Series Hilbert -curves for Dual-band and Circular Polarization Applications

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Fractal Antenna with Series Hilbert-curves for 2.45/5.35 GHz Dual-band and Circular Polarization ApplicationsSheau-Shong Bor a, Tsung-Che Lu*a, Ji-Chyun Liu b, Bing-Hao Zeng ca Dept. of Electrical Engineering, Feng Chia Universityb Dept. of Electronics Engineering, Ching Yun Universityc Dept. of Communication Engineering, Yuan Ze University229, Chien-Hsin Rd.,Jung-Li,Taiwan,R.O.Cjichyun@.twAbstract —A novel design of fractal antenna with series Hilbert-curve configuration for 2.45/5.35 GHz dual-band and circular polarization applications is presented in this paper. The compact fractal antenna consists of five series Hilbert-curve and the monopole couplings, and exhibits the desired bands 2.45 GHz (BW=85 MHz, 3.45%) and 5.35 GHz (BW=240 MHz, 4.46%) respectively. It is a compact and available monopole antenna for IEEE 802.11a/b WLAN applications. The circular polarizations for dual-band are demonstrated with AR spectrum and electric field intensities. Results included with frequency responses, radiation patterns and electric field intensities are presented and discussed.1I.I NTRODUCTIONDual-band operations in the 2.4 GHz (2.4-2.484 GHz) and 5 GHz (5.15-5.95 GHz) bands are applied to rapid developments in wireless communications. In practice, IEEE802.11b is one of WLAN with frequency band from 2.4 to 2.484 GHz for the ISM band. Hiper-LAN2 is developed with frequency bands in 5.15-5.35 GHz, 5.47-5.725 GHz and 5.725-5.925 GHz. Bands of 5.15-5.35 GHz, 5.725-5.825 GHz are applied to IEEE802.11a. The U-NII band covers the frequency band of 5.725-5.825 GHz. Besides the individual approaches among 2.4 GHz and 5.0 GHz, the requirements of 2.4/5.0 GHz dual-band antenna are necessary for applications [1]-[14], [17]-[22].For implementations, configurations such as printed dipole [1], F-shaped monopole [3], stack inverted-F monopole [2], [4], [17], double-T monopole [5], [6], rectangular slot [7], reconfigurable geometry [8], double S-shaped monopole [9], triangular slot [10], G-shaped monopole [11], C-shaped monopole [12], double ring slot [13], hock-shaped monopole [14], rectangular dielectric resonator [15], shorted parasitic patches [16], double slot-ring [18], loop monopole [19], slotted-patch monopole [20], ring monopole [21] and circular monopole [22] etc. have been developed for WLAN dual-band applications. In which, the stack inverted-F antenna usually be applied for the PCMCIA network card due to the advantage of low profile, low cost, good 50Ω matching, capable integration with circuits and easy mounted [2]. And, the modified stack inverted-F antenna was proposed to deliver the properties of diversity and improvements of frequency responses, isolation and radiation gain [4], [17]. Both the characteristics of dual-band dependent on the longer and short F-shape arms related to the 2.4 GHz lower band and 5.0 GHz higher band respectively. The compact planar loop monopole antenna was proposed for WLAN applications [19]. Recently, the dual-polarization was introduced for 5.3/5.8 GHz dual-band application [20]. However, the circular polarization of the monopole antenna has not been studied for 2.4/5.0 GHz dual-band applications.1This work was supported in part by the National Science Council in Taiwan for financially supporting this research under the Contract No. NSC 96-2623-7- 231-005-D.An alternative design of fractal antenna with Hilbert-curve configuration for 2.45/5.35 GHz dual-band and circular polarization applications is presented in this paper. Based on the iterative configuration, a novel compact monopole antenna is proposed by using the Hilbert-curve and the series configurations. The compact fractal antenna consists of five series Hilbert-curve and the monopole couplings, and exhibits the desired bands 2.45 GHz and 5.35 GHz with wider bandwidth respectively. The AR spectrum and electric field intensities are applied for demonstrating the circular polarizations for 2.45/5.35 GHz dual-bands.It is a compact and available microstrip antenna for IEEE 802.11a/b WLAN applications.II.A NTENNA C ONFIGURATION AND A NALYSISA. Fractal antenna configurationIn Fig. 1(a) and (b), the fractal antenna with series Hilbert-curve configuration and detailed Hilbert-curve dimensions are proposed respectively. The test coordinate is with the configuration in Fig. 1(c). The 50Ω microstrip feed-line is excited with a probe feed. The FR4 substrate with thickness 1.6mm and relative permittivity 4.4 is used.B. Circular polarization and electric field intensitiesFor circular polarizations, the AR spectrums are presented in Fig. 2(a) and (b). The minimum AR with 1.12 dB at θ = 100°, φ = 0° and the circular polarizations (–3dB BW = 50 MHz) are observed with a symmetrical curve for 2.45 GHz(a)(b) (c)Fig. 1 (a) Configuration of series Hilbert-curve fractal antenna.