Many thanks to SWLing Post contributor, Trevor, who shares this link to a blog post from Daniel Estevez describing the characteristics of HF OTH RADAR:
Most amateur operators are familiar with over-the-horizon radars in the HF bands. They sometimes pop up in the Amateur bands, rendering several tens of kilohertzs unusable. Inspired by Balint Seeber’s talk in GRCon16, I’ve decided to learn more about radars. Here I look at a typical OTH radar, presumably of Russian origin. It was recorded at my station around 20:00UTC on 8 December at a frequency around 6860kHz. This radar sometimes appears inside the 40m Amateur band as well.
Above this post you can see a waterfall plot of the radar signal. It’s the wide red signal. It is 15kHz wide and pretty strong, but severely affected by frequency-selective fading. Looking at the IQ recording in Audacity, it is clear that the radar uses pulses with a 50Hz repetition rate. This explains the characteristic sound of this radar when received with a conventional SSB radio. It sounds similar to AC hum (here in Europe, of course).
