A tunable high‐Q Fabry‐Perot resonator and related instrumentation which has been used to measure the shapes of nitric oxide rotational lines is described. The cavity was used around 150 GHz, but is suitable for any frequency above 120 GHz. The transmitter klystron is phase‐locked to a tunable 15 MHz crystal standard. A sawtooth sweep is applied to this standard resulting in a linear‐frequency sweep of the 150 GHz signal across the cavity resonance. The shape of the cavity response is a measure of the gas absorption. A beamsplitter is used to couple energy into the resonator. It is demonstrated that such resonators are well suited for certain microwave spectroscopy experiments.