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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
A Compact Circularly Polarized Crossed-Dipole Antenna for an RFID Tag Huy Hung Tran, Son Xuat Ta, and Ikmo Park
Abstract—A compact circularly polarized (CP) crossed-dipole antenna for radio frequency identification in the ultra-high frequency is proposed. The antenna consists of two orthogonal dipoles with an arm composed of a meander line and triangular-shaped ending for achieving a compact size. A modified T-match with a meander line is used for impedance matching between the antenna and the tag chip. In this design, CP excitation is achieved by incorporating two semi-circular curves inserted between the orthogonally arranged dipole arms. The final design, with the dimensions of mm , yields a 10-dB impedance bandwidth of 37 MHz (892–929 MHz) and a 3-dB axial-ratio bandwidth of 11.4 MHz (905.2–916.6 MHz). The measured results show that the proposed tag antenna can provide a maximum reading distance of approximately 7.6 m with an effective isotropic radiated power of 3.28 W over the CP operational bandwidth. Index Terms—Circular polarization, crossed dipole, meander line, radio frequency identification, tag antenna.
I. INTRODUCTION
I
N RECENT years, radio frequency identification (RFID) technology in the ultra-high frequency (UHF; 860–940 MHz) band becomes mainstream applications that help the speed of handling manufactured goods and materials [1]. RFID systems are composed of at least three core components: RFID tags, RFID readers, and databases that associate arbitrary records with tag-identifying data. It is obvious that a tag antenna plays a key role in overall RFID system performance factors because passive tags obtain energy from the incoming radio frequency communication signal. Therefore, the tag antenna has substantial effects on the reading distance, the overall size, and the compatibility with the tagged object of RFID systems. To date, many miniaturized tag antennas, such as a planar meander-line antenna [2], loop-fed antenna [3], modified folded dipole antenna [4], [5], monopole with helical strips and vias [6], and printed symmetric inverted-F antenna with a quasi-isotropic radiation pattern [7]–[9], have been proposed for the RFID system. However, all the above-presented
Manuscript received October 22, 2014; accepted November 19, 2014. Date of publication December 05, 2014; date of current version March 02, 2015. This work was supported by the ICT R&D Program of MSIP/IITP under Grant 14-911-01-001. The authors are with the Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, Korea (e-mail:
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2376945
miniaturized tag antennas are linearly polarized (LP); consequently, only half of the transmitted power is received by tags because of the polarization mismatch phenomenon between the reader and the tag. As a result, the maximum reading range of the antenna decreases. To overcome this hurdle, the concept of circular polarization (CP) is applied to increase the power received by the RFID tag and simultaneously increase the reading range of the antenna. Various types of passive RFID tag antennas with CP radiation have been studied, including designs based on planar circular patch antennas [10], square microstrip patch antennas [11]–[13], and loop antennas [14]. However, the aforementioned antennas have drawbacks that include large size, the need for a high-dielectric substrate, or multilayer substrates, which make antennas bulky. In this letter, a compact planar CP tag antenna is proposed for use in the RFID-UHF bands. It is composed of two orthogonal dipoles with an arm that is a meander line with a triangularly shaped ending to provide for a compact size [15], [16]. The antenna is fed by a T-match network, which is the most common matching network used for efficient matching of UHF RFID tags [17]. However, unlike the conventional structure, a modified T-match with a meander line is used to give a compact size and easy conjugate impedance matching between the antenna and the RFID chip. Two semi-circular curves are inserted between the orthogonally arranged dipole arms to obtain the conditions of CP generation [18]. The proposed antenna is characterized computationally and validated experimentally. II. ANTENNA GEOMETRY AND DESIGN Fig. 1 shows the geometry of the passive CP RFID tag antenna, which is built on a square Rogers RO4003 substrate with a dielectric constant of 3.38, a loss tangent of 0.0027, and a thickness of 0.508 mm. The antenna consists of two orthogonal dipoles that have an arm composed of a meander line and a triangular-shaped ending to give a compact size. In order to achieve a complex conjugate matching between the antenna and the RFID tag chip, a T-match network loaded with a meander line was employed in the feeding structure. For CP radiation, the antenna was developed based on the idea of designing two orthogonal dipoles with a single feed; the lengths of the dipoles were chosen so that the real part of their input admittances are equal and the angle of the input admittances differs by 90 at the desired frequency [17]. To meet these conditions, two semi-circular curves were inserted between the orthogonally arranged dipole arms. The proposed antenna was designed to incorporate the UCODE G2XM chip, which has an input impedance of at 915 MHz and a minimum threshold-activated
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TRAN et al.: COMPACT CP CROSSED-DIPOLE ANTENNA FOR RFID TAG
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Fig. 1. Geometry of the proposed RFID tag antenna.
