High-Gain Printed Dipole Antenna

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19. S.G. Johnson and J. Joannopoulos, Bloch-iterative frequency domain can provide a high antenna gain of about 6.8 dBi for operation in methods for Maxwell’s equations in a planewave basis, Opt Express 8 the 2.4-GHz WLAN band (2400–2484 MHz). Design consider- (2001), 173–190. ations of the proposed antenna are described, and results of the 20. N. Susa, Threshold gain and gain-enhancement due to distributed- constructed prototype are presented and discussed. feedback in two-dimensional photonic crystal lasers, J Appl Phys 89 (2001), 815–823. 2. DESIGN CONSIDERATIONS OF THE PROPOSED 21. C. Wilsen, H. Temkin, and L.A. Coldren, Vertical-cavity surface- ANTENNA emitting lasers: Design, fabrication, characterization and applications, Cambridge University Press, Cambridge, UK, 2001. Figure 1 shows the geometry of the proposed high-gain printed dipole antenna. Note that the dimensions given in the figure are for © 2005 Wiley Periodicals, Inc. operating in the 2.4-GHz band. The antenna has two identical radiating arms printed on an inexpensive 0.4-mm-thick FR4 substrate and is fed at points A and B by a 50⍀ mini coaxial line with a balun attached. Also note that in this design the antenna is HIGH-GAIN PRINTED DIPOLE ANTENNA designed to have a narrow width of 10 mm, which is comparable to that of conventional coaxial antennas. This narrow width makes Yung-Tao Liu, Ting-Chih Tseng and Kin-Lu Wong Department of Electrical Engineering the proposed antenna suitable for replacing the conventional co- National Sun Yat-Sen University axial antennas in practical applications. Kaohsiung 804, Taiwan The antenna can be divided into five sections: element 1, element 2, elements 3a and 3b, element 4, and element 5. These Received 14 January 2005 five sections (from elements 1 to 5) can be considered to corre- spond to the half-wavelength elements A to E of a conventional ABSTRACT: A printed dipole antenna showing high gain (about 6.8 2.5-wavelength dipole antenna, as shown in Figure 2. It can be dBi) radiation in the azimuthal plane is presented. The dipole antenna seen that the currents in elements B and D have a phase opposite has a narrow width of 10 mm and a total length of about 202 mm (with to those of elements A, C, and E. In this case, large destructive 1.64 wavelengths of the frequency at 2442 MHz, the center frequency of effects caused by the radiation contributed from the five elements the 2.4-GHz WLAN band). In each one of the two radiating arms of the dipole antenna, a meandered line is inserted as a phase-reversal device. With this arrangement, the proposed dipole antenna performs as a collinear array antenna with three in-phase half-wavelength radiating elements, leading to constructive radiation in the azimuthal plane and small side lobes in the elevation plane. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 214–218, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 20948

Key words: dipole antennas; printed antennas; collinear array antennas; high-gain dipole antennas

1. INTRODUCTION Conventional dipole antennas are usually constructed from coaxial lines and operated as a half-wavelength resonant structure [1]. This kind of coaxial dipole antenna usually shows an antenna gain of about 2.2 dBi only. However, for some practical applications such as in the access point for wireless local area networks (WLANs) [2], a much larger antenna gain may be required. To achieve a high-gain dipole antenna, one can use several half-wavelength coaxial-line sections, in which their inner and outer conductor connections are reversed at each junction, to form a coaxial collinear antenna with in-phase radiating sections in order to achieve an enhanced gain [3, 4]. A planar or printed type of this design with seven 50⍀ microstrip-line sections has been recently demonstrated, and an antenna gain of about 5.0 dBi has been obtained [5]. Another promising method for obtaining a high-gain dipole antenna is to use a meandered-line section to act as a phase-reversal device [6–8] in the radiating arms of a long dipole or monopole antenna. This arrangement has a simpler conﬁguration than the design in [3–5] and can also provide a collinear-array antenna structure with in-phase radiating elements, thus making possible an enhanced antenna gain for the dipole antenna. Based on the use of a meandered-line section as the phase- reversal device [6–8], we propose in this paper a high-gain printed-dipole antenna suitable for replacing the conventional coaxial antennas for WLAN access-point applications. The proposed printed-dipole antenna has the advantage of low cost in fabrication, compared to coaxial antennas. Furthermore, the proposed antenna Figure 1 Geometry of the proposed high-gain printed dipole antenna

