Design of MF RC500 Matching Circuits and Antennas

ENVIRONMENTAL INFLUENCES
Metallic Antenna Environment
Any alternating magnetic field induces a voltage in metal components positioned nearby the reader antenna. This induced voltage generates eddy currents in the metal plane. These eddy currents cause loss combined with a detuning of the antenna and decreasing of the magnetic field. The result of these effects is a reduced operating distance as well as possible transmission errors.
It is recommended that the distance between antenna and massive metal components is at least as large as the operating distance
To avoid negative influences of a metallic environment a ferrite shielding should be used.

The antenna distance from massive metal should be at least 10cm for full R/W distance, 3cm for reduced R/W distance and for close metal ferrite shielding is a must!
In all cases the tuning of the antenna has to be made with the metal placed in the finally intended position.

Multiple Antennas
Antennas are resonance circuits with a high quality factor and tuned to the operating frequency. According to the reciprocity law a good transmitting antenna is also a good receiving antenna and vice versa. This means that an antenna positioned close to the used reader antenna and tuned to the same frequency dissipates energy from the field. This causes a detuning of the antenna and a reduced operating distance. If two active antennas for an MIFARE® application are positioned in a close distance a communication to the card will be disturbed.

Multiple MIFARE  R/W antennas should be at least 30 cm away from each other if they are magnetically shielded and 10 times of the antenna radius if they are not shielded!

Temperature
The R/W antenna may be detuned as a consequence of temperature drifts of the electrical parameters of the antenna itself and the matching circuit. This will result in a reduction of the transmitting power available at the antenna. The consequence will be a reduced operating distance.
Measurements have shown that these influences can be neglected when appropriate components with low temperature coefficient for the matching circuit (SMD capacitors with NP0 dielectric medium) are used.

ANTENNA SHIELDING, COMPENSATION
Three different concepts shall be discussed.
–Electrical Shielding: The electrical shielding absorbs the electrical field generated by the antenna coil as well as the electrical field of the reader PCB.
–Compensation: Compensation should be used to reduce common mode earth currents.
–Ferrite: Shielding Ferrite Shielding should be used if metal has to be placed very close to the antenna itself. This metal, e.g. metal housing of the terminal generates eddy currents. The effect of the eddy currents is a dramatically reduced operating distance. A ferrite shielding should be used to reduce the generated eddy currents.
Note: Ferrite shielding will not increase the operating distance above values achievable in non metallic environment.

ELECTRICAL SHIELDING
Directly Matched Antennas
An electrical shielding should be used to reduce the electrical field generated by the antenna coil itself. To build a shielded antenna on a PCB at least one with 4 layers should be used with the shielding loop on the top and the bottom layer. These loops must not be closed. The loops provide electrical shielding and improves EMC behaviour. The shielding has to be connected in one point to system ground. The coil is routed in the first inner layer. The centre tap of the coil is done with the marked Via to GND. The connection of the coil ends to the matching circuit shall be routed close together, to avoid additional inductance.

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Figure 5-1. Electrical shielding for a directly matched antenna

50 Ω Matched Antennas
An electrical shielding should be used to reduce the electrical field generated by the antenna coil itself. To build a shielded antenna on a PCB at least one with 4 layers should be used with the shielding loop on the top and the bottom layer. These loops must not be closed. The loops provide electrical shielding and improves EMC behaviour. The shielding has to be connected in one point to system ground.

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Figure 5-2. Electrical shielding for a 50 W matched antenna using a triax cable.

On the top and bottom layers of the PCB a shielding plane is placed directly above the active antenna loop which is an inside layer of the PCB. These shielding planes must not be closed loops! The shielding should be connected with a triax cable.

COMPENSATION

To compensate the stray capacitance of the antenna another turn with an open end is added. Due to the transformer’s principle the induced voltage in the open loop is inverted. The stray capacitance of the active and the compensation loop have nearly the same value. The effect will be, that the current across these capacitance has nearly the same magnitude but opposite direction. By that a compensation of these currents is done. These currents can reach values in a range of mA at 13.56 MHz, so compensation is necessary to avoid problems with ground currents.

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Figure 5-3. Compensated 50Ω antenna

To compensate the stray capacitance of the antenna another turn with an open end is added. Due to the transformer’s principle the induced voltage in the open loop is inverted. The stray capacitance of the active and the compensation loop have nearly the same value. The effect will be, that the current across these capacitance has nearly the same magnitude but opposite direction. By that a compensation of these currents is done. These currents can reach values in a range of mA at 13.56 MHz, so compensation is necessary to avoid problems with ground currents.

FERRITE SHIELDING
The benefit of a ferrite is to shield an antenna against the influence of metal. A metal plane could be part of the housing of the reader or a ground plane of the reader PCB itself, which has to be connected very near to the antenna. If metal is placed very near to the antenna the alternating magnetic field generates eddy currents in the metal. These eddy currents absorb power, and lead to detuning of the antenna due to a decreased inductance and quality factor. Therefore for operation of an antenna in metallic environment, it is necessary to shield the antenna with ferrite.
The following examples should give an impression on the influence of ferrite for the distribution of a magnetic field.
For easy simulation a circular antenna has been used in all case. A circular antenna is rotation symmetrical to the x-axis. Therefore the simulation can be reduced to a two dimensional mathematical problem. The simulations shows on the one hand the field distribution of a non disturbed antenna. Common for all examples: Radius of the RWD antenna 7.5 cm, 1 turn, wire width 1mm copper.
Figure 5-4 shows the two dimensional field of the circular antenna. The right part shows the field distribution. The highest field strength is generated in the area of the coil. The left part shows the magnitude of the field strength H over the distance d. Marked is the line of a minimal field strength of HMIN = 1.5 A/m according to ISO 14443.

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Figure 5-4. Non disturbed field distribution of a circular antenna

Figure 5-5 shows the field distribution of the same antenna with a metal plane near to the antenna. Compared to the disturbed field it is obvious that the magnitude of the field strength has decreased leading to a decreased operating distance.

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Figure 5-5. Field distribution of a circular antenna with a metal plane

Now, as shown in Figure 5-6 a ferrite plane (μR=40) is positioned in between the metal plane and the antenna coil itself. The field strength very near to the ferrite increases, but this increasing of the magnitude is not combined with an increasing of the operating distance. This is marked once again with the HMIN value according to ISO 14443.

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Figure 5-6. Ferrite shielded field distribution of a circular antenne

These simulations show how the use of ferrite reduces the generated eddy currents in a metal plane. The ferrite generates an additional field component and the effect for the design of the antenna is a fixed detuning of the antenna itself. Figure 5-7 gives recommendations how to dimension the ferrite to find the optimum dimensions between ferrite plane and metal plane. To calculate the optimum dimensions of the ferrite plane and the optimum distance and overlapping is very hard and not recommended. Application specific tests have to be made to find the best ferrite dimensions.

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Figure 5-7. Estimation of the optimum ferrite dimensions

Test have shown that the best performance is achieved when the overlapping of the antenna coil and the ferrite is in a range of 5 mm. That gives a balance between needed stray field to communicate to the card and the shielding of the ferrite. Applying the distance estimation to specific applications, it is recommended to make test to find the best solution. Once again it has to be mentioned that ferrite does not increase the operating distance compared to a non-disturbed field.