2013 Asia-Pacific Microwave Conference Proceedings
[P3-15]
Wide-beam Circularly Polarized Composite Cavity-Backed Crossed Scythe-Shaped Dipole Son Xuat Ta1, Jea Jin Han1, 2, Richard W. Ziolkowski3, and Ikmo Park1 1
Department of Electrical and Computer Engineering, Ajou University 5 Woncheon-dong, Youngtong-gu, Suwon 443-749, Korea 2
Danam Systems 799 Gwangyang 2-dong, Dongan-gu, Anyang 431-767, Korea 3
Department of Electrical and Computer Engineering, University of Arizona, 1230 East Speedway Blvd., Tucson, AZ 85721 USA. E-mail:
[email protected]
Abstract — In this paper, a wide-beam circularly polarized (CP) composite cavity-backed crossed scythe-shaped dipole is proposed. Each dipole arm contains a meander line whose end is shaped like a scythe chine to accomplish a significant reduction in the radiator size. The scythe-shaped dipoles are crossed through a 90° phase delay line of a vacant-quarter printed ring in order to generate CP radiation and broadband impedance matching. The crossed scythe-shaped dipole is incorporated with a cavity-backed reflector to improve the radiation pattern in terms of the CP radiation beamwidth and front-to-back ratio. The proposed antenna yields broad impedance-matching and 3dB axial-ratio bandwidths, a high front-to-back ratio, and a wide beamwidth. Index Terms — Circular polarization, scythe shape, meander line, phase delay line, cavity-backed reflector.
(a) I. INTRODUCTION Circularly polarized (CP) antennas are receiving increasing attention in many fields of wireless communications, such as global navigation satellite systems (GNSS), satellite communications, radio frequency identification, and wireless local area networks. The CP radiation reduces multipath effects and provides flexibility in the orientation angle between the transmitter and the receiver. A conventional way to construct a CP antenna is to use two crossed linearly polarized dipoles excited with equal amplitude and a 90° phase difference. Based on this technique, broadband CP crossed dipole antennas were presented with a variety of feeding structures, including vacant-quarter printed rings [1], Wilkinson power dividers [2], microstrip line to double slotline transitions [3], and T-junction power dividers [4, 5]. However, these antennas are bulky because of their use of straight or bowtie dipoles. In this paper, we present a composite cavity-backed crossed scythe-shaped dipole antenna with broadband characteristics and wide-beam radiation. The scythe-shaped dipole with a meander line is designed for a significant reduction in the size of the primary radiating element [6] –
(b) Fig. 1. Geometry of the proposed antenna: (a) scythe-shaped dipole and (b) side view including cavity-backed reflector.
[8]. A vacant-quarter printed ring with broadband impedance matching characteristics is used as a 90° phase delay line for the crossed dipoles to achieve CP radiation [1], [6] – [8]. The cavity-backed reflector is not only used to achieve the unidirectional radiation pattern, but also to improve the 3-dB axial-ratio (AR) bandwidth and beamwidth enhancement of the CP radiation. Compared with the composite cavity-backed crossed arrowhead-shaped dipole antenna [6], the proposed antenna has a wider impedance-
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2013 Asia-Pacific Microwave Conference Proceedings
TABLE I PERFORMANCE CHARACTERISTICS OF THE CROSSED SCYTHE-SHAPED DIPOLE ANTENNA FOR DIFFERENT CAVITY HEIGHTS Hc (mm)
Impedance bandwidth (GHz)
3-dB AR bandwidth (GHz)
CP center frequency (GHz)
0
1.495–1.830
0
1.605
3-dB AR beamwidth at the CP center frequency x-z plane y-z plane 0º 0º
10
1.49 –1.840
0
1.605
0º
0º
20
1.485–1.830
1.585–1.605
1.595
36º
146º
30
1.480–1.835
1.555–1.625
1.585
110º
174º
40
1.480–1.890
1.530–1.615
1.570
198º
209º
Fig. 3. Optimized results of the composite cavity-backed crossed scythe-shaped dipole antenna Fig. 2. Simulated electric field distribution in the cavity aperture for different phase angles of the excitation.
