What are these downchirp transmissions ?
Hi,
Has someone any clue about these unstabilized carriers, exponential down chirping, often periodic in the 11m band ? Some have a "wobbling" structure when heard in SSB. It seems to come from central or eastern Europe.
73, Thierry
Comments
Hi! If you are referring to these "scratches" or "worms" on the waterfall, then most likely it is the work of an industrial installation for high-frequency plastic welding. There is a plastic processing factory not far from me. I see such interference too.
On the other hand, it is believed that this is the work of "Ionosondes". We have discussed this topic for a long time, but have not come to a clear conclusion about the origin of such interference.
Hi @studentkra ,
Yes, your explanation makes sense, as we are in an ISM band. I thought this kind of setup was shielded enough to prevent listeners to receive it thousands of kilometers away. 😄
But the high power used, and higher bands opening allows reception far away, and also they don't need a frequency stabilization. Everything is clear now !
Definitely not the shape of ionosondes, with a -linear- frequency sweep, sometimes 3 of them a few seconds apart. Neither the digisondes with their phase modulated bursts, jumping upward, easy to identify.
Thank you for your answer.
73, Thierry
Looks like Citizens Band radios to me. There are a lot of truckers running big rigs with under-current power supplies that will do that. When the high-band propagation is good the 11m band (27MHz) is full of those "variable-frequency" transmitters. That the tuckers don't know their transmitters are doing that is the amazing thing.
@F5AFY ,
Yes. It seems that these plastic welding machines have a lot of power. I've also studied their specifications. The frequency range is 27-28 MHz. I'm even receiving a fifth-harmonic signal at 137-138 MHz, which interferes with the reception of NOAA and Meteor satellites.
I think there was a discussion about this a long time ago. There are indeed industrial drying machines, amongst others as @studentkra mentions, running huge RF power that cycle on & off to dry wood and other materials. Since they're built as cheaply as possible frequency stability is probably not high on the agenda. They're not supposed to leak RF either, but you know how that goes..
@bwilson Hi, I know unlocked PLL can do funny things and what you suggest certainly occured in time, but these chirps are so numerous and frequent, and often periodic with a few seconds cycle that it makes very probable an industrial process.
"Fish Hook Swisher" they call it, in this forum discussion :
Somewhere else, I read that it can use 10s or 100s of kilowatts, so shielding these owens and their supplies is not an easy task.
Thanks all for this interesting discussion !
These are typical of RF welding / Drying / Curing processes. Very high power RF sources are also used during some types of semiconductor manufacturing processes.
Another source, is from the cheap Chinese "Radio Frequency Facial Machine" or "wands". These typically produce between 5 to 25 watts, but some are rated at several hundred watts.
All of the above can usually be observed around most of the ISM allocations, and they are a good propagation indicator on the low VHF bands.
Here are a few that should really be in the 40MHz ISM band, most likely from Asia.
When I used to live in an industrial town, we had one in a factory that welded seams on industrial clothing, that operated on 27MHz, and the Interference Investigators were forever chasing the owners, as they often removed safety covers, so that they could run the machine for longer before it overheated. The stray RF signals and harmonics used to interfere with the 2m amateur band and local taxi services over a very large radius. Once you have heard the distinctive warbling, humming, and drifting signals, you can easily spot them again.
Regards,
Martin
Hi Martin,
Thank you for your contribution. There is no doubt now for me, about the origin of these emissions.
The forum link I posted above even give videos of these machines.
And yes, it's a good indicator of band opening.
73, Thierry