alt Hacker News> It can also be improved; I know BLE is not _that_ effective under rubble.
It's a tough problem to solve because you're up against the laws of physics and the very boring (and often counterintuitive) "Antenna Theory". Bluetooth is in the UHF band, and UHF isn't good for penetrating anything let a lone concrete rubble.
To penetrate rubble effectively you really want to be in the ELF-VLF bands, (That's what submarines/mining bots/underground seismic sensors use to get signals out).
Obviously that's ridiculous. Everything from ELF to even HF is impossible to use in a "under the rubble" situation because of physics[1]. Bluetooth (UHF) might be "better than nothing" but you're losing at least 25-30 dBs (which is like 99.99% signal) in 12 inches of concrete rubble. VHF (like a handheld radio) can buy you another 5 inches.
Honestly I think sound waves travel further in such medium than RF waves.
[1]: Your "standard reference dipole" antenna needs to be 1/2 or 1/4 your wave length to resonate. At ELF-VLF range you need an antenna that's 10k-1k feet long. You can play with inductors and loops to electrically lengthen your antenna without physically lengthening it, but you're not gonna get that below 500-200 feet. The length of a submarine is an important design consideration when deciding on what type of radio signal it needs to be able to receive/transmit vs how deep it needs to be for stealth.
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Using an AirTag on my dog's collar and WifiMan on my phone to track BLE signals, I could pick it up (just... not enough to interact with it, but enough to detect it above -100dbM) about 8 - 10m with a clear line of sight, and about 2m when she was in dense scrub.
So yeah, I imagine concrete is going to be even worse than a shrubbery, that said, might get some interesting propagation paths if there's metal surfaces in the rubble to reflect waves.
My ham license let's me transmit on 2200 meter and 630 meter bands. I'm not allowed to use a 1100 meter or 315 meter dipole, either.
And I can only eirp 1000mW. I mean, legally. I can't eirp anywhere near that can you imagine the losses?
You could make the phones communicate via sound, not sure how it would impact the battery nor its communication range. Lots of details would have to be worked out. You have to find the frequency that communicates the longest. Perhaps do short pulses of sound to save battery life.
What about muon imaging?
What about Rydberg sensors for VLF earth penetrating imaging, at least?
From "3D scene reconstruction in adverse weather conditions via Gaussian splatting" https://news.ycombinator.com/item?id=42900053 :
> Is it possible to see microwave ovens on the ground with a Rydberg antenna array for in-cockpit drone Remote ID signal location?
With a frequency range from below 1 KHz to multiple THz fwiu, Rydberg antennae can receive VLF but IDK about ELF.
IIRC there's actually also a way to transmit with or like Rydberg antennae now; probably with VLF if that's best for the application and there's not already enough backscatter to e.g. infer occlusion with? https://www.google.com/search?q=transmit+with+Rydberg+antenn....
Thanks for the amazing breakdown.
That means LoRa and cellular network won't help either... That's good to know.
Yeah, sound seems like a good option. There's two points I'm struggling with though: 1. Disasters often are accompanied with a lot of noise which I'm afraid would drown or interfere with the other sound waves. 2. Mobile devices might not be able to detect the sound waves that are broadcasted.
I'll need to do more homework about those.
We'll have more flexibility with recovery for sure, so maybe that can address the shortcomings of the sound signaling...?
It still has to be cheap-ish though. Because first responders already have MUCH better tools [1].
Again, thank you so much for this comment. If you have any other suggestions or feedback, please don't hesitate to open an issue on github. Your input is awesome.
[1]: https://www.dhs.gov/archive/detecting-heartbeats-rubble-dhs-...