I happened to be looking at the various receivers at the Northern Utah WebSDR - as I'm wont to do (since I maintain them!) and noticed a few strange-looking signals that hadn't been there before:
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| Figure 1: Obvious QRM (interference) in the 20 meter amateur band. |
The first thing that I did was to check other receivers - both on-site and across the U.S. - to make sure that this wasn't some sort of local problem (overload, image, nearby source) and found it elsewhere - but the selective fading visible in the waterfall display make me quite sure that this was ionospherically propagated. The errant signal was practically nonexistant in the Eastern U.S. - but with the known skip distance of 20 meters, that might have meant that those receivers were closer to the source, geographically.
When tuned in using AM, there was a very obvious audio tone (approximately 363 Hz) associated with the signal, but the RF signal itself wasn't stable. The tell-tale sign that this was more likely a spurious signal of some sort was the fact that this signal seemed to appear at intervals - roughly 66 kHz - so I decided to "follow the money", tuning lower in frequency and finding stronger and stronger instances.
Figure 2:
YouTube clip with audio from the errant spurious signal.
The above YouTube link is intended to convey what these spurious signals sounded like: The original modulation, in Spanish, can be clearly heard underneath the audio-frequency oscillation.
Adjacent to the 20 meter amateur band is the 22 meter Shortwave Broadcast Band and there I found the culprit: A Radio Habana signal with the same sort of tone on it, symmetrically flanked by the same sidebands. Using the TDOA feature of the KiwiSDR network clinched the diagnosis. I tuned to one of the lower-frequency components of this signals and came up with the results, below:
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| Figure 3: Several TDOA runs on the WebSDR network all yielded the same results: The errant signal appeared to be coming from western Cuba. The main signal was not actually on 13563: It was slightly higher up the band - I just picked this particular spurious component because it was one of the strongest ones and "in the clear" - not atop another signal. |
Clearly, the program material (I could hear the announcers mention Radio Habana Cuba on several occasions) matched the location!
While writing this, I noticed that the spurious signal suddenly disappeared at around 1503 UTC: Perhaps someone noticed the problem and switched the errant transmitter off (or fixed something) - or maybe whatever it was that had been failing finally gave up the ghost?
Interesting!
This page stolen from ka7oei.blogspot.com
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