SISTEMA FISCAL ESPANYOL

Following the above section, a TTP SSRC antenna having k = 0.06 was fabricated on FR4 substrate with εr = 4.4 and h = 0.8 mm, Figure 6.3(b), to operate at 13.56 MHz. As the real part of the impedance peaks at the SRF at around 7 kΩ, this is too far from 50 Ω, to match the antenna with a PI or T matching network as a very high Q factor would result [70]. In order to solve this problem the operating frequency of 13.56 MHz is considered at the beginning of the real impedance curve when SRF = 14.5 MHz (R = 90.2mm) as shown in Figure 6.4. The impedance of the TTP SSRC antenna at 13.56 MHz is around 50+615j Ω when SRF = 14.5 MHz that the inductive part of 615j Ω is eliminated by just adding two series capacitors of 39 pF to match the antenna to 50 Ω line, as shown in Figure 6.3(b).

Also, based on the old model shown in Figure 5.1 (a), a planar single loop HF RFID antenna having the same radius of R = 90.2 mm with the TTP SSRC having k =0.06 dimension is designed and fabricated to operate at 13.56 MHz. Following the Thomson formula by adding a 390 pF capacitor to the input of the single loop a resonance LC circuit is created to operate at 13.56 MHz and also a 50 Ω matching circuit is designed as described in [9, 55] and illustrated in Figure 6.3(a). From the measurement and simulation results of return loss shown in Figure 6.5, the Q factor of both antennas are calculated using the fcentre /(fhigh-3dB - flow-3dB) expression and are obtained as 8 and 25 for TTP SSRC antenna and single loop antenna, respectively. It is seen that the Q factor of the TTP SSRC antenna following the new model of Figure 5.1(b), fTX = SRF, is nearer to the specifications of the standard 13.56 MHz systems at 15 ~ 30 range ([10, 33, 54], ISO 14443, ISO 15693).

Figure 6.1 Geometry of the TTP SSRC antenna ; (a) Top and bottom layers, (b) Side view.

Figure 6.3 Geometries of the (a) Planar single loop antenna (b) TTP SSRC antenna (both are designed to operate at 13.56 Mhz and are matched to 50 Ω input).

Figure 6.5 Simulated and measured return losses of the TTP SSRC antenna and single turn antenna both operating at 13.56 MHz, Figure 6.3.

The relative magnetic field intensity for the single loop and the TTP SSRC antenna is measured at different fixed distances away from the antenna while the probe is adjusted to three different positions A, B and C, in the transverse direction. The fixed distances range up to 1 metre as shown in Figure 6.6. From Figure 6.6 (b) it is seen that the measured H-field is more uniform and has less changes in front of the TTP SSRC antenna in the three points than the single loop planar antenna in Figure 6.6 (a). It is seen than that the maximum difference of the measured H -field in the three points is changed from 8 dB in the old antenna to about 1 dB in the new antenna. Therefore tagged objects can be identified even with changing the position of the tag at the surface of the object and also the distance from the loop can be predicted with more accuracy in front of the TTP SSRC antenna. This is crucial for the smart shelf application such that it can determine the position of the item along the shelf wherever the tag is placed within the item, for example a book. This will be crucial in a reliable system ascertaining the order in which a set of books on a shelf are stacked. Also, it is seen that the measured relative magnetic field intensity of the TTP SSRC antenna is about 5 dB stronger than the single loop antenna in front of the three points at the constant distance levels that follows the discussion of Section 5.2.1.

As mentioned before in Section 2.4, if a conductor loop is located in the vicinity of the other one, through which a current is flowing, there would be a mutual inductance between the loops. The effect of the mutual inductance on the induced voltage is analysed in Section 2.6. Here, the effect of mutual inductance is measured in terms of the changes on the measured magnetic field intensity. In order to measure relative magnetic intensity when adjacent tags are placed in front of the TTP SSRC reader antenna, a set up as shown in Figure 6.7 is considered. Tag 1 is connected to

a spectrum analyser and placed at distance M in front of the TTP SSRC antenna while tag 2 is placed at distance P from tag 1, towards the reader antenna. In four individual measurements, tag 1 is fixed at M = 10, 15, 20, 30 cm and the position of tag 2 is changed in the 0 ~ 10 cm range. The measurement results obtained by the spectrum analyser are shown in Figure 6.7 with about 1 dB difference for points A and C. It is seen that the adjacent tag 2 affects the measured magnetic field by < 1 dB when is placed less than 1.5 cm distance from the tag 1 and it has no effect on the measured magnetic field far away from this distance. This could be used to identify many tagged objects in front of the reader antenna where the measurements are affected less than 1 dB due the mutual inductance between the tags together and also the reader antenna.

Figure 6.6 The measured relative magnetic field intensity for the single loop antenna, (a), and the TTP SSRC antenna, (b), at different distances in front of three points of A, B and C, up to 1 metre distance.

Figure 6.7 Measuring relative magnetic intensity when adjacent tags are placed in front of TTP SSRC reader antenna.

6.3 Adjusting Q Factor and Dimension of TTP SSRC Antenna by