Second harmonic radiation at 70 GHz was generated using a 35‐GHz magnetron in bulk ferroelectric LiNbO3and LiTaO3located in a rectangular waveguide. The measured second harmonic output was comparable to theoretical predictions with peak powers as high as 9.8 W for LiTaO3and 7.6 W for LiNbO3for an input power of 24 kW. The output power varied periodically as a function of the sample length due to cavity resonance effects.