Because the Jacobian connecting wave number to frequency conversion is nonlinear, in the presence of background waves even without a mean current a wave number spectral component does not appear at the expected intrinsic frequency in the frequency spectrum measured by a stationary probe. The advection of the wave number component by the orbital current of background waves produces a net downshift in the encounter frequency. Comparison with laboratory measurements shows that the effect is important in low-wind conditions. As wind speed increases, the frequency downshift due to orbital advection becomes less recognizable.