We describe a tunable Fabry–Perot type filter which has application to radio astronomy in the frequency band 80–110 GHz. The filter causes image sideband suppression when used with a heterodyne spectral line receiver having an intermediate frequency of 4.75 GHz. The transmission loss in the signal sideband is about 0.4 dB, while the image sideband rejection is more than 15 dB; there appears to be little if any problem with scattering or distortion of the antenna radiation pattern.