While viewing the 40 m band with my SDRplay receiver I saw a strong ionosonde signal sweeping across the band, as shown in the above screenshot. The top part shows the signal on the panadapter view (horizontal axis is frequency, vertical axis is amplitude). The bottom part shows the waterfall display (horizontal axis is frequency, vertical axis is time, progressing downward). Signal amplitude is represented by brightness. The sweep shows here as a diagonal line. Other stations on the band are operating on fixed frequencies.
Data collected by the ionosondes around the world are publicly available. See my blog posting: Graphs of Maximum Usable Frequency from Ionosonde Data for more information. The collection and analysis of data from ionosondes is managed by the GLOBAL IONOSPHERE RADIO OBSERVATORY.
I captured a video of the SDRplay window that includes a sweep, as seen below. There is no sound in this recording. Normally one hears only a short chirp as the sweep passes through the receiver's audio bandwidth. On this view the 2.8 kHz audio bandwidth is the thin vertical column in the center. Other active signals on the band are seen as the up-and-down white vertical spikes.
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