antennas for 8.2 MHz Tag detection

[AndreyG]

Hello!

I am trying to figure out design of 8.2 MHz EAS tags detection system (that is for passive tags most frequently used in retail). Also known as Checkpoint tags.

Appreciate any pointers, materials and design examples describing specifically RF-part and antennas.

 

[Warpspeed]

Your best bet would probably be a multi turn loop antenna tuned to resonance.

The loop could be made quite small, or surround an entire enclosed area, depending on your requirements.

 

[AndreyG]

Thank you Tony,

Most of practical systems have rather large antennas. And area of detection - distance between the antennas is not much longer then antenna height.
They are the typical items in stores, shopping malls etc.
Picture below shows basic principle of operation.
There are some tricky details though: figure-8 turns, two figure-8 turns, combinations of different wire layouts.

I am learning all that as we speak. Appreciate pointers and examples

 

[Warpspeed]

Yes, there are many applications such as small hand held readers, door entry readers and whole large area (warehouse) readers.

Basically it is as you suggest, a multi turn coil with dimensions suitable for the application.

A single loop ("O" loop?) will be sensitive to nearby signals, but also to signals at a distance, but with reducing strength with distance.

A crossed over double loop ("8" loop) will be sensitive to very near signals, but be much less sensitive to signals at a distance. Crossing over two loops like that significantly cancels distant signals.

It would be quite effective where you have several door loops operating side by side as in your illustration, so each door loop would then be more independent than with a single "O" loop design.

If you have only one door to cover it probably does not matter.

 

[AndreyG]

Also there are different loops - some transmit and others receive. It can be one transmitter and two receivers or two transmitter and one receiver. And when arranged as single unit there still two independent loops inside the unit.
Most pictures I saw so far use single turn loops or several loops folded inside each other: o-type as you said and different figure-8's. Yet every 'figure' powered up separately.
It is totally different then 13.56MHz systems.

 

[FvM]

I presume the double loops are necessary to comply with EMI standards which define limit values for magnetic field in 10 m distance.

 

[betwixt]

I have seen systems like this with nothing more than a coil and diode in series as the 'tag'. I'm not sure but maybe some of these systems rely on pumping at fundamantal frequency and looking for harmonics to detect nearby tags.

Brian.

 

[AndreyG]

You are right Brian: the Tag is very simple device. Not even a diode – it is coil and capacitor making parallel tank tuned at 8.2 MHz. The transmitter transmits swept frequency (typically from 7.4 to 8.8 MHz). Output of the receiver is flat when there is no Tag between the two and outputs pulses at the sweep repetition frequency when Tag is present (due to change in transfer function at Tag resonant frequency.) That’s the basics. On top of it are mysterious details meant to optimize performance I suppose. Various aspects of performance. That’s the different antenna ‘figures’, phase shifts and … didn’t digest it all yet, digging through various documents.

 

[Warpspeed]

I have seen systems like this with nothing more than a coil and diode in series as the 'tag'. I'm not sure but maybe some of these systems rely on pumping at fundamantal frequency and looking for harmonics to detect nearby tags.

Brian.

I have seen systems like that, and those were my exact thoughts too. They seem to void these tags at the cash register by blowing them up with excess power. Having peeled off a few tags out of curiosity, they all appear to have been very violently burned out.

I did not know that they swept the frequency, but it sounds like an excellent way to do it. I suspect there are several different types of system in operation on widely different frequencies.

 

[AndreyG]

Some examples of antenna arrangements

 

[biff44]

some RFID tags DO have a diode in them. It doubles the frequency, so if the receiver receives ANY 2nd harmonic, then an alarm goes off.

But RFID tags that have data transmitted, like a short serial number or tracking number, often do just load a coil, and you can detect if the coil is loaded or not in the receive coil....kind of like building a giant coil Q meter.

Some tags (usually the ones above 3oo) MHz can actually phase or amplitude modulate the data and return it.

 

[FvM]

some RFID tags DO have a diode in them. It doubles the frequency, so if the receiver receives ANY 2nd harmonic, then an alarm goes off.

That's what I also have heard about 8.2 MHz EAS systems. There could be also a non-linear capacitor generating 3rd harmonic. Unfortunately I didn't yet see an overview about existing EAS systems and it's parameters. May be AndreyG is interested to compile it.

But RFID tags that have data transmitted, like a short serial number or tracking number, often do just load a coil, and you can detect if the coil is loaded or not in the receive coil....kind of like building a giant coil Q meter.

