Impedance matching with an adjustable segmented transmission line

Abstract

A capability for impedance matching between the RF probe and the spectrometer is a standard requirement for NMR. Both lumped element and branched transmission line methods are widely used for this purpose. Here, we propose to use the segmented transmission line structure which is well known in wireless communications. It relies upon reflections between transmission lines of different characteristic impedances that are serially connected to match the impedance of a coil or resonator to the characteristic impedance of the NMR spectrometer. In our implementation, two quarter wave length dielectric slugs are placed within a coaxial transmission line. Adjustment of the positions of the slugs allows the variable tuning and matching needed for NMR probes, eliminating the need for variable capacitors and inductors. As a demonstration of the usefulness of this approach, we have incorporated a variable segmented transmission line into a home-built Variable Angle Spinning probe. Finally, we discuss the range of possible application for segmented transmission line networks in NMR probe design.