A Bent, Shorted, Planar Monopole Antenna for 2.4 GHz WLAN Applications
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A Bent, Shorted, Planar Monopole Antenna for 2.4 GHz WLAN Applications

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A simple, bent monopole antenna well useful for WLAN applications in the 2.4 GHz band is presented. The monopole antenna has a rectangular radiating plate in general and is short-circuited to a small ...

A simple, bent monopole antenna well useful for WLAN applications in the 2.4 GHz band is presented. The monopole antenna has a rectangular radiating plate in general and is short-circuited to a small antenna ground and an assembly plate. The assembly plate is not only used as a supporting plate for antenna installation but also regarded as antenna ground. With a low profile of the monopole and use of the coaxial-line feed, the antenna has much flexibility in the placement inside a wireless device. Good radiation characteristics have been observed too.

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    A Bent, Shorted, Planar Monopole Antenna for 2.4 GHz WLAN Applications A Bent, Shorted, Planar Monopole Antenna for 2.4 GHz WLAN Applications Document Transcript

    • TABLE 3 Relative Permittivity and Loss Tangent of TABLE 4 Solutions to axb c (Power Curve Fit) for Commonly Used Dielectric Materials Dielectric Material Used Loss Material a b c Material Er tangent Duroid 2.159 0.999 0.0730 Duroid 2.2 0.0009 Rogers 3003 1.317 1.008 0.0093 Rogers 3003 3.0 0.0013 Epoxy 1.068 1.005 0.0350 Epoxy 4.0 0.0180 FR4 0.886 0.995 0.0470 FR4 4.4 0.0200 Arion AR600 0.630 1.003 0.0210 Arion AR600 6.0 0.0035 Rogers TM10 0.435 1.000 0.0230 Rogers TM10 9.2 0.0022 REFERENCES the change in plane impedance that resulted. The impedance values 1. J.R. Miller, G. Blando, K.B.A. Williams, and I. Novak, Impact of PCB are solver values. The TE(1,0,1) mode E-field pattern (see Fig. 3) laminate parameters on suppressing modal resonances, DesignCon, is the solver output for the frequency of interest. For the base 2008, pp. 7–9. geometry that frequency is 1.56 GHz. The frequency for each 2. L. Guang-Tsai, R.W. Techentin, and B.K. Gilbert, High-frequency scaling example is 1.86 GHz. characterization of power/ground-plane structures, IEEE Trans Micro- The impedance calculation comparison involved six different wave Theory Tech 47 (1999), 562–569. materials with r values of 2.2, 3.0, 4.0, 4.4, 6.0, and 9.2, respec- 3. M. Xu, H. Wang, and T.H. Hubing, Applications of the cavity model to tively. These materials (Table 3) were selected as typical for lossy power-return plane structures in printed circuit boards, IEEE Trans Adv Packaging 26 (2003), 73– 80. construction of microwave and high-speed digital boards. Five different dielectric thicknesses were used for each material, 100, © 2008 Wiley Periodicals, Inc. 50, 25, 10, and 3 mils. The arithmetic mean of the ratio of the HFSS and calculated values for these five heights over the range of r values is shown in Figure 4. The solutions for the plane imped- ance magnitude converged at an r of 4.0 and were divergent on A BENT, SHORTED, PLANAR either side of that value. At values less than 4.0, the calculated MONOPOLE ANTENNA FOR 2.4 GHz results underestimate the impedance, and at values greater than 4.0 WLAN APPLICATIONS the calculated solution overestimates the impedance. The calcu- lated results were curve fit to the HFFS data whose solution is of Saou-Wen Su1 and Fa-Shian Chang2 1 the form aXb c, where a, b, and c are the unknown coefficients Network Access Strategic Business Unit, Lite-On Technology Corporation, Taipei County 23585, Taiwan; Corresponding author: to be solved for, and X is the calculated solution matrix for (5). susw@ms96.url.com.tw Table 4 lists the coefficients for each material. 2 Department of Electronics, Cheng Shiu University, Kaohsiung County 83347, Taiwan 6. CONCLUSION Closed form solutions for resonant frequency and unloaded Q Received 19 June 2008 correlate well to 3D solver data given that the correct boundary conditions are used. Lambda-epsilon scaling can be used to arbi- ABSTRACT: A simple, bent monopole antenna well useful for WLAN trarily shift and dampen resonances in a power/ground plane applications in the 2.4 GHz band is presented. The monopole antenna cavity. The 2D wave equation correctly captures the effects of has a rectangular radiating plate in general and is short-circuited to a changing the dielectric height and permittivity and the impedance small antenna ground and an assembly plate. The assembly plate is not magnitude was accurate for r of 4.0 which was the case under only used as a supporting plate for antenna installation but also re- garded as antenna ground. With a low profile of the monopole and use interest. of the coaxial-line feed, the antenna has much flexibility in the place- ment inside a wireless device. Good radiation characteristics have been observed too. