Two-dimensional RFID reader pad using free access transmission line

Transcription

1 Two-dimensional RFID reader pad using free access transmission line Takuya Okura a) and Hiroyuki Arai Graduate school of Engineering, Yokohama National University 79 5, Tokiwadai, Hodogaya, Yokohama, Kanagawa, Japan a) Abstract: Two-dimensional RFID reader pad is proposed using free access transmission line. This pad has stable coupling in arbitrary direction of receiving antenna by a circular polarized receiving element and a diamond shaped transmission line. This paper presents a prototype reader pad and its characteristics. Keywords: RFID, reader pad, transmission line, microstrip structure Classification: Antennas and Propagation References [1] T. Okura and H. Arai, One-dimensional free access transmission line for RFID reader, IEICE Commun. Express, vol. 2, no. 1, pp. 7 11, Feb [2] M. Haneishi, K. Hirasawa, and Y. Suzuki, Kogata Heimen Antenna, Denshi Joho Tsusin Gakkai, [3] Nasimuddin, X. Qing, and Z. N. Chen, Compact Circularly Polarized Symmetric-Slit Microstrip Antennas, IEEE Antenna and propagation Magazine, vol. 53, no. 4, pp , Aug Introduction This paper proposes a 2.45 GHz band two-dimensional RFID reader pad using free access transmission line. Free access transmission line proposed in [1] has stable coupling with the receiving antenna in fixed direction. Therefore, the coupled power becomes unstable when the antenna direction has changed due to the polarization mismatch between the transmission line and receiving antenna. To overcome this disadvantage, we propose a diamond shaped transmission line and a circularly polarized antenna reducing coupling characteristics by the receiving antenna direction. 2 Two-dimensional RFID reader pad and the coupled power with linearly polarized antenna Fig. 1 shows the free access transmission line and the direction angle of receiving antenna. The coupled power to receiving antenna is measured by the linearly polarized patch antenna. 205

2 In a linear polarized microstrip patch antenna, a side length of antenna a eff is represented by the following formula [2], ( a eff = a r t ) (ε e +0.3)[(a r /t)+0.262] (1) a r (ε e 0.258)[(a r /t)+0.813] ε e = ε r +1 2 a r = c 2f r εr (2) + ε ( r t ) a r where ε e is the effective dielectric constant, the resonant frequency f r is 2.45 GHz, the substrate thickness t is 0.8 mm, and its dielectric constant ε r is 2.6. From these equation, the side length of antenna a eff is 38.8 mm. Moreover, we obtain that the side length of antenna a is 37.9 mm from the simulation results. The output of 10 dbm from a signal generator is applied to transmission line, other end of the line is terminated with a 50 Ω dummy load and coupled power is measured using a spectrum analyzer. The receiving antenna direction β is 0,45,and90 and the antenna is placed with 1 mm gap. (3) Fig. 1. Free access transmission line and the coupled power with linearly polarized antenna β=0 : black, β=45 : red, β=90 :blue The average coupled power for each antenna direction is 16.2, 21.1, and 33.4 db, respectively, and the difference between average value for each direction is 17.2 db. The measured S 31 along the y axis in Fig. 1 show that the coupled power becomes very weak in case of that the polarization of receiving antenna does not coincide with the polarization of transmission line. To decrease the difference between the coupled power for 0 and 90, we bent the line into 45 to make a diamond shaped line as shown in Fig. 2 (a) 206

3 so that the antenna direction for the transmission line is same. The method of determining the resonant length c of this line is the same as the free access transmission line indicated by [1]. The length of line d is obtained by dividing microstrip line connecting the filters into three parts and the length of line e is determined as that the length of transmission line is constant. We measure the coupled power of receiving antenna on the xy plane as shown in Fig. 2 (b) and define the deviation of S 31 as following, N = S 31 Ave. (4) where Ave. is the average of S 31. The receiving antenna direction β shown in Fig. 2 (a) is 0,45,and90 and the antenna is placed with 1 mm gap. The average coupled power for each antenna direction is 26.6, 27.4, and 25.3 db, respectively, and the difference between the average value for Fig. 2. Two-dimensional RFID reader pad and the deviation using linearly polarized antenna (a) a=210, b=105, c=43.6, d=6.5, e=15.5, substrate thickness=0.8 [mm], α=45, dielectric constant: 2.6 (c)-(e) ±3dB: red, ±6dB: orange, ±9dB: yellow, other: blue 207

