KE7IZL
Member
I received what appears to be an FSK signal just above 124kHz. I have this small air-core magnetic coil antenna (not one tuned for LF or VLF reception, but it's useful for picking up near-field VLF and LF magnetic signals like holding it near a switching supply and picking up its electronic noise, while rejecting far-field actual radio broadcasts at these frequencies). It's literally just a spool of like 24-gauge wire (copper wire appears to be about 1mm in diameter) that's mostly used up now (and so I repurposed it as a magnetic antenna). The coil has about 25 turns on it and a diameter of about 3 inches and a thickness of about an inch.
As an experiment, I decided to just leave the coil sitting out (not holding it within an inch of a device to measure its near-field magnetic field) to see what things it could pick up that might be slightly more distant. And I found this curiously strong signal just above 124kHz using my AirSpy HF Discovery SDR. It appears to be an FSK signal with what initially appeared to be a standard 2 tones spaced 200Hz appart. But then I noticed that 200Hz below the lowest of those main tones it sometimes for a very brief period (a fraction of the time it normally would be on a tone), it had another tone. So it actually was 3-FSK not 2-FSK. Then I noticed there was no particular pattern to when it switched tones. No identifiable data rate, even though the it clearly seemed to be an FSK modulated signal. I received the signal in USB mode tuned to 124kHz, and the highest tone is at an audio frequency of 1kHz, the middle tone at 800Hz, and the low tone at 600Hz.
Since the antenna I used isn't designed for anything really other than near-field monitoring (even nearby AM radio stations barely are picked up), I assume it's a signal generated by some nearby (maybe within a few feet of the antenna) consumer elctronic device (possibly as an unintentional transmission from the device's internal circuits). I'm hoping someone here might be able to identify what might be generating this signal, so I've attached the WAV file that I recorded from HDSDR to this post.
Note that the attached file is a ZIP file containing the WAV, file as this forum won't let me upload WAV files directly. Also note that I had to resample the signal (original was 48000SPS, and that made the file too large to upload even when zipped, so I resampled it to 8000SPS, which still has more than enough bandwidth for this signal, and then it was small enough to upload).
As an experiment, I decided to just leave the coil sitting out (not holding it within an inch of a device to measure its near-field magnetic field) to see what things it could pick up that might be slightly more distant. And I found this curiously strong signal just above 124kHz using my AirSpy HF Discovery SDR. It appears to be an FSK signal with what initially appeared to be a standard 2 tones spaced 200Hz appart. But then I noticed that 200Hz below the lowest of those main tones it sometimes for a very brief period (a fraction of the time it normally would be on a tone), it had another tone. So it actually was 3-FSK not 2-FSK. Then I noticed there was no particular pattern to when it switched tones. No identifiable data rate, even though the it clearly seemed to be an FSK modulated signal. I received the signal in USB mode tuned to 124kHz, and the highest tone is at an audio frequency of 1kHz, the middle tone at 800Hz, and the low tone at 600Hz.
Since the antenna I used isn't designed for anything really other than near-field monitoring (even nearby AM radio stations barely are picked up), I assume it's a signal generated by some nearby (maybe within a few feet of the antenna) consumer elctronic device (possibly as an unintentional transmission from the device's internal circuits). I'm hoping someone here might be able to identify what might be generating this signal, so I've attached the WAV file that I recorded from HDSDR to this post.
Note that the attached file is a ZIP file containing the WAV, file as this forum won't let me upload WAV files directly. Also note that I had to resample the signal (original was 48000SPS, and that made the file too large to upload even when zipped, so I resampled it to 8000SPS, which still has more than enough bandwidth for this signal, and then it was small enough to upload).
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