Circularly Polarized Dual-band Crossed Dipole Antenna ... - IEEE Xplore

7th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics – Metamaterials 2013 Bordeaux, France, 16-21 September 2013

Circularly Polarized Dual-band Crossed Dipole Antenna on an Artificial Magnetic Conductor Reflector Son Xuat Ta and Ikmo Park Department of Electrical and Computer Engineering 5 Woncheon-dong, Youngtong-gu, 443-749, Suwon, Republic of Korea [email protected] Abstract – This paper presents a circularly polarized (CP) dual-band crossed dipole antenna incorporated with an artificial magnetic conductor (AMC) surface that serves as a reflector. Here, two crossed asymmetric dipoles are utilized as the primary radiation elements for a CP dual-band operation. Further, a novel AMC is designed with four T-shaped slits in a unit-cell patch to significantly reduce its second resonant frequency. Consequently, the first and second resonant frequencies of the proposed AMC structure are easily adjusted for the desired dual-band antenna operation. I. INTRODUCTION In recent years, antennas based on artificial magnetic conductors (AMCs) have been intensely developed with the objective of enhancing their performance in terms of increasing their bandwidth and producing a unidirectional radiation pattern with a miniaturized profile. Various types of AMC surfaces such as a mushroomlike high-impedance surface (HIS) [1, 2], a reactive impedance surface (RIS) composed of metallic patches [3, 4], and a three-layered electromagnetic band-gap (EBG) surface [5] have been introduced as either artificial substrates of microstrip (MS) patch antennas or reflectors of different types of antennas. Moreover, several combinations of AMC structures, such as a dual-band MS patch antenna [6] and dual-band artificial substrates [7] or a wideband monopole over a dual-band RIS reflector [8], were reported for multiband operation with a unidirectional radiation pattern and a low profile. However, the aforementioned antennas over an AMC surface were presented for linearly polarized (LP) radiation. It should be noted that CP radiation provides more flexibility in the orientation angle between the transmitting and the receiving antennas than LP radiation. In addition, CP antennas are popularly utilized to reduce the multi-path effects in terrestrial wireless communication systems. Therefore, AMC-based CP antennas have received considerable attention. In this study, a dual-band CP crossed asymmetric dipole is incorporated with a novel AMC reflector to achieve broadband characteristics and a unidirectional radiation pattern with a miniaturized profile. A meander line with a barbed end is utilized in the asymmetric dipole arm to reduce the size of the radiator, and CP operation is achieved by crossing the dipoles through a 90° phase delay line of a vacant-quarter printed ring [9, 10]. Furthermore, the novel AMC structure contains four T-shaped slits in the unit-cell patch in order to significantly reduce its second resonant frequency; therefore, it can be used as a dual-band meta-surface by utilizing its first and second resonant frequencies.

II. DESIGN OF THE AMC STRUCTURE Figure 1 shows the geometry of the proposed dual-band AMC unit cell that was modeled and investigated using the ANSYS-Ansoft high-frequency structure simulator (HFSS) software. An AMC unit cell was printed on a grounded 12 × 12 mm RT/Duroid 6010 substrate with a relative permittivity of 10.2 and a thickness of 2.54 mm. In order to reduce the second resonant frequency, four T-shaped slits were symmetrically inserted into the square patch of the AMC unit cell. The proposed AMC structure was optimized such that its first and second resonant frequencies were around 2.45 and 5.5 GHz, respectively. The optimized design parameters were as follows: P = 12 mm, W = 11.2 mm, Wp = 0.4 mm, Lp = 1.8 mm, Ws = 1 mm, and Ls = 5 mm. Figure 2 shows the simulated phases of the AMC

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design. From this figure, it can be seen that the proposed structure yielded a much lower second resonant frequency than that resulting from the design without T-shaped slits. The resulting value of the second/first resonant frequency was approximately 2.25. Additionally, the proposed AMC enabled the control of the resonant frequencies, with ease, through the adjustment of the design parameters of the T-shaped slit. Finally, the bandwidth of the proposed AMC for the ±90° phase of the reflection coefficient was 2.30–2.50 GHz and 5.30– 5.75 GHz at the first and the second resonances, respectively.

Figure 1. Unit-cell models for the AMC.

Figure 2. Simulated reflection phase of the AMC design.

III. DUAL-BAND CROSSED ASYMMETRIC DIPOLE ON AN AMC REFLECTOR Figure 3 shows the geometry of the dual-band crossed asymmetric dipole antenna over the AMC surface. The crossed dipoles were suspended 8 mm above the AMC reflector. The primary radiator was designed on both sides of a 30 × 30 mm RT/Duroid 5880 substrate with a relative permittivity of 2.2 and a thickness of 0.508 mm. In order to achieve dual-band operation, each dipole arm was divided into two branches with different lengths. The longer branch of the dipole arm worked at the lower band that comprised a meander line with a barbed end for significantly reducing the size of the radiators [10]. To produce CP radiation, the dipoles were crossed through a vacant-quarter printed ring that served as a 90° phase delay line. A 6 × 6 metal patch array forming an AMC reflector with dimensions of 72 × 72 × 2.54 mm was chosen for the proposed design, on the basis of a compromise between the overall size and the stability of the antenna’s performance. The design parameters of the AMC unit cell were identical to those of the optimized AMC structure described in the previous section. The optimized design parameters of the radiator with the AMC reflector were as follows: W1 = 30 mm, W2 = 17.4 mm, Wc1 = 16.5 mm, Wc2 = 10 mm, Ri = 2.5 mm, Wr = 0.4 mm, Wb = 2 mm, S = 0.2 mm, Lb = 8 mm, Li = 3 mm, wi = 0.2 mm, gi = 0.5 mm, and ws = 1 mm.

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(a) (b) Figure 3. Geometry of the proposed antenna: (a) top view and (b) details of the crossed asymmetrical dipole.

Figure 4 shows a comparison between the performance of the crossed asymmetric dipole antenna in free space (without a reflector) and that of the crossed asymmetric dipole antenna over the AMC reflector. As shown in Fig.

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4(a), the impedance bandwidths of the antenna with an AMC reflector are 2.25–2.60 GHz and 4.80–5.70 GHz, which are slightly narrower than the corresponding bandwidths of 2.35–2.85 GHz and 4.80–5.95 GHz for the antenna radiating in free space (without an AMC reflector). However, the CP radiation bandwidths of 2.30–2.46 GHz and 5.04–5.36 GHz for the antenna with the AMC reflector were broader than the corresponding values of 2.38–2.46 GHz and 5.07–5.36 GHz for the antenna in free space, as shown in Fig. 4(b). 9

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3 Without reflector AMC reflector 0

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(a) (b) Figure 4. Comparison of the performance of the crossed asymmetric dipole antenna on the AMC reflector with that of the dipole antenna in free space: (a) reflection coefficient and (b) axial ratio.

VI. CONCLUSION This paper has introduced a dual-band CP crossed asymmetric dipole antenna over a novel AMC reflector. The proposed antenna has broad bandwidths of 2.20–2.60 GHz and 4.90–5.85 GHz for impedance matching with a –10 dB reflection coefficient, and 2.30–2.46 GHz and 5.04–5.36 GHz for an axial ratio of less than 3 dB. Owing to these advantages, the proposed antenna can be widely applied to WLAN, RFID, and gap-filler applications.

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