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The paper presents a method for selecting the carrier frequency of an ultra low power, size-constrained, Colpitts oscillator transmitter in order to minimise transmitter power consumption. The configuration studied uses a single-turn loop antenna which also acts as the inductor of the frequency-defining LC tank. The characteristics of the antenna have been modelled to determine its optimal electrical size in terms of maximising power transfer for a given constraint on antenna dimensions. This analysis reveals that the loop antenna has a particular electrical size, corresponding to a circumference-to-wavelength ratio of about 0.2, for which it both radiates efficiently and possesses a high Q-factor. Evaluation of the frequency-dependent losses of the other circuit elements has allowed the preferred carrier frequency to be determined as a function of antenna size. The results indicate that cm-scale devices should be able to achieve short range, low data-rate transmission (e.g., 10 kb/sec over 1 m) with total transmitter powers of the order of 10 μW.