(b) Detailed dimensions of Hilbert-curve. (c) Coordinate.(a)(b)Fig. 3 Electric field intensities.band. Meantime, the minimum AR with 1.85 dB at θ = 0°, φ = 0° and the circular polarizations (–3dB BW = 75 MHz) are observed for 5.35 GHz. Thus the proposed antenna can be applied to circular polarization applications which represents one of the availabilty and usefulness in contrast to the conventional monopole antennas for 2.45/5.35 GHz dual-bands.(a)Fig. 3 illustrates the simulated electric field intensities among the planar structures, which provide a clearly physical insight on understanding the circular polarization of the proposed antenna. A typical circular polarization patch antennas usually consist of two individuals with horizontal and vertical locations, and a two-phase signal with 90° difference. Due to five Hilbert-curves are connected in vertex with orthogonal location, and the signals flow the curves with zig-zag path, therefore the circular polarization can be achieved.Fig. 3(a) shows that the series Hilbert-curve are excited at the lower -3dB resonance f=2.425 GHz and the higher -3dB resonance f=2.475GHz respectively. Fig. 3(b) shows that the series Hilbert-curve are excited at the lower -3dB(b)Fig. 2 AR spectrums. (a) 2.45 GHz band, (b) 5.35 GHz band(a)Fig. 5 Radiation patterns for 2.45 GHz.(b)Fig. 4 Simulated and measured results of S 11 spectrum(a) at 2.45 GHz band (b) at 5.35 GHz band.resonance f=5.290 GHz and the higher -3dB resonance f=5.365 GHz respectively. The currents flow with zig-zag way along the Hilbert-curves, therefore the circular polarization can be observed.III. S IMULATION & RESULTSA. S-parameters frequency responsesTo analyze the resonant frequencies, the S 11 return loss spectrums are simulated and measured in Fig. 4 (a) and (b) for 2.45/5.35 GHz dual-band respectively. By using the commercial software of HFSS tool, the simulation results are also presented and analyzed. It is evident that the simulated and measured results of frequency responses are in agreement. In measurement, while the return loss is smaller than -10dB, the frequency responses cover two bands, from 2.425 to 2.51 GHz (bandwidth = 85 MHz, 3.45%), and from 5.26 to 5.50 GHz (bandwidth = 240 MHz, 4.46%). For applications, the frequency responses are covered in the operation bands of the IEEE802.11a/b and the U-NII band.Fig. 6 Radiation patterns for 5.35 GHz.B. Abbreviations and AcronymsIn field analyses, the radiation patterns are obtained by an automatic measurement system in an anechoic chamber. For the field coordinates shown in Fig. 1(c), the under-tested antenna is located on the X-Y plane, and the feeding line is located along the Y-axis. Thus, radiation patterns with X-Y cut, X-Z cut and Y-Z cut are obtained.Fig. 7 Radiation power gains.The three cut patterns with resonant 2.45 GHz and 5.35 GHz are represented in Fig. 5 as well as in Fig. 6 respectively. In Fig. 5, broadside patterns are observed in the X-Y cut and X-Z cut (solid line φ = 0°, θ = varied), and omnidirectional patterns are obtained in the Y-Z cut (solid line φ = 0°, θ = varied) for 2.45 GHz band. Related smaller directive radiation patterns are shown in X-Y cut, X-Z cut and Y-Z cut (dash lineθ = 0°, φ = varied). It can be applied for the monopole applications. The peak radiation power gains are presented in Fig. 7 with 2.0-3.4 dBi.In Fig. 6, slightly broadside patterns are observed in the X-Y cut and X-Z cut (φ = 0°, θ = varied), and omnidirectional patterns are obtained in the Y-Z cut (φ = 0°, θ = varied) for 5.35 GHz band. Related larger directive radiation patterns are shown in X-Y cut, X-Z cut and Y-Z cut (θ = 0°, φ = varied). It can be applied for the quasi-monopole applications. The peak radiation power gains are presented in Fig. 7 with 0.6-1.1 dBi.IV.C ONCLUSIONThe fractal antenna with series Hilbert-curve configuration for 2.45/5.35GHz dual-band applications is presented in this paper. The structure is smaller in size and easy to fabricate in PCMCIA circuits. The configuration of the five series Hilbert-curve configuration determines the lower band of 2.45 GHz and the higher band of 5.35 GHz simultaneously. Its operations simultaneously cover two bands of 2.45 GHz (85 MHz, 3.45%), and 5.35 GHz (240 MHz, 4.46%) for return loss <-10dB. Both simulation and measurement results agreed with the verified frequency responses. The AR spectrum and electric field intensities are applied for demonstrating the circular polarizations for 2.45/5.35 GHz dual-bands.It is a compact and available microstrip antenna for IEEE 802.11a/b WLAN applications. 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