A new millimetre wave magnetic resonance spectrometer, using broad-band components, is described. A `magic tee' provides both a balanced harmonic generator and a detector of the harmonic signal which has passed through the sample in the spectrometer. Studies have been made of a simple reflection system with no resonant cavity at about 4.7 mm wave-length, using standard 8 mm components. With a power of about 30 MW incident on the harmonic generator crystals, a time constant of 1 sec and a magnetic field modulation frequency of 630 c/s, approximately 9 × 10¹⁵ paramagnetic spins of 1 gauss line-width can be detected. The device is primarily intended for antiferromagnetic resonance studies and is in principle capable of operating at much shorter millimetre wavelengths.