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Comment by columbo | Hacker News Reader

Couldn't you do this with RFID tags and a scanner on your phone? I don't know if this is legitimate or not, but something like this: http://agbeat.com/tech-news/rfid-enabled-devices-keep-tabs-o...

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Regardless of all of the technical arguments about RFID range, the idea behind this product is that it beeps, so you don't need fancy locating equipment.

I don't know of any passive RFID tags with enough left-over power to run a loud beeper.

You would need a strongly directional antenna and user input (or an antenna array) to find the tag using the phone itself, which would be a relatively bulky and expensive piece of equipment.

If you're just trying to find something using radio, a 100% passive system like http://en.wikipedia.org/wiki/RECCO is simpler and cheaper anyway.

I'm not sure why they say these are RFID-enabled; all the tags they show are BTLE. The difference being that BTLE devices are active transmitters (they have a battery), and RFID tags are passive, having their energy provided via radio waves or magnetic coupling.

The problem with RFID is that past a few feet, you need a lot of energy to power them (it falls off with the inverse-square law), which your phone just won't (and shouldn't) put out. So RFID is dead in the water for this sort of thing, but BTLE solutions abound and are quite cheap, despite that they need to have a battery.

IIRC RFID actually falls off with R^4 because the power gets transmitted to the RFID tag and then it's reduced by the same factor again when it transmits back.

The power is obviously lost on the way back, but this doesn't actually factor into the distance you can read from, practically speaking. If you can get power to the device, you can use a super high-gain antenna to receive its transmission. Same thing goes with NFC chips; the one I have implanted can only be powered feasibly from about 10 inches away (physically impossible beyond about half a meter, due to flesh and the fact that it's magnetic coupling), but if something else powers it, you can read it from an exceptionally large distance.

Well, yeah, by the same logic you could could power it with a high gain antenna as well and boost your distance. The falloff is still 1/R^4 without a third power source, you've just multiplied it by a constant.

The limiting factor is (afaik) getting enough energy into the tag to run its components. If you were working with much higher distances and powers then the transmission strength of the tag might matter, but in this case it's basically a one-way transmission problem.

compare O(n^2 + .0001 n^4) where n is 5

Implanted?

RFID works at very close range (think swipe cards to get into an office). Readers with larger ranges do exist, but they get very pricey.

RFID is basically an application of radar, and as such suffers from an inverse fourth power dropoff with range. Radio is inverse square, but when you reflect it off something, you suffer that inverse square twice. So while doubling the range drops your received radio power by a factor of 4, it drops your received RFID response power by a factor of 16. To have more than a tiny range, you need a lot of transmitting power and a really good receiver.

No, my father designs RFID readers and antennas. The long-range UHF readers reach 9-15 feet (with passive credit-card size tags).

The cost of that type of system would be no where close to acceptable for personal use.

Yeah, but how big is the reader?

A lot bigger and stronger (rf) than my cellphone- I'm sure.

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