Laboratory measurements to quantify the effect of long waves on the modulation of short waves and the effect that this modulation has on radar reflectivity have been conducted at the Air-Sea Interaction Saltwater Tank (ASIST) facility of the University of Miami. The critical parameters for the modulation of radar reflectivity were directly observed using non-intrusive techniques. The local water surface elevation and the slope vector were sampled using laser techniques at a point. The two-dimensional distribution of surface slopes were also imaged at a rate of 120 Hz using an optical imaging slope gauge. Experiments were run with various wind velocities modulated by mechanical long-waves. Co-incident with these measurements, the radar backscatter was sampled over the same region of the surface using a dual-polarized C-band Doppler radar. When there were no long waves, the short wave structure as observed by the 2-D slope gauge was well developed with peaks in surface slopes corresponding to the wavelength of the peak wind waves and having a clear group structure. This organization was in sharp contrast to the variability of the slopes observed when a 1 Hz mechanical long-wave was generated in the same direction as the wind waves. The slopes linked to the wind wave were suppressed, while there was modulation at the 1 Hz long-wave frequency. Significant modulation of the C-band HH polarized radar return was evident at the long wave frequency.