Saturday, February 22, 2014

31.65 MHz: Strangest Noise Yet

Previous solar peaks saw a lot of very baffling and intriguing noises via low-VHF skip. This stuff really appeals to hard core radio geeks.  Nothing like a new noise to make things interesting.

This go-round it's been pretty quiet. I think the reasons are lower solar activity and fewer stations using this largely vacated band.

On February 22, a fairly interesting noise appeared. Superficially it looked like a backward squiggle.  The classic squiggle appears to be caused by industrial RF welding and heating equipment, and it always goes downward.  This one went upward.

Notice the more normal squiggles on the frequency. When there's propagation, these appear all over the band from 30 to at least 33 MHz.

Lately I've been listening to "over 30" squiggles. They're not very squiggly at all.  They tend to be wider than those found on the 10-11 meter band. When received in FM, they make a low, purposeful growl at 50 or 60 Hz, depending on the power line frequency in the country of origin. Here's a typical audio plot right at the time the squiggle crossed the radio passband:

It's not known for sure why the power line frequency would appear, but there are several fairly obvious reasons for it.

The reverse squiggle in question, however, sounded different.  It occasionally made a vocal sound when crossing the radio passband. I decided that I needed to hear more of it.  I started using the SDR to track the signal, in FM mode, as it went upward.

This was not easy, but practice made perfect. Again and again, I chased this across the SDR screen.  Now you know how I spent a perfectly nice Saturday when I could have been engaging in healthy outdoor activity.  Oh well.

Tracking it produces something that has to have started as a voice radio transmission. At times, one hears the exact cadence and audio response typical of Spanish speakers using the "freeband" below 10 meter amateur. A couple of especially good tracking jobs even gave evidence of an echo box in use.

No point posting links to sound files, since these would still sound like weird QRM.  Rest assured, however, that they do NOT sound like industrial machinery.  Here's the recovered waveform, minus as much of the noise as can be removed without changing the signal.

The waveform envelope is very much like male human speech.  So are the frequencies heard. It's just zero intelligibility in anyone's language.

Cause?  I suspect a spur (spurious emission), though it could be some kind of weird intermod with "real" squiggles.

Dizzy stuff, this.