Field measurements of non-linear characteristics, related to the dual-frequency technique applied to the ocean surface, were carried out in the North Sea. The data observations were made with a CW multi-frequency C-band radar onboard the offshore research platform ‘Forschungsplattform Nordsee’. It is verified that the cross-product spectrum contains additional information on distinct non-linear features, when compared to existing theories previously published in the literature. In particular, a relatively sharp second order Bragg resonance line is found on the right-hand side of the pronounced first order peak. This Doppler line occurs, within error bars, in the spectrum at √2 times the first order resonance frequency