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From the answers to this question it's clear that RFID is passive and (therefore) doesn't work in the far field.
I was wondering if you couldn't use it remotely if you make an actual transmitter of the tag, sending a 125kHz carrier AM modulated with the tag's code. Wouldn't the reader pick up this signal?

edit Olin Lathrop mentioned one wavelength as a rule of thumb for the limit of the near field. But at 125kHz this is 2400m! Is the rule of thumb still valid for these low frequencies?

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    \$\begingroup\$ I doubt it - the reader isn't a receiver - it just watches the fluctuations in its own signal. \$\endgroup\$ – Majenko Jun 28 '11 at 10:47
  • \$\begingroup\$ @Matt - Then the question is: can it tell it's own signal apart from others at the same frequency? It's an interesting question. +1 \$\endgroup\$ – stevenvh Jun 28 '11 at 10:50
  • \$\begingroup\$ @stevenvh I would say yes it can, since its 'signal' is contained within the near-field of a coil, not radiated out through an antenna. \$\endgroup\$ – Majenko Jun 28 '11 at 10:52
  • \$\begingroup\$ @Matt - The coil is an antenna, and any antenna will have a near field and a far field effect. I can imagine (though don't actually know, otherwise this would be an answer) that loading the signal in the near field looks much like receiving one from the far field. Maybe we should wait until Olin is awake :-) \$\endgroup\$ – stevenvh Jun 28 '11 at 11:04
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You can certainly transmit 125 kHz with a signal on it, but most likely this won't be picked up as you expect by the reader.

The details depend on the exact reader design. The few I've seen the inner workings of didn't look at signal strength in their vicinity because they were always transmitting. They looked for changes in load on the transmitter. Basically the change in load was high pass filtered, then that signal attempted to be interpreted as the expected digital data stream. The tag sends its signal by varying the load on its receiving coil. As you can imagine, lots of things can go wrong with this so there are lots of retries and CRC checksums on each message.

I've heard some fancy readers modulate the outgoing field to encode a challenge number. The tag hashes this with its stored internal unique number and responds with the result. That prevents someone from easily copying a tag by getting it to dump its ID once. However, these kinds of systems are much rarer and I've never seen the details of such a reader.

In both cases as far as I know, the reader is looking for varying load on its antenna, which is basically a coil. Another outside signal at the same frequency would interfere contructively or destructively, depending on what the phase relationship happened to be. It's possible that the destructive interference could look like a higher load, but I've never tried that nor heard of anyone trying that. If you get the carrier phase wrong, I suppose the bits would look inverted, which probably would not be a valid message.

All in all I don't think this will work very well, if at all.

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  • \$\begingroup\$ is correct that many systems implement an authentication scheme (mutual or one way) so you may need to extend the range of both the reader and the tag. \$\endgroup\$ – Jon L Jun 28 '11 at 16:53
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The reader 'transmits' a strong magnetic field (and a weak electric field) which is why it decays rapidly with distance from the coil. The tag shunts it's own coil at a kew kHz in order to transmit data and this has the effect of changing the Q and/or the centre frequency of the transmitter's tuned circuit, and the reader sees this as a change in amplitude of it's own drive signal. So the reader will not really be sensitive to a distant EM source. If it were a good RF receiver, it would not be able to detect a tag beneath the noise given the very low depth of modulation - which is typically less than 1%.

You might disable the reader's own oscillator and receive something with a powerful enough transmitter but if the reader relies on it's own oscillator for synchronous detection this won't work either!

Postscript: Olin's answer just beat mine but he's correct!

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If you're looking to re-use an RFID antenna to also do long-range RF, check out http://www.dash7.org/. Apparently because it works on the 433.94 MHz frequency, it can use the same antenna as a 13.56 MHz RFID reader. This could be NFC or ISO 15693. It still seems to be in its early stages but could be very exciting if it takes off.

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