There are chips that support this protocol:
https://www.qorvo.com/products/p/DW3110#parameters
Using all those pieces, it's possible to build a credit card size thin tracker that doesn't bulge the wallet like AirTag.
Who is building this? If you have the expertise in the field can you help me understand what it takes to build this?
[1] https://developer.apple.com/nearby-interaction/specification/
The right design would have the ability to reply to another device with an advertising beacon, which could work in a limited set of circumstances, with some tweaks to how the AirTag protocol currently works. Like, if you stuck your wallet in your pocket next to your phone or on a table next to your laptop.
RF ambient energy harvesters also exist, which use more generalized RF to gather energy in a generic way. Where RFID sends the signal intentionally that powers the RFID reply beacon, you could theoretically find something in the ISM that gives you enough power to somewhat reliably harvest that instead.
https://www.digikey.com/en/articles/rf-energy-harvesting-bat...
Not sure about the advertising beacon interval but if it's flexible enough you might be able to join the two concepts. The limitation is likely figuring out the right ISM band, and the right package (coil size) to harvest enough energy reliably to send out your advertising beacons.
Distance and directionality (phase) once again affects energy output, but also, you would need to build a coil to a specific size to the frequency you're trying to harvest. Not sure these ever match up for this application. Higher frequencies would allow for smaller coils, but with lower power output efficiency.
The math behind this is wrapped up in the definition of the Q factor, and this is very similar to how you understand radio propagation in general, if you're looking for something to research further.
But, my ELI5 to myself: BLE beacons are already using ISM, so if your goal is to retransmit an ISM signal that beats the noise floor you're using to harvest RF in the same range, efficiency is going to have to be phenomenal in your system design (or take advantage of some power differential).
There's been some interesting projects and papers on this in the last 10 years or so, and the UW had an interesting retransmission concept a year or two ago they built. Links appear to be broken now though.
Bicycle thieves know to look for and remove airtags now, I would definitely pay good money for something more covert, that looked like it was just a part of the bike.
Just because you can get a chip that fits in there, does't mean you can build a product...
I've already had to track it a couple times.
https://chipolo.net/en-us/products/chipolo-card-spot?cl=head...
Also, the Tile Slim tracker is very comparable:
https://www.tile.com/product/black-slim?pack-size=1&category...
It doesn't implement the U1 protocol, rather it's a Bluetooth LE-based tracker that works with any phone, but it fits your form factor. Buy one and peel it apart, or look at their FCC filing photos:
https://fccid.io/2ABXLT1601S/Internal-Photos/2ABXLT1601S-Int...
or a teardown:
https://youtu.be/Vl6EdSlNKQo?t=61
It uses flexible PCBs, thin rigid PCBs, and an extremely thin battery like this one:
https://www.ultralifecorporation.com/default.asp?LINKNAME=LI...
plus the processors, piezo buzzer, antenna, as well as injection-molded plastic shell.
To build this from scratch, you need an electrical engineer, a programmer, a manufacturing engineer, a PCB manufacturer, and an assembly manufacturer. Be aware that hardware startups are hard, you can't just throw up a webserver and scale later, you are probably needing at least 800 hours of skilled engineering effort, probably double that if you're new to electronics manufacturing, just to get a comedy-size PCB conglomeration which would get you tackled by security the airport, might have 3 days of battery life, costs $200 each, and can be found by an iPhone using the Find My network. Then you have to miniaturize it and get the BOM cost under $15 (which won't happen at quantities under 1,000) and the assembly tooling semi-automated so you can build it in under 120 seconds of hands-on operator time and then you can sell it for $29.99, until someone on Aliexpress rips off your design and sells it for $24.99. See https://www.ycombinator.com/library/47-product-advice-for-ha... for some less bitter advice.