RapidNFC's ScanStrength is a graded index designed to indicate the relative scanning distance performance of the different NFC tag chips that we sell on the store. The scale runs from 1 to 10. The higher the grade, the greater the average relative scanning distance.
Each tag is placed into a specially designed low relative permittivity testing rig. We then measure the scanning distance using three of the most popular NFC enabled phones from Samsung, Nokia and BlackBerry.
To ensure accurate and reliable results, the tests are performed a set number of times with differing phone orientations and using multiple copies of the same NFC tags or product.
Results are then collated and a ScanStrength relative performance index from 1 to 10 is applied.
We use a relative index rather than actual distances because the performance distances between phones vary substantially. Even the orientation or angle of the phone can affect scanning performance. Providing an actual distance is likely to be misleading.
However, as a guide, minimum distances are usually less than one centimetre and maximum scanning distances are many centimetres. In other words, there's a substantial performance difference between tags.
A large number of factors affect scan distance performance. One of the most common misconceptions is that the larger the antenna (aerial), the better the performance. When using mobile phones to read NFC tags, this is usually not the case. A larger antenna often requires more energy and therefore the phone needs to be closer.
A second factor is the antenna design which in many cases, such as keyfobs or wristbands, may be restricted.
And then there is the chip itself. In much the same way as mobile phones have different battery performances, there are significant differences in chip performance and the energy required to power them. The greater the energy required, the closer the energy source needs to be. This can be attributed to chip architecture, such as the Ultralight C's cryptography functionality, but also to overall design of the chip itself.
Generally, we would advise that you select the right chip for your application first, then decide on the shape or format. Clearly, if you are purchasing wristbands or keyfobs your options are more limited, but for NFC stickers, you can use ScanStrength as part of your decision process alongside design (printed, clear, white) and size.