Coil arrangements

A coil arrangement for generating a high frequency magnetic field in a generally cylindrical volume, e.g. for use in an NMR imaging apparatus, comprises a plurality of conductors extending generally parallel to the axis of the volume at spaced positions around the volume and metal half rings and ring whereby the conductors are connected in electrically parallel paths for energisation from a common source. The paths have a high self-resonant frequency compared with the source frequency, e.g. by being arranged each to comprise a single turn only, and the impedances of the paths are arranged to differ, e.g. conductors of different cross-section, so that when energised the fields produced by the individual conductors combine to produce a resultant desired field distribution. The difficulty of providing a sufficient number of series connected turns distributed so as to obtain a desired field distribution without having a low self-resonant frequency is thus avoided. Combinations of conductor sizing, separate impedance elements, and unequal conductor spacing may be used to obtain the desired field distribution. (author)