined at respective stand-off points, the antenna comprising:

• a power source for injecting a current across a first insulated section of the pipe, the first insulated section providing an electrical discontinuity in the pipe;
• a first electrode for conducting the current from a first location of the pipe to the casing;
• a second electrode for conducting the current from the casing to a second location of the pipe;
• a second insulated section insulating a selected length of the pipe between the power source and the first insulated section;
• and
• a set of further discrete insulated sections insulating the respective stand-off points.

An antenna locatable downhole on a pipe surrounded by a casing, the pipe having a plurality of segments joined at respective stand-off points, the antenna comprising:

• a power source for injecting a current across a first insulated section of the pipe, the first insulated section providing an electrical discontinuity in the pipe;
• a first electrode for conducting the current from a first location of the pipe to the casing;
• a second electrode for conducting the current from the casing to a second location of the pipe;
• a second insulated section insulating the pipe between the first and second electrodes.