Antenna measurements in a RFID application

Introduction

The SARK-110 antenna analyzer is a convenient tool for the measurement of antennas for HF RFID applications because features such as the graphical representations of the impedance, the determination of the sign of the reactance, and good accuracy characteristics. Besides, starting from firmware version v0.8.3.2 the analyzer implements a very convenient feature for this kind of applications, which is the automatic calculation of the equivalent circuit model of small loop antennas or coils.

The purpose of this application note is describing the step by step procedure of the use of the SARK-110 for this application using a practical example. This procedure will consist in the measurement of a 13.56 MHz RFID reader printed coil antenna, the determination of the equivalent circuit model and the matching to the reader output impedance, 50-ohm in this case.

The RFID antenna used in this application note is realized as a printed coil on a FR-4 board substrate. The series equivalent circuit is used to simulate the antenna in a RF simulation software and calculate the matching network. I have used the freely available RFSim99 simulator, but any other suitable simulation software can be used. After designing the matching network, the simulation results are verified with the measurement results.

Antenna

The design of the reader antenna is not in the scope of this application note. Manufacturers such as NXP, TI, or Microchip provide detailed antenna design guides included in application notes. In this example, the antenna is realized as a printed coil on a FR-4 board substrate, which also includes the placements for the matching network components. In order to facilitate the connection to the analyzer, it was soldered a board edge type SMA connector to the board.

In the first stage, this connector is connected directly to the coil for the measurement of the antenna characteristics and the determination of the equivalent circuit model. Once this process is completed, the components of the matching network are soldered to the board and the SMA connector connected to the input of the matching network.

Note: for the measurements, it is important that the antenna be at the final mounting position to consider all parasitic effects such as metal influence on quality factor, inductance and additional capacitance.

Figure 1, connection of the antenna coil to the analyzer

Equivalent Circuit Measurement

The antenna needs to be connected to the analyzer by using an appropriate test fixture that does not influence the antenna parameters. In this case it has been used a short (5cm) SMA to MCX adapter cable as illustrated in the Figure 1.

It is essential that the analyzer is calibrated before each measurement. The open, short and load calibration loads must be connected to the end of the test port extension cable, in this case to the SMA plug (see Figure 2). Please refer to the user’s manual on how to carry out the steps for OSL calibration.

Figure 2, calibration loads connected to the end of the test port extension

Once the calibration process is completed, the analyzer can be connected to the antenna via the test port extension cable. As a first verification step, we will visualize the graphs of the impedance characteristics of the antenna. We will start visualizing the reflection characteristics in the Smith Chart mode. In this case the analyzer centre frequency was set to a value of 36 MHz and the span to 71 MHz. The screen capture of the Smith chart is shown in the Figure 3. The marker is positioned around the self-resonance point of the antenna, which is around 52 MHz in this case.

Figure 3, antenna impedance characteristics in the Smith chart

The Figure 4 shows the measurements results in the Scalar chart mode, that it is configured to show the modulus and the phase angle of the impedance. The marker is positioned at the resonance point as it was done in the Smith chart.

Figure 4, antenna impedance characteristics in the Scalar chart

The next step will be the determination of the equivalent circuit model of the antenna. This can be determined “manually” by doing impedance measurements at 1 MHz, at the operating frequency of the antenna (13.56 MHz), and at the self-resonance point of the antenna. These results are then taken as input for some calculations and the model can be derived. However, the SARK-110 does automatically all this process and provides the results of the equivalent circuit.

The procedure is as follows:

Figure 5, Single Frequency mode

Figure 6, «CModel» menu options

Figure 7, Setting operating frequency

Figure 8, antenna equivalent circuit model screen

The contents of the measurement screen are described in the Figure 9:

Figure 9, antenna equivalent circuit model screen explained

The internal procedure done by the analyzer is as follows: the analyzer measures the impedance at 1 MHz and at the desired operation frequency (13.56 MHz). Then it automatically scans for the antenna self-resonance frequency and measures the impedance at this point. The following parameters are extracted from these measurements:

After that, the antenna capacitance is calculated with the following equation:

The series equivalent resistance of the antenna (see Figure 10) at the operating frequency Fop = 13.56 MHz is calculated with the following equation:

Figure 10, series equivalent resistance calculation

Now we can start entering the model in the simulator software. Figure 11 shows the schematic in RFSim99 and Figure 12 the simulation results of the antenna equivalent circuit model.

Figure 11, schematic of the antenna equivalent model in RFSim99

Figure 12, simulation results of the calculated antenna model in RFSim99

Calculating the matching network

At this stage, the antenna circuit equivalent has been determined so it is available the necessary information for the design of the matching network for the adaptation to the output impedance of the RFID reader. In this example, the RFID reader output impedance is 50-ohm.

The Figure 13 illustrates the three-component matching network. The resistor RQ is a quality factor damping resistor required to reduce the Q factor of the circuit since normally is higher than the required. The two capacitors transform the complex impedance of the coil to the desired impedance of 50-ohm at the operating frequency.

Figure 13, matching network

The matching procedure for this matching network is explained in the Figure 14 with a Smith chart. The complex impedance at the operating frequency of the antenna coil which must be matched is marked with “A” in the figure. The capacitor C2 transforms the impedance to the point B depending on its value. The resistor RQ defined by the operating bandwidth transforms the impedance to the point “C”. This point is located on the circle in the chart with the property Re (Z) = 1. From point “C” o “D” the serial capacitor C1 is used to reach the matching point.

Figure 14, matching procedure

The bandwidth of the antenna can be defined by the Q factor. The relationship between the Q and bandwidth is defined by the following equation:

The value of the resistor RQ in the matching network can be calculated by the following equation:

The XL value is the reactance of the antenna at the operating frequency, in this case the antenna has value XL=114.6; see Figure 15.

Figure 15, XL value in the equivalent circuit model screen

In this example, I selected a RQ value of 1.3K that calculates to a Q value of 11.4 and a bandwidth of about 1.2 MHz. Then following the matching procedure above, I obtained values of 47pF for C1 and 52pF for C2.

These values were introduced and the RFSim99 schematic and run the simulation to see the Smith chart (see Figure 15 and Figure 16). The marker is set at 13.57 MHz which is nearly the centre of the chart, corresponding to the perfect match.

The reason of not implementing a perfect match was due to the convenience of using standard capacitor values. It is usual in matching network implementations, implementing capacitors in parallel in order to get precise capacitances.

Figure 16, schematic of the antenna equivalent model and matching network in RFSim99

Figure 17, simulation results of the matched antenna model in RFSim99

Finally, the calculated components can be populated in the prototype antenna board in order to verify with the analyzer that the measurements are correct and match to the simulation results.

The Figure 18 and Figure 19 show the measurements of the antenna with the matching network, in both Smith and Scalar chart formats. The measurement results match quite accurately the results of the simulation.

In some cases, it will be needed some fine tuning of the components, following the matching procedure and then verifying the results with the analyzer.

Figure 18, measurement of the matched antenna in a Smith chart

Figure 19, measurement of the matched antenna in a Scalar chart

Conclusion

The SARK-110 antenna analyzer allows the easy determination of the equivalent circuit model of an antenna coil for RFID applications. This facilitates the calculation process for the determination of the matching network component values and simulating the results using a RF simulation software such as RFSim99. The analyzer can then be used to verify the matched antenna characteristics very quickly.

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