214 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 46, No. 3, August 5 2005 Figure 3 Measured and simulated return loss for the proposed antenna Also note that, since the half-wavelength elements 2 and 4 are densely meandered, their lengths (only 16 mm in this design) along the dipole arm are now much smaller than a half-wavelength (about 61 mm at 2442 MHz). Given this characteristic, along with

Figure 2 Current distribution on a conventional 2.5␭ dipole antenna are expected, which leads to undesired radiation patterns for practical applications. By compressing elements B and D into densely meandered lines (element 2 from points C to D and element 4 from points E to F in Fig. 1), the currents with a phase opposite to that of elements A, C, and E will be constrained in the meandered line and oriented mainly in the horizontal direction. In this case, the radiation contributed from elements 2 and 4 will be diverted to be mainly with E␾ (horizontal) polarization. Furthermore, due to the dense meandering, large coupling between adjacent segments (spacing 1 mm in this design) in elements 2 and 4 is expected, which can lead to a much decreased radiation efﬁciency for the currents in elements 2 and 4. It can thus be expected that the radiated power contributed from elements 2 and 4 will be much less than that from elements 1, 3, and 5. In this case, elements 2 and 4 can be considered as good phase-reversal devices with small or negligible destructive effects on the E␪ (vertical) radiation contributed from elements 1, 3, and 5. The proposed dipole antenna can thus be treated as a collinear array antenna with three in-phase half-wavelength radiating elements (elements 1, 3, and 5), Figure 4 Simulated surface-current distribution at 2442 MHz for the which leads to enhanced antenna gain for the antenna. proposed antenna

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 46, No. 3, August 5 2005 215 Figure 5 Simulated return loss as a function of the feed gap d between Figure 7 Simulated return loss as a function of the length t of elements elements 3a and 3b (other dimensions are as in Fig. 1) 1 and 5 (other dimensions are as in Fig. 1) the presence of the FR4 substrate (with relative permittivity 4.4), which will also reduce the resonant length of the half-wavelength type. The simulated results are obtained using the Ansoft simula- elements 1, 3, and 5, the proposed dipole antenna shows a total tion software High-Frequency Structure Simulator (HFSS), and length of only 202 mm, about 1.64 wavelengths at 2442 MHz. good agreement between the measurement and simulation is seen. To achieve good impedance matching for the proposed an- From the measured results, the impedance bandwidth reaches tenna, it is found that the feed gap d between elements 3a and 3b about 220 MHz (2330–2550 MHz), thus allowing the proposed is an important factor. The optimal feed gap in this study is found antenna easily cover the 2.4-GHz band for WLAN operation. to be 2 mm, and the effects of various feed gaps on the impedance Figure 4 shows the HFSS simulated surface current distri- matching will be discussed in section 3. As for varying the width bution at 2442 MHz for the proposed dipole antenna. It is w of elements 3a and 3b, it has a large effect on the achievable clearly seen that elements 1, 3, and 5 have in-phase and verti- impedance bandwidth for the proposed dipole antenna. To achieve cally-oriented excited currents. Null excited currents are also a large impedance bandwidth to easily cover the 2.4-GHz band seen near points C, D, E, and F, which indicates that phase (2400–2484 MHz), the width w is chosen to be 10 mm in this reversals occur and the out-of-phase currents are now com- design, the same as that of the FR4 substrate. With regard to pressed in elements 2 and 4. These characteristics agree with elements 1 and 5, their width has a relatively small effect on the the prediction described in section 2. impedance bandwidth of the antenna and is thus ﬁxed to be 1 mm To analyze the effects of various dimensions on the antenna in this design; their length t, however, shows a large effect on the performances, a parametric study for the proposed antenna was resonant frequency of the antenna. The optimal length t is deter- also conducted using Ansoft HFSS simulation software. Figure 5 mined to be 56 mm (about 0.46 wavelength at 2442 MHz), close shows the simulated return loss as a function of the feed gap d to a half-wavelength at 2442 MHz. The effects of the width w and between elements 3a and 3b. A large effect of the feed gap on the length t will be discussed in more detail in section 3. impedance matching is seen, and the optimal feed gap in this design is determined to be 2 mm, which is the same as the result 3. RESULTS AND DISCUSSION obtained for a printed dipole antenna shown in [9]. Figure 6 shows Based on the design dimensions given in Figure 1, a prototype of the simulated return loss as a function of the width w of elements the proposed antenna was constructed and tested. Figure 3 shows 3a and 3b. The impedance bandwidth is seen to increase with an the measured and simulated return loss for the constructed proto- increase in w. This behavior is very similar to the known result that the thicker the wire dipole, the wider is its bandwidth [10]. Similar results have also been obtained for a printed monopole antenna with a wider strip width [11]. For this reason, the width w is selected to be the same as that (10 mm) of the FR4 substrate in this design.

