A method to characterize small antennas in their actual operating conditions is presented. The proposed procedure is based on the radar cross section (RCS) measurement technique using backscattering modulation. A theoretical description of the measurement setup based on the signal flow graph representation is presented. The experimental validation includes the measurement of one and two port antennas, as well as the measurement of a small integrated antenna. Results show how the effect of the environment on the antenna impedance can be adequately measured with the proposed system.