The design of RF and millimetre-wave receivers demands low-cost features and compact architecture. Self-oscillating mixers (SOMs) are excellent choices for such a design as both the oscillating source and the mixer are combined in one single device. Moreover, subharmonic operation reduces LO frequency requirements and therefore opens up interesting perspectives for high-end millimetre-wave systems. A fully-balanced third-harmonic SOM structure is presented, operating at 5.8 GHz with high measured conversion gain (max. 8.75 dB) that is used as a frequency-scaled prototype for future millimetre-wave circuits. To the authors' knowledge, this is the first presentation of such a circuit structure.