Ferrite

An iron-based material used to absorb certain unwanted RF frequencies; used to allow NFC tags to work on metal surfaces.

GoToTags calls these NFC tags “On-Metal NFC tags”.

Effects of Ferrite with NFC

If metal is placed close to the NFC antenna the alternating magnetic field generates some eddy currents in the metal. These eddy currents create a magnetic field in opposite direction; it absorbs power, and leads to detuning of the antenna due to a decreased inductance and quality factor. Therefore, for proper operation in close metallic environment, it is necessary to shield the antenna with a ferrite sheet.

Field distribution around the antenna wire in an ideal environment without any metal near the antenna

Field distribution of the same antenna but with a metal plane near to it; the magnitude of the field strength has significantly decreased compared to the open air case which leads to a decreased operating distance
Effects of adding a ferrite sheet between the metal plane and the
antenna coil itself; the field distribution is still modified but the operating distance
recovers its original open air level

Ferrite Placement

In order to reach a proper shielding, the ferrite sheet must at least fully cover the antenna surface. It is even needed to have an overlay but not too much because otherwise it would tend to reduce the stray field strength. This trade-off is illustrated by the picture below:

Notes

Some of the above content was copied from NXP public documents and is their copyright.

Updated on August 15, 2018

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