Tag antenna design for ST25TV tag IC optimised for 10cm range

[PKrig.1]

Reader IC: ST25R95

Reader Antenna: 47mmx34mm, 4 loop, PCB antenna based on the reference antenna design from the X-Nucelo-NFC03A1

Tag IC: ST25TV02K-AD6H3

Desired read range: 10cm

I'm trying to optimise a NFC tag PCB antenna for a 10cm read-range for the system above. There are constrains on the dimensions of the tag antenna size. Ideal tag size for our application is roughly 25mmx50mm, however using off the shelf tags I only get close to 10cm with tags larger than 50mmx50mm or 30mmx80mm.

As I'm using the ST25TV I've seen that I should tune the antenna for 23pF and hence Lant = 5.95uH. Using the eDesign tool I can play with various parameters to get this resultant inductance.

However, there are many options like trace width, trace spacing and number of loops which play a part in the performance and read range of the resulting tag. Of course, tag antenna size / surface area play a significant part.

Are there any rules of thumb that can help optimise for smaller size and long read range?

As an example the below tags have the same antenna dimensions 47mmx47mm, but different read range:

Top: ~10 cm

Bottom: ~7 cm

[Henry Crane · Accepted solution]

Hello Peter,

many different parameters will affect the read range of a tag.

For a given reader and and given tag IC:

1. the tag tuning (resonant ) frequency shall close a possible to 13.56MHz(or slightly above): for ST25TV with 23pF tuning capacitance, this crietria leads to an equivalent inductance of 5.9µH
2. the total surface of antenna turns shall be maximum to get maximum voltage from field strength. this shall be achieved respecting 1). For a given equivalent target inductance value, the larger the antenna is, the less is the number of turns, but the larger is the total surface of the antenna turns.

So, roughly, for a given reader, a given tag size will lead to a given read range.

Increasing the read range requires to increase the antenna dimension.

Concerning the form factor, a square antenna is recommended compared to a rectangular one, unless required by application available space.

In case tag size is limited, increasing the reader size will help. You could design a reader antenna with 1.2µH inductance (same as X-Nucelo-NFC03A1 antenna inductance). Thus, you could evaluate new reader antenna without the need of retuning it.

feel free to tell us more about your investigations,

Best Regards,

Henry Crane, RFID support team