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This application note describes how the correct antenna and measurement distances are chosen when HF RFID or NFC tags are tested with the Voyantic Tagformance HF. Using incorrect antenna size or measurement distance may lead into inaccurate results.
2 Voyantic Tagformance HF Antennas
There are two antennas included in the Tagformance HF system:
▪ C60 antenna is a loop antenna with 4 loops, loop diameter 60 mm
▪ C145 antenna is a loop antenna with 2 loops, loop diameter 145 mm.
3 Selecting the Antenna Size
There are no exact rules for selecting the correct antenna size for a certain tag. The antenna selection and measurement distance should always be verified for each tag.
As a rule of thumb, the best antenna sizes for tags of different size (referring to size classes in ISO/IEC14443-1:2008/Amd. 1:2012) are:
▪ C145 is better for Class 1-3 tags
o Class 1 is a typical smart card size, antenna dimensions between
64 mm x 34 mm and 81 mm x 49 mm
o Circle Class 2 and 3 tags have antenna diameter 32 mm – 50 mm
o Square Class 3 tags have antenna dimensions bigger than 35 mm x 24 mm,
but smaller than 50 mm x 40 mm
o Square Class 2 tags have antenna dimensions between 51 mm x 13 mm
and 81 mm x 27 mm
▪ Class 4 is at the borderline, circle antenna dimensions between 24 mm and 41 mm, square antenna dimensions between 35 mm x 13 mm and 50 mm x 27 mm
▪ C60 is better for Class 5 and smaller tags. Antenna dimensions smaller than 35 mm diameter in circle tags, and smaller than 40.5 mm x 24.5 mm with square tags.
The above list may not apply to every tag. It does not remove the need to verify the correct antenna and distance for each tag to be tested.
4 Optimal Measurement Distance for Tagformance HF Antennas
From the Tagformance HF antenna point of view, at optimal measurement distance the magnetic field distribution is even across the antenna area.
For C60 antenna the optimal distance is 15.5 mm.
For C145 antenna the optimal distance is 37.5 mm.
At optimal distances the error in the field strength measurement is 14% maximum. The maximum error increases when the measurement distance is further from the optimal. When the tag is positioned exactly at the center point of the antenna, the error is minimized.
The optimal measurement distance from the Tagformance HF antenna point of view is not always the best distance for each tag. The measurement distance must be verified.
5 Verifying the Antenna and Defining the Measurement Distance
The antenna and measurement distance can be verified with the threshold measurement and magnetic field strength. The measurement should be repeated at least in three points: at intended measurement distance, at 10 mm closer distance and at 10 mm longer distance.
The following figure (Figure 1) shows example results, when a wrong antenna size is used. When the distance between the tag and the antenna is increased, the magnetic field strength curve shifts. This indicates that at the smaller distance the tag was loading the antenna. In the example the tag is moved outside of the read range before the loading effect is minimized.
With a correctly sized antenna, the magnetic field strength curves overlap almost completely when the good measurement distance is reached (Picture 2). The same ISO14443A Class 1 tag was used in both tests.
6 Summary of Antenna and Distance Selection Process
1. Choose the antenna based on the size of the tag
2. Measure the magnetic field strength 10 mm below the optimal distance, at the optimal distance and 10 mm above the optimal distance.
- If the magnetic field strength does not indicate loading at the optimal distance, use the optimal distance in measurements.
- If loading is indicated, increase the measurement distance until the loading effect is removed; and test also with the other antenna.
- If the tag cannot be tested (too long range), move the tag closer until result can be reached; and test also with the other antenna.
- If neither antenna can be used with the optimal distance, use the antenna and the distance without loading effect, closer to the optimal measurement distance to that antenna.