The performance of any given modulation on high-frequency channels reflected from the ionosphere will depend on the attenuation and time-frequency dispersion representative for the propagation medium. Spread-spectrum modulation has the basic function of spreading an information carrying signal over a very wide bandwidth. The ionospheric mode will, however, tend to limit the maximum useful bandwidth because of dispersion. The purpose of the experiment was to obtain some practical knowledge of the effect of time dispersion on spread-spectrum modulation and to obtain an indication of the bandwidth offered by the propagation medium.