9024 kHz, DSB, shortwave oddity, 03 April, 2020, 0420 UTC

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Jun 18, 2010
Mojave Desert, California, USA
This is an oddity on shortwave that I don't think I have seen before. If it is common then somehow or another I have missed it.

About 0420 UTC, 03 April, 2020, I noticed what appeared to be a DSB transmission on 9024 kHz. DSB is not a commonly used mode these days, so I looked a bit closer.

The signal did appear to be DSB, it also appeared to be a repeating audio loop. About every 21.6 seconds (start to start) the loop repeated, with about 1.6 seconds pause between audio segments. The audio was not understandable, either it was distorted, processed in some way, or encrypted. Longer observation showed that every 11th repeated segment included a longer pause, about 3.1 seconds. This made a repeating cycle of 11 segments about every 239.6 seconds.

Every segment transmitted appeared identical. Just the same 21.6 second segment (time includes the audio pause) over and over, with every 11th including an increased length pause.

I have uploaded a video of this signal to my YouTube channel. If you watch the video I include spectrograms of the audio loops, and visual comparisons of multiple examples. I also demodulate the signal using multiple modes, USB, LSB, AM, and NFM, to show the sound in each mode. And the last minute or so of the video is the signal demodulated with LSB in the left audio channel and USB in the right audio channel, for comparison.

Video here:

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