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 455– 457, 2009; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mop.24106 Key words: antennas; monopole antennas; plate antennas; WLAN an- tennas 1. INTRODUCTION There has been a great success in developing WLAN technology over the past few years. Many Wi-Fi-enabled consumer-electronic devices are easily seen and available in the market. Moreover, the laptops nowadays are almost equipped with 802.11a/b/g wireless functionality as a basic, required specification. For these wireless applications, the antenna can play an important role among some key components in determining RF performance. Because of di- verse industrial design appearances of wireless devices, there Figure 4 HFSS to calculated mean values versus permittivity. [Color cannot easily exist one antenna design to fit all devices, especially figure can be viewed in the online issue, which is available at www. the antenna properties are prone to vary from free space to device interscience.wiley.com] housing. Therefore, many kinds of WLAN antennas have widely DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009 455
    • been studied in academia. Among many of them, the coaxial-line- fed antennas of a small form factor have favorably been introduced [1–7]. Owing to their flexibility and mobility, these kinds of antennas are very attractive to fit into many kinds of WLAN devices. In this letter, we demonstrate a new design of a low- profile, short-circuited monopole antenna for WLAN operation in the 2.4 GHz (2400 –2484 MHz) band with good impedance band- Figure 2 Photo of a constructed prototype made of a 0.3-mm-thick alloy width. The antenna is fabricated by stamping a single, flat metal and fed by a 50- mini-coaxial cable of length 30 mm. [Color figure can plate only, which consists of a bent monopole, a shorting portion, be viewed in the online issue, which is available at www.interscience. a small antenna ground, and a flat assembly plate that is connected wiley.com] to the antenna ground. The assembly plate in this design is not only used as a supporting plate for affixing the antenna to a wireless to meet the bandwidth requirement of the WLAN 2.4 GHz oper- device’s internal surface but also treated as antenna ground. De- ation. tails of a design example of the proposed antenna are described, To test the design prototype in experiments, a short 50- and the experimental results thereof are elaborated and discussed. mini-coaxial cable of length 3 mm with an I-PEX connector is utilized (see photo of a working sample in Fig. 2). The inner 2. ANTENNA DESIGN conductor of the coaxial cable is connected to feed point A, and the Figure 1(a) shows the configuration of the proposed, bent mono- outer braided shielding is connected to ground point B. Notice that pole antenna in detail. The antenna mainly comprises a bent a small portion of 2 mm 4 mm protruding from the antenna monopole, a shorting portion, an antenna ground, and an assembly ground is used to accommodate ground point B. In addition, the plate. The bent monopole, formed by bending a radiating plate, is near optimal value of the feed gap in between the bent monopole fed at one corner and short-circuited, at a partial side of the and antenna ground is 1 mm in the design. monopole, to the antenna ground and assembly plate through the shorting portion. Both the bent monopole and shorting portion are 3. EXPERIMENTAL RESULTS AND DISCUSSION 5 mm in height and also in width and located above the antenna Figure 3 shows the measured and simulated return loss of a design ground (5 mm 40 mm). The assembly plate with dimensions 10 prototype. It can be first seen that in general, the experimental data mm 40 mm is then perpendicularly connected to the antenna compare favorably with the simulation results, which are based on ground. In this case, the proposed antenna can firmly be affixed to the finite element method (FEM). The measured impedance band- the internal surface of a wireless device by various mechanic width, defined by 10 dB return loss, reaches about 195 MHz methods. Detailed dimensions of the antenna in a flat plate struc- (2368 –2563 MHz) and meets the bandwidth specification for 2.4 ture are also given in Figure 1(b). Notice that for matching the GHz WLAN