matching bandwidth, wider 3-dB AR bandwidth, and wider CP radiation beamwidth. II. ANTENNA DESIGN AND CHARACTERISTICS Figure 1 shows the geometry of the proposed antenna. The antenna is composed of two printed dipoles, a coaxial line, and a cavity-backed reflector. The cavity is a rectangular box with base dimensions of 120 × 120 mm2 and a height of Hc = 40 mm. The printed dipoles are suspended in the center of the cavity at a height of H = 40 mm from its bottom. The coaxial line passes through the center of the cavity in order to feed the primary radiating elements. The dipoles are printed on both sides of a circular RT/Duroid 5880 substrate with a relative permittivity of 2.2, a loss tangent of 0.0009, and a thickness of 0.508 mm. The outer conductor of the coaxial line is connected to the dipole arms on the bottom side of the substrate. The inner conductor of the coaxial line is extended through the substrate and connects to the dipole arms on the top side. Each dipole arm contains a meander line whose end is shaped like a scythe chine; thus, it is called a scythe-shaped
dipole. The antenna is optimized via the ANSYS-Ansoft high-frequency structure simulator (HFSS) to have a CP center frequency at 1.58 GHz, where the CP center frequency is defined as the frequency with a minimum AR value. The optimized antenna design parameters are as follows: Rd = 25 mm, Wb = 4 mm, Rb = 6 mm, Wr = 1.2 mm, Wc = 20 mm, wi = 0.8 mm, gi = 0.6 mm, Li = 8 mm, ws = 2 mm. It has been shown that the radiation of a cavity-backed antenna is determined more directly by the electric field distribution in the aperture than by the current on the radiator [3]. Accordingly, the electric field distribution in the aperture is examined and illustrated in Fig. 2 with excitation phase angles of 0°, 45°, 90°, and 135° to understand the CP behavior of the proposed cavity-backed antenna. It can be seen that a right-hand rotated electric field is concentrated within the cavity aperture, and a strong electric field is present near the scythe-shaped dipole arms. This indicates that the radiation of the composite cavity-backed crossed scythe-shaped dipole antenna is right-hand circularly polarized (RHCP). The antenna characteristics for different heights (Hc) of the cavity-backed reflector were studied; these results are summarized in Table I. The proposed antenna exhibits the maximum 3-dB AR beamwidth at the CP center frequency. For an increase of Hc in Table I, the impedance bandwidth for
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2013 Asia-Pacific Microwave Conference Proceedings
the x-z and y-z planes. The antenna exhibits a gain of 7.5 dBic, a front-to-back ratio of 23.7 dB, and half-power beam-widths (HPBWs) of 107° and 108° in the x-z and y-z planes, respectively. Figure 5 shows the AR of the antenna versus θ at 1.58 GHz with a very wide CP radiation beamwidth. The beamwidths for AR < 3 dB are 195° and 214° in the x-z and y-z planes, respectively. III. CONCLUSION
Fig. 4. Radiation pattern of the antenna at 1.58 GHz.
A circularly polarized composite cavity-backed crossed scythe-shaped dipole antenna was introduced with broad bandwidth and wide beamwidth radiation characteristics. The scythe-shape dipole with a meander line was presented for a significant reduction in the radiator size. To achieve CP radiation, a vacant-quarter printed-ring with broadband impedance matching characteristics was used as the 90° phase delay line. The improvement in the CP radiation was obtained by a composite scythe-shaped dipole in conjunction with a cavity-backed reflector. The proposed antenna exhibited broad bandwidths of 1.475–1.835 GHz for the –10-dB reflection coefficient and 1.530–1.615 GHz for a 3-dB AR, a wide HPBW (>100°), and a very wide CP radiation beamwidth (>190°). With many advantages, the proposed antenna can be widely employed for GNSS and satellite communication applications. REFERENCES
(b) Fig. 5. Axial ratio versus theta angle at 1.58 GHz.
a –10-dB reflection coefficient hardly changes, but the CP radiation characteristics significantly improve. Moreover, the CP center frequency decreases, the 3-dB AR bandwidth is enhanced, and the CP radiation beamwidth widens. In addition, the HFSS simulations show that the antenna with Hc < 18 mm yields AR > 3 dB, and Hc = 40 mm produces a 3-dB AR beamwidth > 190° in both the x-z and y-z planes. These results indicate that the CP radiation of the scythe-shaped dipole is significantly improved by the use of the cavitybacked reflector. The optimized results of the proposed antenna are shown in Figs. 3–5. As shown in Fig. 3, the impedance bandwidth for the –10-dB reflection coefficient is 1.475–1.835 GHz, while the 3-dB AR bandwidth is 1.53–1.615 GHz with a CP center frequency of 1.58 GHz (AR of 0.76 dB). Figure 4 shows the 1.58-GHz radiation pattern of the antenna with RHCP, a symmetrical profile, and a wide beamwidth in both
[1] J. W. Baik, T. H. Lee, S. Pyo, S. M. Han, J. Jeong, and Y. S. Kim, “Broadband circularly crossed dipole with parasitic loop resonators and its array,” IEEE Trans. Antenna Propag., vol. 59, no. 1, pp. 80–88, Jan. 2011. [2] K. Mak and K. Luk, “A circularly polarized antenna with wide axial ratio beamwidth,” IEEE Trans. Antenna Propag., vol. 57, no. 10, pp. 3309–3312, Oct. 2009. [3] S. Qu, C. Chan, and Q. Xue, “Wideband and high-gain composite cavity-backed crossed triangular bowtie dipoles for circular polarized radiation,” IEEE Trans. Antenna Propag. vol. 58, no. 10, pp. 3157–3164, Oct. 2010. [4] L. Wang, H. Yang, and Y. Li, “Design of a new printed dipole antenna using in high latitudes for Inmarsat,” IEEE Antennas Wireless Propag. Lett., vol. 10, 358–360, 2011. [5] J. Zhang, H. Yang, and D. Yang, “Design of a high-gain circularly polarized antenna for Inmarsat communications,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 350–353, 2012. [6] S. X. Ta, J. J. Han, and I. Park, “Compact circularly polarized composite cavity-backed crossed dipole for GPS applications,” J. Electromag. Eng. Sci., vol. 13, no. 1, 44–49, Mar. 2013. [7] S. X. Ta, H. Choo, and I. Park, “Planar, lightweight, circularly polarized crossed dipole antenna for handheld UHF RFID reader,” Microwave Opt. Technol. Lett., vol. 55, no. 8, pp. 1874–1878, Aug. 2013. [8] S. X. Ta, I. Park, and R. W. Ziolkowski, “Dual-band widebeam crossed asymmetric dipole antenna for GPS application,” Electron. Lett., vol. 48, no. 25, pp.1580–1581, Dec. 2012.
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