Choice of antenna

Maximum power transfer occurs when the Near Field Communication (NFC) tag antenna and the NFC poller antenna have the same shape and size.

To achieve the best performance, the choice of NFC tag antenna should be based on the application and the typical poller NFC antenna for that application. It could be a good idea to test the NFC tag with many NFC pollers.

Overcoupling is a physical effect in NFC which degrades performance. This occurs when the Poller and the Listener antenna have identical geometry and are aligned and placed at near zero distance. In some NFC products this problem is avoided due to the thickness of the casing that separates the antennas. Other NFC products use an asymmetrical antenna shape to ensure that when two identical devices are put together there is not a perfect overlap.

Figure 1. Asymmetrical antenna shape
asymmetrical antenna shape
The overcoupling effect should be taken into account when developing a product for certification. The NFC Forum has announced an initiative called the Tag Certification Program that is meant to establish a standard for NFC tag technology. Overcoupling can degrade performance or even lead to failing test cases when the antenna of the device matches the geometry of the Reference Poller antennas used during certification. A small difference in aspect ratio or in a single dimension is usually enough. You should avoid the following antenna geometries in a product.