Sorry for the delayed response everyone. Thanks for your help!
If you want to know if it's through the air or through mains, try to find some battery to run it from.
That's exactly what I tried afterwards. On batteries there's no problem (although I've yet to formally check it's running at exactly the same power.)
You're sending enough energy through the air to light a neon bulb, I'm not all that surprised that it's causing equipment to malfunction. How far away is the router?
To be fair it only lights it if I'm adequately grounding it, but yeah.
Router: Very near. Less than 4 feet, certainly (it's on the other side of a wall, hence the ambiguity.) Same with the smart bulb (although that seems to lose it's settings at the drop of a hat, to be fair.)
I'm not surprised either. What bugs me is that by battery it was fine, suggesting mains acting as a conduit. Also, mobile, other equipment, not noticeably affected.
I would suspect that your modems power supply is flaky.
Also because its connected to both the phone line and any ethernet cable within your house it provides a massive antenna for any EMI in the area, its the one thats going to be most vulnerable to interference out of any other device in the house.
You could also be rebooting neighbours modems so be careful. Not to mention possibly damaging some devices.
Some elderly people use emergency bracelets.... hint hint.
I expected some RFI using it, hence just a few goes with it, although I didn't expect it to be as powerful as this.
Our internet is VDSL2 (copper to local telecoms outdoor cabinet) so I totally imagined it impacting that. It doesn't, it just tends to cause the router (which is also the VDSL2 modem) to reboot.
Risk of damage would presumably be related to distance, inverse square law stuff, would it really present a damage risk beyond just very nearby items?
Talking of emergency bracelets etc... with the fibre internet rollout here in the UK, the powers that be have only recently cottoned on to that the dact that the fibre setups that carry the telecoms here being installed are mains powered with no backup means all these emergency systems won't work during power cuts, unlike POTS did. FFS.
I didn't consider about the modem's supply being flaky! I might have a look at that and inside power circuitry. I think it's probably a linear mode one too by virtue of it's age. It's an old 24/7 device that will have seen lots of transients over the years.
High voltage high frequency is very dangerous for any electronics.
Both radiated and conducted EMI will be powerful enough to kill devis, cause RF bursn and disturb medical like pacemakers.
Can happen even for the tiny Chinese junk coils shown.
We run TC far from any devices and on separate mains feed.
The real thig is here:
https://themaverickobserver.com/tesla-eccentric-and-electric-teslas-tower-of-power-in-colorado-springs/
Old friend Greg LEYH: Modern largest TC
https://www.lod.org/
ZAP!
Enjoy!
Jon
Heh, the only (other) Jon I know also used to be a TC junky! (Jon in UK is normally spelt John.) Thanks for your input too, Jon.
I'm normally strictly a 12V 0.5A max kinda gal who reads every manual first; if I *have* to e.g. probe mains, it's with a Fluke, connected by screw terminals, at a distance, turning mains on remotely, flinching as I do it...
...you'll not be surprised to know, then, that the coil kit was a present from a friend with the devil in him. This little coil has peaked my curiosity now! I honestly didn't expect it to work, or I at least expected it burn the coil out in seconds or something.
Now I'm desperate to know what's going on with the RFI causing such an issue via the mains but not battery and, for that matter, what rough base frequency this runs at, and what voltage the coil is peaking at, and so on. Aaargh. And I can't do any of that without turning it back on, which I can't really do willy nilly because it's clearly more of a nuisance than I was expecting! I'm thinking about having one more quick go with a mains filter device I have on the PSU (capacitors plus common mode choke and MOVs IIRC), to see if it does prevent the noise that resets the router, and will pop the oscilloscope on as I imagine it'll pick up the noise through the air and then I might be able to see what frequency it is.
Thanks for your input everyone. So, it seems this tiny thing does have the capacity to reek havok after all, and problems in the vicinity are to be expected. Oops!