The coherence of successive dwells from an Over-the-Horizon (OTH) High-Frequency (HF) radar (3-30 MHz) is investigated using a technique based on the spectrogram. Although the data are coherent within a dwell, it is not known if coherence is preserved from one dwell to the next, due to possible limitations of the signal processor. (Incoherence imposed by the propagating medium is not considered here.) Land clutter consisting of a sufficiently clean complex sinusoid should reveal the coherence of data across dwells. Unfortunately, the ionosphere imposes multipath and distortion such that it is difficult to obtain sufficiently clean dwells, but a few cases indicate a linear spectral phase offset on the second dwell consistent with a virtual shift of the time origin that can, in principle, be easily compensated for.