ST25R39 (Reading next to metall surface )

[Stasevich]

Is there any way to get an antenna working in reader mode (chip 3911) when placed very close to a metal surface (around 2–3 mm)?
I did some experiments with NFC in modern smartphones — their antennas sit right on top of the battery surface, and everything works fine there. But when I tried the same idea with the 3911 and 3916 evaluation kits, the antennas turned out to be pretty much ineffective.

[AScha.3]

>when placed very close to a metal surface 

What metal ? magnetic (steel) or not (copper, Lithium...as the battery is) ? with high loss or low impedance?

This might be the important point, try non magnetic and an LiPo near the antenna .

Also size and shape of the conductive area rel. to the antenna form and size is important,

to modify the 3-D form the field gets as result , not just being an absorber for the field.

Just think on the RF antennas, small difference in size makes a reflector or a director for a certain frequency:

...and the optimum distance to get the desired effect.

[Ulysses HERNIOSUS]

Hi,

typically you would be using a ferrite. https://www.st.com/content/st_com/en/support/learning/essentials-and-insights/connectivity/nfc/nfc-touchpoints.html could be a start for your investigation here. 

BR, Ulysses