As relates to automatic matching of antenna impedance for a radiofrequency transmit or receive circuit, an impedance matching network is inserted between an amplifier and antenna. The current i and output (resp. input) voltage V of the amplifier and their phase shift are measured, the complex impedance defined by V/i is deduced; the impedance of the antenna is calculated as a function of this complex impedance and the known existing values of the adjustable impedances of the matching network. New adjustable values of impedances of the matching network to obtain a desired overall load impedance for the amplifier are calculated on the basis of the calculated antenna impedance value, and the matching network is controlled to adjust the adjustable impedances to these new values. The measurement is made at a measurement frequency different from the working frequency to allow automatic matching without interrupting normal operation of the chain.