Some tags (usually the ones above 3oo) MHz can actually phase or amplitude modulate the data and return it.

All RFID tags that return information are using some kind of modulation. All 13.56 MHz systems are using so called "load modulation" with one or multiple subcarriers, generated by diving the 13.56 MHz carrier. See ISO 14443/NFC and ISO 15693 standard for details.

 

[AndreyG]

FvM, since you asked...

Description of the Transmitter Electronics
Each transmitter electronics is able to feed one LUCATRON antenna with a swept HF signal of
8.2 MHz. In order to avoid disturbances between the emitted HF signals, synchronization with other
transmitters must be guaranteed.
The standard version of the Aquila TX board allows the 85 Hz sinusoidal modulated 8.2 MHz HF
carrier signal to be:
− generated locally (oscillator circuit for master applications), or alternatively
− to be regenerated from an optically received swept HF signal of another TX (opto receiver for
slave applications).
The bi-opto version of the Aquila TX additionally allows the onboard generated (master) or received
and then regenerated (slave) swept HF-signal to be converted and optically transmitted in order to
synchronize additional transmitters.
The two opto transmitters used in the bi-opto version allow:
− a master transmitter to synchronize two other optically slaved (and possibly repeating) transmit-
ters.
− an optically slaved transmitter to be used as repeater to synchronize two other optically slaved
transmitters.
By using 3 bi-opto transmitter boards (TX) (one used as synchronization master and two as
synchronization repeaters) and 4 standard TX (as synchronization slaves), clusters of up to 7 TX
may be synchronized. This way up to 7 checkout or 14 exit-gates (in a row) may be configured with-
out need of a master rack.
Since the optically transmitted synchronization signal may be repeated only once, a master rack still
has to be used if more than seven transmitters have to be synchronized.
A block diagram and of the Aquila TX board are shown on the next pages. The board consists of a
digital part, an analog part and a power supply / filter part.
The standard version of the Aquila transmitter is fully equipped with the exception of the two optional
opto transmitters. It can be used as a master-TX for small installations (single or dual gates
equipped with one TX only) or as slave-TX for all larger applications.
The bi-opto version of the Mark 4 transmitter board is almost identical to the standard version. The
only difference is that the two opto transmitters (IR-LED's) are already mounted. The bi-opto TX
board can be used as a master-TX in medium installations (2 to 7 TX) or as synchronization signal
repeating slave-TX for medium to very large installations.



Receiver Electronics
The receiver board consists of a:
- Analog Part
- Digital Part
- Power / Alarm Part
1.1. Analog Part
The first input stage amplifies the received RF signal. If this signal is too large, the gain of the first
input stage can be reduced using jumper J4 (Narrow or Wide position).
The next stage is a band-pass filter having a frequency range between 7.2 to 9.2 MHz. If necessary,
the gain of the RF amplifier following the band-pass filter can be changed with potentiometer R112
(RF-Gain). An AGC is not built in, this gives a controlled RF amplification. The amplitude of the tag
signal is pre-regulated by a fixed resistor. The DSP synchronization is done through "air", that is
extracted from the received transmitter signal. A beat note circuit is implemented. This circuit is
inhibiting spikes, radio transmitters and other signals with a very high Q factor.
1.2. Digital Part
The analog tag signal is A/D converted and sampled. The DSP (40MIPS) filters the demodulated LF
signal and stores the result in a memory. It processes this data and if all alarm criteria are met, it
triggers an alarm.
DIL and rotary octal switches allow adjusting the software parameters and test positions.
1.3. Power / Alarm Part
Power is supplied to the receiver electronics by applying 20-26 VDC or 18-20 VAC to the power
supply/power filter part. The integrated filter is used to reduce any interference picked up on the
incoming line from the power supply.
An audible alarm (buzzer) is mounted on the filter part. Outputs for the antenna lamp and an exter-
nal alarm are provided.
The volume of the buzzer is adjustable with the Volume potentiometer (R426). A jumper (J10) on the
filter part allows setting the buzzer for continuous or intermittent tone. The duration of the audible
alarm is about 2 seconds. The duration of the alarm light is about 10 seconds.



And there are also Antenna matching circuit description to which is missing.
Antenna board:

And here not everything is clear yet. Antenna board connects to two loops. Each loop through it's own transformer. there is a capacitor for each loop to complete the tank and tuning cap - just one. 200 Ohm resistor on the output. I'd love to understand this board fully. Any hints?