Introduction to transmission lines basic principles and applications of quarter‐wavelength cables and impedance matching

The basic electrical properties of transmission lines are reviewed, and it is shown how these properties determine the characteristic impedance of the line. For resonant lines, the formation of standing waves is explained. The variation of the impedance of the line as a function of its length is shown to be caused by the standing waves of voltage and current. Various equations for the standing‐wave ratio and the concomitant power losses are derived for systems that are impedance mismatched. Applications in NMR spectroscopy include the use of transmission lines as filters, as impedance‐matching devices, and for directing pulses toward the probe and away from the preamplifier. Finally, a simple laboratory experiment demonstrates the properties of open and shorted quarter‐ and half‐wavelength cables.