A resonance coil or resonator, inside which the sample to be investigated is placed, is one of the basic components of NMR devices (spectrometers and tomographs). In the state-of-the-art NMR devices based on superconducting magnet-solenoids, the internal operating volume and (logically) the resonator are shaped as cylinders, the cylinder axis coinciding with the direction of constant magnetic field Ç 0 . The resonator is used to produce uniform alternating magnetic field Ç 1 , which is normal to Ç 0 . As is well known, a uniform magnetic field perpendicular to the cylinder axis can be induced by a current passing along the cylinder surface, if the current amplitude is I 0 ~ sin θ , where θ is the angle measured from the direction of field Ç 1 (see, e.g., [1]).The best-known structure for utilizing this principle is the birdcage resonator [2]. It consists of N (usually, N = 8 or 16) parallel conductors A , uniformly distributed over the cylindrical surface, the ends of which are connected with annular conductors B . Depending on the version (lowpass, highpass, or hybrid, a combination of the first two), the resonator contains N , 2 N , or 3 N fixed capacitors, placed between the halves of conductors A and/or B (Fig. 1). Fine tuning is performed by adjusting a variable capacitor either connected in parallel with one of the fixed capacitors or used in place of it. A drawback of this structure is that the resonator frequency and the uniformity of the magnetic field it produces depend on the capacitance of a large number of capacitors. This requires that the capacitances of the capacitors be carefully preselected, since a large spread in capacitance values deteriorates the uniformity of the field. In addition, the field uniformity is impaired when the capacitance of the capacitor used to tune the resonator to the resonant frequency is varied. This limits the tuning range and requires that the capacitance of the fixed capacitors be changed when the type of sample under investigation is changed. Various methods for the simultaneous (mechanical) trimming of all the capacitors in conductors A or B have been proposed, but these methods entail considerable complication of the resonator design.Another well-known structure is the saddle coil, which can be easily retuned over a wide range of frequencies by adjusting a single variable capacitor [3]. The uniformity of the field produced by this coil is not high, however, due to the small number of active conductors A ( N = 4). Figure 2 shows a new design for a probe coil with N = 6, tuned by adjusting a single variable capacitor. The required current distribution is achieved by branch-Abstract -An NMR probe coil that produces a high-frequency magnetic field as uniform as the field provided by the well-known birdcage resonator, is described. Unlike the resonator, however, the coil can easily be tuned by varying a single variable capacitor. (a) (b) A B Fig. 1. (a) Lowpass and (b) highpass modifications of the birdcage resonator.(a) (b) B 0 B 1 2 3 4 5 6 1 1 2 3 4 5 6 Fig. 2. (a) 3...
