A bucked transmitter comprising: a transmitting electrical circuit (300, 301, 301a, 301b, 302, 302a, 302b, 304, 304a, 304b, 305, 306, 307) comprising a transmitter loop; a transmitter loop framework to support the transmitter loop, the transmitter loop framework being substantially rigid; a transmitter adapted to energize the transmitting electrical circuit with a transmitting electric current having a known waveform so as to form a primary magnetic field; a plurality of radially separated, substantially planar bucking loops (2, 3, 110, 111, 112, 113, 114, 306, 307), each bucking loop being dimensionally smaller than the transmitter loop such that the radius of the bucking loops is smaller than the radius of the transmitter loop, arranged coaxially along an axis substantially parallel to the local direction of the primary magnetic field and energized with current by a current controller (301a, 301b); a substantially rigid support to which each of the bucking loops is affixed, the support being attached to the transmitter loop framework, wherein the bucking loops are stably located with respect to geometrical aspects of the primary magnetic field; a nulling axis which extends bi-directionally in a perpendicular direction from a plane parallel to the bucking loops to a termination point at each end of the nulling axis; wherein a bucking magnetic field is formed within a bucked volume by current in the bucking loops, the bucked volume being substantially centered on the nulling axis, the termination points are located where the bucking magnetic field fails to substantially annul the primary magnetic field, and the bucking magnetic field is substantially in a direction opposite to, and is substantially equal in magnitude with, the primary magnetic field, such that the bucking magnetic field substantially annuls the primary magnetic field within the bucked volume. a transmitting electrical circuit (300, 301, 301a, 301b, 302, 302a, 302b, 304, 304a, 304b, 305, 306, 307) comprising a transmitter loop; a transmitter loop framework to support the transmitter loop, the transmitter loop framework being substantially rigid; a transmitter adapted to energize the transmitting electrical circuit with a transmitting electric current having a known waveform so as to form a primary magnetic field; a plurality of radially separated, substantially planar bucking loops (2, 3, 110, 111, 112, 113, 114, 306, 307), each bucking loop being dimensionally smaller than the transmitter loop such that the radius of the bucking loops is smaller than the radius of the transmitter loop, arranged coaxially along an axis substantially parallel to the local direction of the primary magnetic field and energized with current by a current controller (301a, 301b); a substantially rigid support to which each of the bucking loops is affixed, the support being attached to the transmitter loop framework, wherein the bucking loops are stably located with respect to geometrical aspects of the primary magnetic field; a nulling axis which extends bi-directionally in a perpendicular direction from a plane parallel to the bucking loops to a termination point at each end of the nulling axis; a bucking magnetic field is formed within a bucked volume by current in the bucking loops, the bucked volume being substantially centered on the nulling axis, the termination points are located where the bucking magnetic field fails to substantially annul the primary magnetic field, and the bucking magnetic field is substantially in a direction opposite to, and is substantially equal in magnitude with, the primary magnetic field, such that the bucking magnetic field substantially annuls the primary magnetic field within the bucked volume.