The design, performance, and operation of a broadband (3–26.5 GHz) high resolution microwave spectrometer is described. In comparison to previously developed molecular beam Fabry–Perot resonator spectrometers the design presented here implements some significant improvements: a coaxially oriented beam resonator arrangement (COBRA) formed by a confocal pair of mirrors incorporating an electromechanical valve and employing two pairs of microwave antennas, and a multioctave Fourier‐transform microwave (FTMW) instrument providing the pulsed excitation source with microwave pulse phase‐inversion scheme and the low‐noise receiving system employing image‐rejection downconversion with superheterodyne as well as quadrature detection. The entire apparatus, fully automated for scanning operation, covers a frequency range of more than three octaves. The novel design of the FTMW instrument does not require any changes of the spectrometer hardware in order to reach all regions of its spectral range. While operated in high resolution mode the COBRA‐FTMW spectrometer is achieving a linewidth (half‐width at half‐height, HWHH) relative to the molecular emission frequency of better than 10−7HWHH. The sensitivity and resolution of the spectrometer is demonstrated by several examples. ©1996 American Institute of Physics.