Abstract: A symmetric RF coil for use in a probe for NMR apparatus comprises an odd number of turns having generally straight portions and arc-shaped portions wound alternately in series on the curved surface of a cylindrical sample region. An additional pair of arc-shaped segments connect to the outermost turn to form continuous end rings, and to split the current on the outermost turn to the continuous end rings. Such an RF probe coil possesses material symmetry provided by the additional arc-shaped segments and RF symmetry as a result of the symmetry in current flow generated by the split in current of the outermost turn to the continuous end rings. The symmetric odd-turn RF coil achieves NMR performance at the same level of even-turn coils with high RF homogeneity, and is suitable for high resolution NMR applications. The coil inductance of the present invention bridges the gap between even-turn coils, and facilitates improved optimization of the resonance circuits over the entire operating range.

Abstract: A variable external capacitor produces greater functionality for NMR probes by reducing the size of components and efficiently arranging the components. The variable external capacity has a cavity that allows adjustment tools to be extended through the variable external capacitor to adjust additional components that may be stacked above the variable external capacitor. The cavity may be used to house reactance such as a fixed capacitor or inductor connected in parallel with the variable external capacitor. The variable external capacitor includes a polytetrafluoroethylene housing configured to minimize its size and reduce lead lengths of the NMR circuit. A silver deposit plate on the polytetrafluoroethylene housing constitutes a stationary capacitor plate. A drum disposed inside the cavity and movably spaced from the stationary capacitor plate functions as a movable capacitor plate. An anti-rotation rail supports the drum to prevent rotation.