Fig. 2. Simulated current distribution of the tag antenna at 910 MHz for two phase angles (a) 0 and (b) 90 .
power of 15 dBm. Hence, the requirement is that the input impedance of the proposed antenna must be approximately for the complex conjugate matching between the tag chip and the antenna. The input impedance of the antenna is easily adjusted by using the modified T-match network in the feeding structure. The design and development of the proposed antenna were investigated by using the commercially available electromagnetic simulation software Ansoft High-Frequency Structure Simulator (HFSS). Its optimized design parameters are as follows: mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, and mm. As mentioned above, good CP radiation of the antenna is obtained by the insertion of two semi-circular curves between the orthogonally arranged dipole arms, i.e., the two orthogonal unequal dipoles were tuned to have overlapping resonances that exhibit a 90 phase difference between them at the desired frequency of 910 MHz. CP excitation of the antenna is illustrated in Fig. 2, which shows the simulated current distribution of the antenna at 910 MHz for the two phase angles 0 . and 90 . As shown in the figure, the currents are mainly distributed on the horizontal dipole at a phase of 0 , whereas currents mostly concentrate on the vertical dipole at a phase of 90 . This arrangement explains the CP behavior of the proposed antenna. However, the antenna is not symmetric, and it leads currents to flow through T-match in different manners at 0 and 90 phases. At
Fig. 3. Simulated and measured input impedance of the antenna: (a) resistance and (b) reactance.
the 0 phase, the currents flow in the horizontal dipole, and the direction is from right to left. The currents are divided into two paths with comparable amplitudes and phases when they pass a T-match [Fig. 2(a)]. At the 90 phase, the currents flow in the vertical dipole, and the direction is from top to bottom. The high currents flow in the meander line of the T-match, but the significant amount of currents also flow to the opposite direction in the bottom line of the T-match [Fig. 2(b)]. Therefore, the current distribution on the meander line of the T-match is larger at phase 90 than it is at the phase 0 III. SIMULATED AND MEASURED RESULTS Fig. 3 shows the simulated and measured input impedances of the antenna; the measured results were in good agreement with the simulated results. Good conjugate matching between the input impedances of the antenna and the UCODE G2XM chip was observed. The resistance and reactance components of the antenna were fairly close to those of the chip in the 890–925-MHz range. These results indicate that the antenna has a good impedance-matching bandwidth. This is confirmed in Fig. 4, which shows the simulated and measured reflection coefficients ( ) that were derived from simulated and measured input impedances of the proposed antenna. The value was determined by using the following basic formula [19]:
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Fig. 4. Simulated and measured
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
and simulated AR. Fig. 6. Photograph of the fabricated sample.
Fig. 5. Simulated radiation pattern at 910 MHz: (a)
-plane and (b)
-plane.
