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RF based electronic article surveillance (EAS) tag operating at 8.2 Mhz

(by Matthias Franz, HB9EFY Aug-2009)

translation from the German original







8.2 Mhz RF surveillance tag

A few days ago I found on the street one of these electronic article surveillance tags. Being a radio amateur I picked it up in order to see what's inside. At home I carefully opened it with a plier. Beside 2 metal bushes a metal spring and 3 little metal balls it contained a nice LC tank circuit (picture 01 and 02).

Even you can easily find out the resonanz frequency of such a LC tank in the internet I wanted to messure it myself using the equipment I have at home.
LC tank from EAS
[01] LC tank from an EAS system
RF 8.2 Mhz oscillating circuit
[02] RF 8.2 Mhz oscillating circuit

The LC tank is made of a flat coil with 8 turns and a ceramic capacitor. The copper wire is wrapped with HF silk. As you can see on the photo the capacitor is a 101 type meaning its capacity is 100pf. Mathematical the coild should have around 3.76 uH to reach the resonance frequency of 8.2 Mhz.

The 8.2 Mhz are mentioned on the website of the company producing these RF tags. However - let's test this first.
To test the resonance frequency I first used an old fashion dipmeter.

The dipmeter I used is however not an old tube powered one. It's a quite new circuit from a German company called QRP Project®™. The dipmeter is working with a NF modulated carrier wave not with an unmodulated carrier as the old ones do.

The functional principle is to analyze the NF signal. By using this principle the dipmeter does not need to be calibrated. The resonance is signalized by a red LED and a NF ton you can hear when using a headphone.

To easily read the frequency, the circuit has a build-in frequency counter. The resonance frequency I measured was 8.11 Mhz.
To see this video you have to activate javascript and you need to install the Adope flash player ®™

Finally, picture 04 shows the last test with a HF network analyzer (NTW) from Funkamateur.de®™. The NTW was loose coupled to the LC tank with two 50 Ohm antenna-coils (picture 03).

The diagram shows the sweeping curve from 7.2 Mhz to 9.3 Mhz. The peak at approximately 8.13 Mhz is the resonance frequency of the LC tank.

sweeping the LC tank
[03] Sweeping the LC tank
Sweeping diagram
[04] Sweeping diagram of the LC tank

The final results are as follow:

The resonance frequency is in both cases a bit lower than the 8.2 Mhz the tank is made for. However, in the RF tag, there are these "locking" metal parts, preventing the tag to be removed from e.g. the clothes. These metal parts are increasing the frequency a bit.

That's the reason the measured frequencies are a bit lower than the official 8.2 Mhz this specific system is using. In addition the cheap ceramic capacitor certainly does not ensure the best accuracy.


Greetings,
Matthias, HB9EFY

[ HB9EFY(at)yahoo.de ]



Source:

QRP Project:
- DipIt, ultimate dipmeter

Funkamateur.de:
- Netzwerktester NWT01U USB

Copyright:

Text, pictures and diagrams of the "
Electronic article surveillance " - project are copyright protected by endorphino.de (Matthias Franz) in August 2009.

The following names:
Funkamateur.de ®™,
Berg Sicherheitstechnik ®™,
QRP Project ®™,
are protected trade names and/or registered trademarks.

The video player JW FLV MEDIA PLAYER is licensed as Creative Commons.

Useful links:


Basics "Electronic article surveillance":
Wikipedia: http://en.wikipedia.org/wiki/Electronic_article_surveillance

Producer of the RF tag "Hard tags RF":
Berg-Sicherheitstechnik: http://www.berg-sicherheitstechnik.de/en_warensicherung-zubehoer.php





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