Any idea what might be causing this signal to appear in the "panadapter" spectrogram of my Omnia SDR around the 40m band?
Every twenty seconds, I get a strong "focused" signal that then rapidly spreads out its power — like all the power is concentrated at one frequency but then gets "spiralled" out within a certain (relatively narrow) bandwidth. This signal seems somewhat symmetrical, and is repeated every 12kHz or so across my waterfall.
Its exact characteristics/"sound" seems to change occasionally — for example tonight it repeated fairly consistently every 20 seconds for a long time, then went quiet for a few minutes, then gave one isolated (and slower) buzz. A few minutes later, it began repeating every 20 seconds again. This sort of thing happened a few months ago too.
I live near a power substation, and have some neighbors who weld, but neither seem a likely culprit, particularly given the usual streaks of consistent repetition of a shaped signal.
Given its symmetrical and repetitive nature, am I right to suspect there is some sort of intermodulation involved here — i.e. the actual noise source is simply a strong sweep that's getting mixed with something in my SDR? Then what are some ways I could determine its actual frequency and get a better/cleaner look at it? And any guess as to the source based on this information? Might it be some sort of radar, or more likely just somebody's malfunctioning digital panini press?