TABLE 1 Comparison of the Simulated Radiation Performances of the Proposed Antenna with Various Lengths t of Elements 1 and 5

Length t of Directivity Gain Center Frequency Elements 1, 5 [mm] [dBi] [dBi] [MHz] 60 6.87 6.65 2420 56 7.13 6.85 2442 50 6.44 6.28 2500 45 6.34 6.23 2550

Figure 6 Simulated return loss as a function of the width w of elements NOTE—The directivity and gain are computed at the center frequency of 3a and 3b (other dimensions are as in Fig. 1) the antenna’s impedance bandwidth.

216 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 46, No. 3, August 5 2005 Figure 8 Measured radiation patterns at 2442 MHz for the proposed antenna

Figure 9 Simulated radiation patterns at 2442 MHz for the proposed antenna

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 46, No. 3, August 5 2005 217 REFERENCES 1. B. Drozd and W.T. Joines, Comparison of coaxial dipole antennas for applications in the near-field and far-field regions, Microwave J 47 (2004), 160–176. 2. K.L. Wong, Planar antennas for wireless communications, Wiley, New York, 2003. 3. T.J. Judasz, W.L. Ecklund, and B.B. Balsley, The coaxial collinear antenna: current distribution from the cylindrical antenna equation, IEEE Trans Antennas Propagat 35 (1987), 327–331. 4. T.J. Judasz and B.B. Balsley, Improved theoretical and experimental models for the coaxial collinear antenna, IEEE Trans Antennas Propa- gat 37 (1989), 289–296. 5. R. Bancroft and B. Bateman, An omnidirectional planar microstrip antenna with low sidelobes, Microwave Opt Technol Lett 42 (2004), 68–69. 6. C.J. Tsai and M.H. Chong, Collinear antenna structure, U.S. Patent No. 6771227 B2, 2004. Figure 10 Measured and simulated antenna gain for the proposed an- 7. A.V. Buren, Antenna with multiple radiators, U.S. Patent No. 6788261 tenna in the 2.4-GHz band B1, 2004. 8. K.L. Wong, T.C. Tseng, F.R. Hsiao, and T.W. Chiu, High-gain omnidirectional printed collinear antenna, Microwave Opt Technol Lett 44 (2005), 348–351. The effects of the length t of elements 1 and 5 are also 9. C.M. Su, H.T. Chen, and K.L. Wong, Printed dual-band dipole antenna analyzed, and the results are shown in Figure 7. It is observed that with U-slotted arms for 2.4/5.2-GHz WLAN band operation, Electron when the length t decreases, the resonant frequency of the antenna Lett 38 (2002), 1308–1309. 10. W.L. Stutzman and G.A. Thiele, Antenna Theory and Design, 2nd ed., is shifted to high frequencies and the impedance matching is also Wiley, New York, 1998, pp. 172–173. degraded. On the other hand, when the length t increases, the 11. Y.L. Kuo and K.L. Wong, Printed double-T monopole antenna for resonant frequency of the antenna decreases and the impedance 2.4/2-GHz dual-band WLAN operations, IEEE Trans Antennas Propa- matching is also degraded. The antenna directivity and gain are gat 51 (2003), 2187–2192. also computed and listed in Table 1 for comparison. The results indicate that there exists an optimal length t for achieving a © 2005 Wiley Periodicals, Inc. maximum antenna gain. This is largely because, with an optimal length t chosen, the null excited currents will occur near points C, D, E, and F (see Fig. 4), as desired. In this case all portions in elements 1, 3, and 5 are of the same