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From what I can find on the Internet about active RFID systems, it seems the reader sends out a signal at a certain frequency and if a transponder tag that operates at the same frequency is within range, it will activate and send its signal back to the reader. Quoting from the site I found this:

In a system that uses an active transponder tag, the reader (like passive systems) will send a signal first, and then the active transponder will send a signal back with the relevant information. (Source)

I understand this concept at a very basic level but I am wondering why transponders that operate at 2.4GHz are not activated by being within range of a wireless router? If they respond to RF waves at that frequency wouldn't they be activated and send out a signal, albeit one that would just get ignored?

I have no background in this and am just starting to learn about RFID so if anything I said is fundamentally flawed or unclear I apologize.

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  • \$\begingroup\$ RFID tags need to be in very close proximity of the reader. The 2.4GHz signal from a WiFi access point drops of with the square of the distance, and would be too weak unless you brought the tag right up to the WiFi antenna. Even then, the tag would recognize that it wasn't a reader. \$\endgroup\$
    – Mark
    Jul 9, 2016 at 5:15
  • \$\begingroup\$ Ok, thanks Mark. But there are active readers that can have a range of 30+ meters As seen here. Are you saying an access point doesn't produce the same amount of power? And then can you elaborate on HOW the tag would recognize that it isn't a reader? \$\endgroup\$
    – dunkmann00
    Jul 9, 2016 at 16:23
  • \$\begingroup\$ Most likely, the signal sent out by the reader contains encoding that allows the transponder (tag) to reject commonly present signals which do not come from a reader. Also, RFID systems are normally very narrow band. They may not actually even be on the same band as wifi. \$\endgroup\$
    – user57037
    Jul 9, 2016 at 17:44

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The tag is not activated by just any RF signal, even one transmitting on the tag's preassigned frequency. The tag receives an RF signal that has certain information encoded in it. If the RF frequency is within the tag's preassigned frequency, and the received signal contains a code that matches the tag's preassigned "turn on" code, then the tag will in turn transmit it's data.

The chances that a random wifi signal will match both the frequency and codeword requirements of any given tag are vanishingly small. By design.

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  • \$\begingroup\$ Ahh that makes a lot of sense. Thank you for the complete answer AndyW. I was wondering if there was more to activating than just frequency but could not find that information anywhere. As a separate question then, if you do not know what a tag's preassigned activation code is, is there any way to discover it? \$\endgroup\$
    – dunkmann00
    Jul 9, 2016 at 20:22
  • \$\begingroup\$ Tags are manufactured with many different activation codes, and are configured at time of manufacture. So you can't tell just by looking at the tag model what the activation code is. The length of the codewords could even vary, so an exhaustive search usinga particular tag, assuming you could generate an RF signal of the right frequency, could also take a very long time. \$\endgroup\$
    – AndyW
    Jul 9, 2016 at 20:42

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