How much power to detect an RFID tag?

[Mori0502]

Hi,
I'm currently trying to build a security system that's supposed to work like those EAS (Electronic Article Surveillance) systems in retail stores. The basic idea is that I have a transmitter on one side of the exit and a receiver on the other side. And if someone having an RFID tag on them passes the door an alarm goes of.
I already built and tested a Colpitts oscillator at 8.2MHz that connects to a 1.2m loaded antenna as the transmitter and a 1.2m loaded antenna as the receiver. I power the system with 9V and receive about 500mV peak to peak voltage when I place the transmitter and receiver ~1m apart.
For the RFID tag a purchased a 8.2MHz passive tag, normally used in retail stores, consisting of an inductor an a capacitor. When I now place the tag in between my transmitter and receiver, I don't notice a change in the received amplitude though.
Does anyone know why that is? Is my transmitter too weak?

 

[FvM]

Read about the characteristic of industry standard EAS systems. They are using frequency sweep to detect resonance of tag circuit. I won't expect to achieve sufficient sensitivity with unmodulated CW generator.

Generator power must be high enough to detect the resonance signal in noise floor but low enough to keep EMC standards. In the ETSI domain, a maximal H-field in 10 m distance of 9 dBuA/m is accepted for EAS systems in 7.4 - 8.8 MHz band (ETSI EN 300 330-1).

 

[vfone]

Looks like the TX power of the system is in the order of Watts, so just a Colpitts oscillator without any amplifier after it, may not do the job.

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[BradtheRad]

RFID (and wireless power transfer) reminds me of the old-fashioned 'crystal' radio receiver that needs no power source. It's powered by a broadcast picked out of the air.
See where you can make use of any and all techniques those projects use, to optimize your own results.

 

[betwixt]

I very much doubt the security systems use RFID at all. RFID perturbs the EM field to carry digital data, the principle described in post #1 is simply absorbs RF at the resonant frequency so as FvM states, it requires a swept frequency and monitoring of the antenna current to see if a resonant circuit is within range.

Brian.

 

[BradtheRad]

The local pharmacy entrance has objects that I presume is a detector system similar to what the OP describes. Like decorative handholds 4 feet tall to left and right as I walk through. Smooth and white. I can believe they disguise large coils of wire. And OP tells that his antennas are 1.2m (meters?).
Could this compare to the operating principle of a metal detector? Two coils inside the wand, oscillating jointly or separately. They are detuned by any object approaching close. A beat frequency arises which might be in the range to create audible tones in headphones. Similar effects should be possible with the coil-and-capacitor tank circuit as in RFID tags.

 

[FvM]

I very much doubt the security systems use RFID at all.

RFID and EAS are considered as a common application range, e.g. in quoted ETSI standard. RFID is expected to send back data, at least an unique ID, it respectively implements a transmitter circuit with modulator. Simple EAS tags send no information except their presence.