Fig. 7. Measured maximum readable range of the antenna as a function of the frequency.
where and are the input impedances of the antenna and the tag chip, respectively. The simulated results yielded a 10-dB reflection coefficient bandwidth of 37 MHz (892–929 MHz); meanwhile, this figure for measured results was 39 MHz from 890 to 929 MHz. The conventional anechoic chamber cannot be used to measure the antenna far field since the input impedance of the tag antenna is not matched to a 50- coaxial feedline. Therefore, the far-field radiation performances of the antenna were only characterized via the HFSS simulations. Fig. 4 also depicts the simulated axial ratio (AR) value of the proposed design; its 3-dB AR bandwidth was 11.4 MHz (905.2–916.6 MHz) with a minimum AR value of 0.3 dB at 911 MHz. Fig. 5 exhibits the simulated radiation pattern of the antenna at 910 MHz. The antenna radiates a bidirectional wave; the front side radiates a righthand CP (RHCP), whereas the back side radiates a left-hand CP (LHCP). The typical bidirectional patterns with a maximum radiation at and were exhibited in the two principle planes ( - and -planes), and it turns out that the proposed RFID antenna has excellent CP radiation. The proposed antenna was fabricated on a Roger RO4003 substrate with a copper thickness of 17 m via standard etching technology. The UCODE G2XM chip was mounted on the fabricated antenna via standard chip bonding technology. Fig. 6 shows the photograph of the fabricated sample. The accurate tag range of the proposed antenna was measured in a full anechoic chamber with a CP RFID reader antenna.
The measurements were conducted at the RFID/USN Center, Incheon, Korea. During the measurement, the tag antenna was placed at a fixed position from the reader antenna. At each frequency, the minimum power required to communicate with the tag was recorded. The tag range for any transmitter effective isotropic radiation power (EIRP) of interest can be determined by the following equation [19]:
where is the loss of the connecting cable, is the gain of the transmitting antenna, and is the distance from the reader to the tag. In our experiment, dB, dBic, m, and W. The maximum reading distance of the tag was measured for different frequencies and is given in Fig. 7. This result verifies that the longest reading distance occurs within the CP bandwidth; the reading range was approximately 7.6 m within the 905–915-MHz range. To verify the CP performance of the proposed antenna, the measurement was performed under identical conditions for an LP tag antenna, which was designed by removing a vertical dipole. The LP tag’s characteristics are as similar as CP tag’s characteristics in terms of impedance matching, directivity, and efficiency at 910 MHz. As shown in Fig. 8, the maximum reading distance was enhanced significantly by using the CP tag antenna. The maximum
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range of antennas of approximately 7.6 m within the 3-dB AR bandwidth. Based on the simulated and measured results, the proposed antenna conforms to the required conditions and is completely appropriate for RFID applications. REFERENCES
Fig. 8. Reading range measurement by separately using LP and CP tag antennas.
Fig. 9. Measured readable range against (b) -plane.
at 910 MHz: (a)
-plane and
readable distances for LP tag antenna were 5.7 and 7.7 m, respectively. Thus, this further illustrates the CP operation of the proposed tag antenna. In further measurements, the position of the tag was fixed and the transmitter output power was varied by controlled attenuation. Measurement was performed by rotating the proposed antenna along the -direction at 910 MHz for two cases where the reader antenna was placed: 1) at the front side of the tag, and 2) at the back side of the tag. The measured results are presented in Fig. 9 for the two principle planes ( - and -planes). It is apparent that the antenna has the same behavior as the typical bidirectional patterns of the proposed antenna (see Fig. 5). The maximum values at and are equivalent, confirming the excellent CP radiation of the RFID antenna. IV. CONCLUSION A compact, crossed-dipole, CP UHF-RFID tag antenna has been designed and fabricated. A meander-line T-match has been proposed to easily fulfill conjugate matching for the desired input resistance and reactance by tuning design parameters. The optimized structure yielded a good antenna characteristic; its impedance bandwidth is 37 MHz (892–929 MHz), and its CP bandwidth is about 11.4 MHz (905.2–916.6 MHz). The proposed antenna performs the measured maximum reading
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