127.7 kHz seems to be licensed to Tesla Motors.
It could be something as mundane as the PWM switching frequency of some electronic component under load, but neither the 12V power supply I'm using (150 kHz), which is anyway far from being at high load in these tests, nor the DC boost converter (75 kHZ) should be directly causing it. Also, the signal is strongly decreased if I further immerse the electrodes, causing the plasma reaction to stop, and it looks as if the smaller features rise and decay slower than the larger-scale noise.
Yesterday I made a couple animations from the data retrieved from the RF receiver. Every 'snapshot' covers a span of a few seconds of time. I'll try doing more tests later, but I'm not confident that overall this not yet another RF artifact of some sort. The sharp persistent peaks in the second one, for instance, are due to an external signal.
EDIT: so far it does not seem to be dependent on antenna length, DC boost converter voltage setting.
Currently I'm looking more at series of peaks spaced by frequencies in the order of kHz on relatively narrow ranges rather than doing measurements spanning very wide ranges. Violante et al. performed measurements up to 83 GHz in a way that makes looking at the small details difficult.
In any case, I tried using sodium bicarbonate (NaHCO3) as the electrolyte and I obtained about the same results, with peaks having spacing again of about 127.5 kHz. I'm not sure if this is theoretically expected from Holmlid, since his results were with potassium. But again, in completely different experiments.
NaHCO3 gave a strongly yellow light when the reaction was steady, by the way.
Most of the signal appears when electrolysis at medium voltage is performed (and the DC boost converter is already under load), but as voltage is increased (possibly giving slight plasma formation or micro-discharge) or a plasma reaction occurs, then the 127.5 kHz peaks appear as well. It does not seem to be related with the switching frequency of the boost converter as it remains fixed regardless of all the different audible noises the converter's circuitry makes under varying loads and voltage.
EDIT: for what it's worth, I tried checking one of the "bumps" (of those spaced by roughly 50 MHz) at about 1296 MHz and one around 327 MHz, but I couldn't find the same small feature there.
I also tried using a very sharp pencil (graphite+clays containing Si, Al, Fe) as the cathode, which made the plasma reaction easier to produce, but it did not bring any significant change other that it seemed more energetic.
Most of the signal appears when electrolysis at medium voltage is performed (and the DC boost converter is already under load), but as voltage is increased (possibly giving slight plasma formation or micro-discharge) or a plasma reaction occurs, then the 127.5 kHz peaks appear as well.
Can you once shield the antenna from all sides except the experiment. Also try to avoid looking at the wires!
It's not simple to shield the antenna given my crude setup. I tried moving it in real-time to check out the source.
I get a clearer signal if I put the antenna tip close to the plasma. For this I used a 12 cm whip antenna with a loading coil.
I tried to also put it close to the DC boost converter and I get a 75 kHz signal (the working frequency as stated by the manufacturer. See here). Once the reaction is going I only get random noise, however. Also, I think this only decreases under heavy load down to the audible range.
I tried measuring it again at 24 MHz (the lowest frequency that I can set with my RF receiver) and not surprisingly—in retrospect—it was sharper and better defined, and other switching-type noise was too. Below a comparison of such signal with one of the previously posted figures from actual papers.
This also allowed better sensitivity to possible nearby disturbances. I couldn't find (yet) other sources of 127.5 kHz noise, but other than the DC boost converter I found that my computer's power supply has a switching frequency of 70 kHz.
Also, previously I used the anode at +125V and the cathode at -125V. Both HV outputs from the DC boost converter have had their capacitor replaced in the past, but they have differing capacitances now (8.2 and 10 µF). I tried changing the setup so that the anode is at ground voltage and the cathode at negative voltage (at -250V) and only one capacitor is used, but no change was observed. The boost converter seems to operate more smoothly when only one output is used, among other things.
Eventually I tried to use a different program to "listen" at a high rate the time variation of one such peaks with 127.5 kHz spacing with amplitude modulation (AM), which can only be done on very narrow bandwidths. I used a program called CubicSDR for this. It turns out that the noise varies on-off at a 100 Hz rate. It looks as if this could possibly be something from 50 Hz AC, but I have no idea if it's the cause or a result of the 127.5 kHz signal observed during the plasma reaction.
I have attached the wav file to the comment.
From Wiki: "Modern DSP chip consumer radios often use a 'low-IF' of 128kHz for FM".
If the mixer stage of such a receiver is overloaded with broadband input noise, the signal you see might result.
I use an RTL2832U-based software-defined radio, and more in detail this one:
However, after more tests, it turned out that the power supply was the cause. I noticed that turning it off/on would cause the same signal for a short while. That yesterday I saw something reminiscent of 50 Hz AC inside the same signal was a hint. Here are the spikes I got typically when trying to turn it off:
After I tried using a VRLA 12V battery to power up the boost converter, the only noise of similar frequency that I got in the area was that of its 75 kHz PWM operation.
Just to make sure, I then tried to get again the spectrum of the noise generated by the plasma over the entire frequency range (24–1750 MHz) and I got results similar to those in the opening post. So at least this one does not seem to be PSU-caused. The yellow line is the background signal (for the gain setting used).
The left one was taken with the whip antenna at its minimum length, the right one with the antenna fully extended.
In summary, the 127.5 (or 128) kHz repeating signal at low frequencies was only a coincidence and was due to the power supply. The larger-scale features seem still dependent on (probably) the cable and antenna length. At the next (larger) scale level, there appears to be a real signal, which is apparently mostly broadband noise decreasing in intensity with frequency.
EDIT: I tried a different 12V power supply and the ~127 kHz noise did not appear with it during the plasma reaction. So it does not seem to be something dependent on grounding or similar issues.