phase, thus optimal construc- BAND SELECTION IN AN AMPLIFIED tive radiation for the proposed antenna can be expected. Also note SPONTANEOUS EMISSION SOURCE that the optimal length t (56 mm) for achieving a maximum USING THE BACKWARD-FEEDBACK antenna gain (see Table 1) is the same as that for achieving a TECHNIQUE maximum impedance bandwidth (see Fig. 7). Figure 8 plots the measured radiation patterns at 2442 MHz for V. Sinivasagam,1 K. Dimyati,1 R. D. Singh,2 and the constructed prototype. Good omnidirectional radiation pattern M. A. G. Abushagur3 1 in the azimuthal plane ( x–y plane) is seen. In the elevation plane Dept. of Electrical Engineering University of Malaya ( x–z and y–z planes), small or negligible side lobes are also seen. 50603 Kuala Lumpur, Malaysia The measured results in general also agree with the simulated 2 Photronix (M) Sdn. Bhd., G05 radiation patterns shown in Figure 9. Figure 10 presents the mea- 2300 Century Square sured and simulated antenna gain against frequency for the con- Jalan Usahawan, 63000 Cyberjaya, Selangor, Malaysia 3 Rochester Institute of Technology, Rochester, NY structed prototype. Good agreement between the measured and simulated results is observed. A high antenna gain of about 6.6– 6.8 dBi for frequencies across the 2.4-GHz band is obtained, which Received 4 February 2005 is much higher than that of a conventional half-wavelength dipole antenna. ABSTRACT: In this paper, the band-selective behavior in an amplified spontaneous emission (ASE) source of an erbium-doped silica fiber (EDF) is demonstrated. C-band operation is obtained without backward ASE feedback, whereas L-band operation is obtained with backward 4. CONCLUSION ASE feedback. The conversion from C-band to L-band results in a total A high-gain printed dipole antenna has been proposed, con- spectral power improvement of 0.3 dB, while the 980-nm pump power is structed, and tested. The proposed antenna has a simple configu- fixed at 44.5 mW. The average power-density improvement for the L- ration and is easy to implement with a low cost. A constructed band region (1565–1605 nm) was 18.9 dB with a maximum of 33.8 dB prototype suitable for application in the 2.4-GHz band for WLAN at 1565.0 nm, compared to the feedback and nonfeedback systems. The operation has been studied. The prototype has a narrow width of 10 maximum power-density results for the feedback and nonfeedback sys- Ϫ Ϫ mm and a total length of 202 mm only (1.64 wavelengths at 2442 tems are 10.4 and 4.0 dBm at 1559.4 and 1563.4 nm, respectively. A bandwidth of 85.6 nm (1515.4–1601.0 nm) and 67.3 nm MHz) and performs as a good collinear array antenna with three (1545.2–1612.5 nm) was obtained for the feedback and nonfeedback in-phase half-wavelength resonant elements. A high antenna gain systems, respectively, at a power density of Ϫ30 dBm/nm. © 2005 Wiley level of about 6.6–6.8 dBi for frequencies across the 2.4-GHz Periodicals, Inc. Microwave Opt Technol Lett 46: 218–220, 2005; band has been obtained. This antenna gain level is much larger Published online in Wiley InterScience (www.interscience.wiley.com). than that of a conventional half-wavelength dipole antenna. DOI 10.1002/mop.20949

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