operation easily. Notice that the impedance matching input impedance of the antenna, a small gap of distance d between is even better than 14 dB (about 1.5:1 VSWR). Further, when there the antenna feed (point A) and shorting portion is carefully tuned is no assembly plate (see inset in Fig. 4), the achievable bandwidth can still cover the 2.4 GHz band with 10-dB return-loss require- ment. The obtained result is highly beneficial because it helps relax constraints on the mechanic structure in the assembly plate for affixing the antenna to the internal surface of some wireless device. For example, the assembly plate can be perforated by a few small, circular holes for holding welding posts with the antenna operating band remaining the same. Figure 5 gives the far-field, 2D radiation patterns at 2442 MHz in E and E fields. The measurement was conducted at a 3 3 7-m3 fully anechoic chamber, with the great-circle method, at Figure 1 (a) Geometry of the proposed, bent, shorted, planar monopole antenna for 2.4 GHz band. (b) Detailed dimensions of the antenna unbent Figure 3 Measured and simulated return loss; d 2 mm. [Color figure into a flat plate structure. [Color figure can be viewed in the online issue, can be viewed in the online issue, which is available at www. which is available at www.interscience.wiley.com] interscience.wiley.com] 456 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009 DOI 10.1002/mop
    • Figure 6 Measured peak antenna gain and radiation efficiency. [Color figure can be viewed in the online issue, which is available at www. Figure 4 Simulated return loss for the proposed antenna and the pro- interscience.wiley.com] posed antenna without the assembly plate. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] still operate in the 2.4 GHz band with 10 dB return loss. Omnidi- rectional radiation patterns have been obtained with good radiation Lite-On Technology Corp. Other frequencies in the 2.4 GHz band efficiency and TRP about 0.8 dB. The proposed antenna can be were also measured, and similar radiation patterns were obtained. a practical internal-antenna solution for WLAN devices. Notice that the omnidirectional radiation patterns in this study are in the x-z plane. Normally, the omnidirectional radiation occurs in REFERENCES the plane (in this study, it is the x-y plane) that is perpendicular to 1. C.H. Lee and S.O. Park, A compact printed hook-shaped monopole the monopole antenna. It is probable that because the antenna antenna for 2.4/5-GHz WLAN applications, Microwave Opt Technol mainly extends in the y-axis direction, the flow of the surface Lett 48 (2006), 327–329. currents is more in the lateral direction, which in turn changes 2. S.W. Su, J.H. Chou, A. Chen, and L. Tai, Compact patch antenna principal polarization of the antenna. The measured peak antenna mountable above conducting plate for WLAN operation, Electron Lett 42 (2006), 1130 –1136. gain and radiation efficiency are plotted in Figure 6. The gain is 3. V. Deepu, K.R. Rohith, J. Manoj, et al., Compact uniplanar antenna for stable and at about 2.4 dBi, and the radiation efficiency reaches WLAN applications, Electron Lett 43 (2007), 70 –72. about 83%, which corresponds to the total radiated power (TRP) of 4. S.W. Su and J.H. Chou, Compact coaxial-line-fed flat-plate dipole 0.8 dB when the antenna input power is 0 dBm. Notice that the antenna for WLAN applications, Microwave Opt Technol Lett 50 radiation efficiency was obtained in the 3D test system by calcu- (2008), 420 – 422. lating the TRP of an antenna under test over the 3D spherical 5. S.W. Su, J.H. Chou, and Y.T. Liu, A one-piece flat-plate dipole antenna radiation and then dividing the sum by the antenna input power for dual-band WLAN operation, Microwave Opt Technol Lett 50 (default value is 0 dBm). (2008), 678 – 680. 6. S.W. Su and J.H. Chou, Low-cost flat metal-plate dipole antenna for 4. CONCLUSIONS 2.4/5-GHz WLAN operation, Microwave Opt Technol Lett 50 (2008), 1686 –1687. A bent, short-circuited monopole antenna fed by mini-coaxial line 7. J.H. Chou and S.W. Su, Very-low-cost copper-wire antenna for 2.4- has been proposed, and a constructed prototype has been studied. GHz WLAN operation, Microwave Opt Technol Lett 50 (2008), 2107– The achievable impedance bandwidth for the antenna in the 2.4 2109. GHz WLAN band can be well defined by 1.5:1 VSWR. The results also indicate that when there is no assembly plate, the antenna can © 2008 Wiley Periodicals, Inc. Figure 5 Measured 2D radiation patterns at 2442 MHz for the antenna studied in Figure 3. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009 457