Say I want to mark a treasure chest in the middle of a forest so I can come back years later to the approximate area and then use a scanner to locate it again.
Are there inexpensive, general-purpose radio location beacons that don’t require active power (meaning they can be triggered by an external RF signal, but don’t require their own battery)? Something like RECCO reflectors but available to the general public?
NFC (RFID) works like this. They are passive tags that react to an RF signal. They modulate and reflect back a portion of the energy they receive, to provide a specific message. You can buy tags on Amazon for very little money, and there are cell phones available today that can find them if you are close enough. If you want to find them from further away, you’ll need a little more specialized equipment…but that exists too.
Would that work for you?
I didn’t realize they worked from more than a few inches away. What kind of specialized equipment are you talking about, and do you know how much it generally costs?
You can get slightly more range - a yard or two - with Sensormatic theft deterrent tags. You’ll need to do some work to make the equipment portable, since its normally meant to be bolted to the floor next to a door.
It probably wouldn’t be too hard to hack the technology to get more range.
Passive RFID tags generally do have a fairly short range. They make long range ones but those usually have a battery.
RFID tags also aren’t designed to last for years and years. They are designed to last about ten years in a relatively controlled environment. Out in the wilderness the expected life is going to be drastically reduced.
Ironically, if you can still find someone that makes them, the early generation RFID systems would be better for you. Modern RFID tags have a little semiconductor circuit that is powered by a capacitor. The incoming RF energy charges the capacitor, then when it is charged enough semiconductor chip switches on and does its thing (generally either broadcasting a number coded into it or reading the incoming pulses and sending out a number based on that and whatever internal algorithm it is programmed for).
The first RFID tags were much simpler, though. They were just a series of tuned RF diodes. If a tag had several different diodes, each tuned to a particular frequency, all you had to do was sweep through the various frequencies. If you got a reflection from the diode at that frequency, then you knew that diode was installed. By choosing which diodes to clip out of the tag, you could generate a number equal to the number of total maximum diodes in your tag. In other words, if you had 16 possible diodes, you could make a number between 0 and 65535 based on which of those 16 diodes you chose to leave installed in the tag.
These types of tags quickly fell out of favor once the semiconductor chip became cheap and practical. The RF diode type is cheap, but being able to produce larger numbers means more diodes and more frequencies and that gets impractical very quickly.
I don’t know of anyone still making these, though you could roll your own (if you had enough RF electronics knowledge). They would also have the benefit of working at a much greater range than your typical passive RFID tag, and because of their simplicity, they will last a lot longer as well.
Hmmm, it occurs to me that a step recovery diode might be used with an antenna, and maybe a couple of tuned circuits, so that whenever RF energy at an appropriate frequency was received by the antenna, a harmonic was generated, which could be re-radiated and picked up. This avoids the problem of simply looking for a passive reflector, as the harmonics could be far enough away from the exiting frequency that even very low levels could easily detected without being swamped by the exciting energy.
For a cheap-esque transponder, you could get one of the RECCO passive avalanche reflectors; they’re on a whole lot of outdoor gear now. My guess is the detectors are fairly expensive, though. Looks like range through the air is 200m, so not too bad. Less if it’s buried under wet dirt, of course.
[I’m sure you know the other option is just use an accurate GPS when hiding the chest; that will get you within a couple meters if the woods aren’t too thick. Need to write down and keep the coordinates of course]
Map and compass will get you within a meter of the spot- is that close enough?
If a reflector is enough for you, just make one. The RECCO ones are only expensive because they need to be compact enough to fit unobstrusively on clothing.
All you need is three pieces of sheet metal or foil (either will work; the difference is mostly one of durability) fitted together at right angles to each other. Put the point where they all intersect in the middle, and it’ll work in any direction. Alternately, if you only want it to work from one general direction, point one of the openings in that direction.
As a reference point on distance for RFID - the state of Florida is using passive tags for Sunpass to do open toll metering on toll roads. So I have a sticker I place on my car window, and drive down the highway. It notices me, gets my unique ID, and charges my account. (We used to have active ones, and we had to replace the battery sometimes.)
That’s probably way more expensive than you want to deal with. I couldn’t guess what it costs.
So even if I made or got a reflector, what about the transmitter? I doubt I’d have the skill to build one of those…
So how hypothetical is your question?
With access to a lab full of test gear you could steal enough devices to hang together something that would work as a proof of concept. But if you really want to create the system you describe, things become much more limited. More fun in a way, but things like budget, technical ability needed to operate, environmental limitations (does it need to work in adverse conditions) weight, power etc all need to be folded into the requirements.