A method is provided for estimating the polarization ellipticity of an antenna response signal to an incident electromagnetic wave received on a crossed-loop antenna or an Adcock antenna array. The method comprises at least the following steps: measuring the phase offset Δφ between the signals acquired respectively on the cosine and sine path of the antenna; measuring the ratio R between the amplitudes of the signals acquired respectively on the cosine and sine path of the antenna; determining the ellipticity angle τ based on the phase offset Δφ and on the ratio R. The invention applies notably to the discrimination of ionospheric waves from ground and/or sea waves, in particular for a surveillance, eavesdropping, or goniometry system using HF band signals.