Spectroscopy at microwave frequencies is complicated by the narrow bandwidth of microwave components. Fortunately, in paramagnetic systems this difficulty is no great handicap, since the separation of energy levels can be changed conveniently by varying an applied magnetic field. For this reason most magnetic resonance experiments have been performed at a single microwave frequency. The spectrometers in our lab use a single microwave frequency of 9GHz (X-band) and 34 GHz (Q-band). There are a number of considerations to take into account when choosing the highest frequency.

In favor of the highest frequency, there is increased sensitivity, minimization of higher order effects, improved resolution when the sample has more than one g value, and spectroscopy between levels whose zero field splitting is of the order of the microwave quantum.