DESIGN OF MF RC500 MATCHING CIRCUITS AND ANTENNAS
Basic Design Rules
The MF RC500 is a single reader IC. It is designed to achieve operating distances up to 100mm without external amplifiers. The design of the remaining passive RF part is straightforward. Firstly it has to be decided, which of the possible basic concepts meets the application requirements best. The design help shown in Figure 3-1 shall give a support for this decision. Two different concepts are possible to design an antenna and a matching circuit.
–Directly matched antennas can be used to build up small, complete terminals with a minimum distance between a reader and antenna. Possible applications could be an access control reader in a small housing or a handheld reader.
–50W matched antennas can be used for an easy solution to achieve long distances between the reader and the antenna using a coaxial cable. Using a coaxial cable between the reader matching circuit and the antenna itself, distances up to 10 m between these parts are possible.

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Figure 3-1. Design Help
Note: This design help is a first step. To achieve the aspired operating distance the antenna design itself and the environmental influences have to be taken into consideration.

Table 2 compares the different concepts and shows the needed components in more detail. Basically, there is a 50W matched or a directly matched antenna concept. For the 50W matched concept, a high-end solution to achieve an operating distance up to 100mm as well as a low cost solution for operating distances lower than 50 mm is available.
Table 2. Comparison of antenna concepts

Concept 50 W matched Directly matched
Full range Short range  
Reader MF RC 500  
EMC-Circuit same circuitry and values
Receiving circuit same circuit and values
Impedance Transformation using TX1 and TX2 using only TX1
Antenna Cable 50 W coaxial Short wire or directly connected
Antenna matching circuitry same circuit, but different values depending on the antenna size Same circuit, but different values depending on the antenna size
Antenna coil operating distance depends on the antenna size and environmental influences operating distance depends on the antenna size and environmental influences
Antenna shielding Shielding depends on the application, e. g. the housing and

It is recommended to use the shown concepts. The next part will start with an estimation of the achievable operating distance depending on the antenna shape followed by the design of the required circuit itself.

Estimation of the Optimum Antenna Size
The achievable operating distance for a MIFARE® system depends on several factors:
–Reader Antenna size
–Quality of the matching circuit for the given antenna
–Environmental influences
The next design step is to estimate the operating distance depending on the antenna size. The complete calculation can be found in Annex A.

The MIFARE® card is powered by a flux generated by the reader. The achievable energy for the card IC varies depending on the distance between reader antenna and card. As mentioned in chapter 2.3, the MIFARE® system 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 reader antenna and the card coil and both, the size of the reader antenna as well as the one of the card coil itself. Assuming that for a standard application the MIFARE® card has chip card dimensions, the card coil dimensions are fixed.

In Annex A it is shown that the maximum coupling coefficient k for a fixed distance between reader antenna and card coil is achieved, when the radius of the reader antenna is equal to the distance. The calculation is done for a circular antenna. If a rectangular or square antenna is practically in use a circular one with an equivalent area can be used for the estimation.

This result can be taken as a rule of thumb for the design of an optimum antenna for a given application.

Important Notes:
–The estimation, that the radius of the reader antenna should be equal to the achievable operating distance is only a 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.
–This estimation shows, that increasing the antenna radius will not automatically increase the operating distance. The energy transmission from the reader to the card is a limiting factor, which can be expressed with a minimum coupling coefficient of 0.3.
–The formula for the calculation of the coupling coefficient is not depended on the number of turns of the reader antenna.

Figure 3-2 gives an approximation of the R/W distance for different antenna sizes. It shows that the best R/W distances can be achieved with antennas of about 20cm diameter (R=10cm). Larger antennas provide no
bigger operating distance!

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Figure 3-2. Antenna Radius versus operating distance