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Issue #211 February 2008
RFID Payment Terminal
by Carlos Cossio
Start | System Overview | Energy Transmission | 100% Amplitude Modulation | Load Modulation | MIFARE Card | MIFARE and ISO 14443 | Pay as You Go | Hardware Design | Antenna Design Rules | Antenna Size | Directly Matched Antenna | Firmware Design | Catch the Wave! | Sources & PDF
ANTENNA SIZE
The achievable operating distance for a contactless card system depends on several factors, such as the reader antenna size, the quality of the matching circuit for the given antenna, and other environmental influences. The contactless MIFARE card is powered by a flux generated by the reader. The achievable energy for the card IC varies depending on the distance between the reader antenna and the card. As I mentioned before, the contactless MIFARE card is based on the transformer principle. One important parameter describing a transformer is the coupling coefficient K. It can be defined as a geometrical parameter depending on the distance between the reader antenna and the card coil and both the size of the reader antenna and the one on the card coil itself. Assuming that for a standard application the contactless MIFARE card has chip card dimensions, the card coil dimensions are fixed.
As a rule of thumb, the maximum coupling coefficient K for a fixed distance between a reader antenna and a card coil is achieved when the radius of the reader antenna is equal to the distance. The calculation is made for a circular antenna. If a rectangular or square antenna is used in practice, a circular one with an equivalent area can be used for the estimation.
However, there are some important notes to take into account. First, the estimation that the radius of the reader antenna should be equal to the achievable operating distance is only the first step for a successful antenna design. For a complete design, the environmental influences, as well as antenna size limitations due to application-related restrictions have to be taken into consideration.
Second, the estimation shows that increasing the antenna radius will not automatically increase the operating distance. Energy transmission from the reader to the card is a limiting factor, which can be expressed with a minimum coupling coefficient of 0.3. And finally, note that the formula for the calculation of the coupling coefficient is not dependent on the number of turns of the reader antenna.
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