What is ST25R3918 max range with 43x43mm antenna?

[KMadry]

Hi,

I'm new with RFID technology especially when it comes to designing antennas on boards and calculating matching circuits but... I have successfully designed and manufactured my board with ST25R3918 and I have a few questions.

I have a 43x43mm on board antenna with other electronics inside it, I have done 0.5mm spacing between antenna and ground polygon surrounding ST25R, uC and some serial communication ICs. Now I have about 8.5cm range with the 5x5cm tag (ST25TV02KC with ISO15693) from ST25-TAG-BAG-UB but only 3-4cm range with the button tag from that bag. Is there anything that I should do to increase the range? I have thought about two things: Firstly, redesign the "internal" circuitry to increase the spacing between antenna and ground. Secondly, change the RFID reader to ST25R3916B with greater output power (1.6W and ST25R3918 only 0.5W) but I don't know if it is possible to just desolder ST25R3918 and solder ST25R3916B in the same place(Can Auto Tuning circuits be a problem?).

Next question is about the range of RFID reader or TAG placed in a plastic case screwed to the metal base. I have read that there are some ferrites like: Flexible Sintered Ferrite Sheet from Wurth Elektronik that I can apply on metal and it could increase the range or it could be at the same level (8.5cm). Could someone give me some more advice and explain it a bit?

Here is the simplified view of my board:

[Travis Palmer · Accepted solution]

Hello KMadry,

Increasing the spacing between GND and antenna is a good idea.

The magnetic field will induce eddie currents into the GND plane, which will cause losses. The bigger the distance will be, the smaller die eddie currents will get. I would also try to pull the antenna traces to the edge of the PCB. (Octagon shaped antenna)

If you only want to read ISO15693 (ST25TV) tags, you could increase the Q-factor of your antenna.

What was the Q-Factor of your current antenna?

This could be done, by increasing the track width of the antenna. You have to decide based on the antenna parameters of your current antenna, (inductance and Q-Factor) if you can do less turns with thicker traces.

A change from ST25R3916/18 to ST25R3916B can be easily done. There is AN5768 which describes the change. AAT/Varicap can be removed if not required. According to AN5276 the matching can be adapted.

The ferrite is providing a lower permeability to the magnetic flux and therefore "shielding" the metal.

Basically it is like a low ohmic bypass for the magentic field.

You could measure the smith chart and read range with and without metal base and see if there is a noticeable difference. Based on this, you could decide if a ferrite sheet is required to recover the read range.

A read range of 8.5cm is already ok for a 43x43mm antenna. the difference is that the button tag has a much smaller antenna. what would be your target reading distance? More output power (higher Q-Factor and less losses) should already give a boost.

Please let us know, if you have additional questions.

br Travis