4 each direction is improved about 15 db compared with free access transmission line. The measured deviation N for each antenna direction in Fig. 2 (c)- (e) shows that the fluctuations of coupled power in each antenna direction are decreased by diamond shaped line, however the coupled power is still weak at a particular angles of 45,90. 3 Measurement of the coupled power with the circularly polarized antenna From the above results, the coupled power by a linear polarized antenna becomes weak at a particular direction. Then, we try to improve the coupled power by using a circularly polarized antenna [2, 3]. We use perturbed patch shape for circular polarization. In a square patch antenna, the perturbation ΔS represented by the following equation apply to the patch area S to excite circular polarization. ΔS S = 1 (5) 2Q 0 In a rectangular patch antenna, the ratio of long side a and short side b is represented by the following formula. a b =1+ 1 (6) Q 0 In these equation, Q 0 represented by the following equation is the unload Q-factor of square patch antenna, 1 = (7) Q 0 Q r Q c Q d ( ) 3 Q r = 8 ε λ 0 r (8) t Q c = t (9) δ s Q d = 1 (10) tan δ where Q r, Q c,andq d is Q-factor due to radiation loss, conductor loss, and dielectric, respectively, the wavelength in free space λ 0 is mm, the skin depth δ s is mm, and the tangent of loss angle tanδ is From these equation, Q r is 149.2, Q c is 601.5, Q d is 714.3, 1/Q 0 is , ΔS/S is ,anda/b is Moreover, we obtain that ΔS/S is and a/b is 1.01 from the simulation results We design circularly polarized antennas using these parameters and the side length of antenna a is 37.9 mm. First, we use a conventional truncated-corner circularly polarized antenna and the average coupled power for each direction is 22.8, 23.9, and 20.8 db, respectively. This antenna has stable coupling at the direction of 0 and 90, however the deviation N is large at the direction of 45. Next, we change the shape of radiating element and the feeding structure. The rectangular patch antenna is fed on the diagonal line to decrease the reception level variation. The average coupled power for each direction is 208

5 27.1, 23.9, and 24.1 db, respectively. This antenna has stable coupling at the direction of 45, however the average coupled power at the direction of 0 is smaller than other directions and the deviation N is large at the direction of 90. Finally, we propose an antenna combines these two shapes into one body and has single feed on the diagonal line as shown in Fig. 3 (a) to complement each other s disadvantage. We downsize this antenna from above antennas to match the resonant frequency. In the measurements of coupled power by this antenna, we obtain good performance for the antenna is placed with 15 mm gap. Fig. 3. The deviation N using proposed antenna (a) a=35.8, b=35.4, c=34.5, d=3.0, e=50.0, l=19.0 [mm] (b)-(d) ±3dB: red, ±6dB: orange, ±9dB: yellow The average coupled power for each antenna direction is 27.5, 25.6, and 26.8 db, respectively. The results in Fig. 3 show that the fluctuation range of its deviation N is improved about 10 db compared with the linearly polarized antenna while the average coupled power level in each direction remain uniform. Therefore, the coupled power is stable at any position on the reader pad in any receiving antenna direction and we present that this pad is effective as a two-dimensional RFID reader. 4 Conclusion This paper proposed two-dimensional RFID reader pad using free access transmission line. First, we proposed a diamond shaped structure bending the line into 45 to improve the coupled power of receiving antenna at any direction. As a result, the average coupled power level in each direction becomes uniform, however its deviation is still large. Next, we optimized 209

6 the structure of receiving antenna to improve the deviation of coupled power for each antenna direction. As a result, the fluctuation range of its deviation improves about 10 db compared with the linearly polarized antenna while the average coupled power level in each direction remain uniform. We present that this pad is effective as a two-dimensional RFID reader without the receiving antenna direction. 210