Modified helix chip antenna for WiBro and WLAN applications J. Jung, H. Lee and Y. Lim A modified helix chip antenna is presented for WiBro and WLAN applications. The impedance and radiation characteristics of the antenna are investigated. The antenna is formed on both sides of the substrate (3.5 8 0.8 mm, FR4). The measured results show that the proposed antenna has a bandwidth of 290 MHz (2.26 – 2.55 GHz) and a maximum radiation gain of 1.468 dBi.
Introduction: Chip antennas have the advantages of being compact in size and mountable on the surface of the circuit board of a communication device [1]. WLANs provide high-speed wireless connectivity between PCs, laptops and other equipment in corporate, public and home environments. The portable Internet service known as WiBro is now in the process of commercialisation [2]. In this Letter, a modified helix chip antenna for the WiBro and WLAN bands is proposed. The proposed antenna is designed adaptively in an efficient miniaturisation process for broadband operation. In the miniaturisation of the antenna, a meander and a helix section are analysed in terms of the current amplitude and the coupling effect between the radiating elements. Furthermore, by inserting two vertical lines in the antenna, broadband operation can be achieved.
the geometry values of the helix varied. The helix has geometry values that are identical to those of the proposed antenna, and the height of the helix antenna is fixed at 6.7 mm. Fig. 2 shows the simulated results while varying the values of the HP and pitch of the helix on the reactance. Table 1 shows the performance of the antennas shown in Fig. 2. By varying the HP from 0.3 to 3.3 mm while maintaining the pitch of the helix at 0.5 mm, the resonant frequency of the helix antenna is increased from 2.72 to 3.14 GHz, as shown in Fig. 2. In the helix section (with a positive coupling region), the current vectors of each helix are in the same direction. The effective length of the antenna is increased by the positive coupling effect. In addition, the positive coupling effect is proportional to the current amplitude. Therefore, in a l/4 monopole antenna, the resonant frequency of the helix antenna is decreased when the helix section becomes close to the feed section. By varying the pitch from 0.5 to 1.7 mm while maintain the HP at 0.3 mm, the resonant frequency of the helix antenna is increased from 2.72 to 3.1 GHz, as shown in Fig. 2. The positive coupling between each helix is decreased when the pitch is increased, and the positive coupling effect disappears when the pitch becomes greater than the specification length (in this helix antenna, the specification length is 1.5 mm). As a result, when the helix section is near the feed section and the pitch decreases, the resonant frequency of the helix antenna is decreased. Therefore, the helix is efficient in the miniaturisation of the antenna.
Antenna design and results: Fig. 1 shows the geometry of the proposed antenna. It is printed on both sides of an FR4 substrate (with a relative permittivity of 4.2 and a thickness of 0.8 mm). The printed radiating elements on both sides of the substrate are connected by via links (with a diameter of 0.3 mm), and circular patches are printed around the via-holes for smooth connections of the via links. The dimensions of the proposed antenna are 3.5 8 0.8 mm. A test board approximating the circuit board of a general mobile terminal was used to measure the antenna. The test board utilises an FR4 substrate with a relative permittivity of 4.2 and a thickness of 0.8 mm. The dimensions of the ground plane are 40 70 mm. On the test board, patches were printed for the feed and fixing of the antenna.
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Fig. 1 Geometry of proposed antenna a Top view b Bottom view c Test board
The antenna consists of the three sections of a meander, two vertical lines and a stacked helix. In the meander section (with a negative coupling region), the current vectors are in opposite directions. Thus, the effective length of the antenna is reduced and the spacing between the meander sections (which increases the negative coupling) is decreased, thereby resulting in a net decrease in the operating frequency of the antenna [3]. In addition, the negative coupling effect is proportional to the current amplitude. The meander section is therefore top-loaded for the purpose of decreasing the coupling effect. In addition, the impedance bandwidth of the antenna is extended when its current distributions are dispersed. Therefore, the two vertical lines serve to disperse the current distributions for broadband operation [4]. A stacked helix is situated near the feed section for miniaturisation of the antenna. A helix antenna is used to observe resonant frequency, and
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Fig. 2 Reactance for helix antenna with various HP and pitches a Geometry of helix antenna b Reactance
Table 1: Performance of antennas shown in Fig. 2 HP [mm] 0.3 0.8 1.3 1.8 2.3 2.8 3.3 Resonant frequency [GHz] 2.72 2.78 2.83 2.9 2.97 3.05 3.14 Pitch [mm] 0.5 0.7 0.9 1.1 1.3 1.5 1.7 Resonant frequency [GHz] 2.72 2.85 2.92 2.98 3.04 3.09 3.1
Fig. 3, which shows the simulated and measured return loss for the given dimensions, confirms the good agreement between the measured and simulated results. The predicted and measured 10 dB return loss bandwidth was 220 MHz (2.28 –2.5 GHz) and 290 MHz (2.26 – 2.55 GHz), respectively. In addition, the radiation characteristics of the proposed antenna were measured. Fig. 4 shows the measured radiation patterns at 2.3, 2.35, 2.4, 2.45 and 2.48 GHz (WiBro: 2.3– 2.4 GHz; WLAN: 2.4– 2.48 GHz). As shown in Fig. 3, the maximum
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gain of the proposed antenna is 1.468 dBi at 2.45 GHz, and the gain variations are less than 0.868 dBi for the operating frequencies.
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Conclusions: A modified helix chip antenna for the WiBro and WLAN bands is proposed and investigated. To reduce the dimensions of the antenna, the characteristics of the helix have been analysed. The proposed antenna has the dimensions of 3.5 8 0.8 mm. The obtained 10 dB return loss bandwidth was 290 MHz and the measured maximum gain was 1.468 dBi.
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# The Institution of Engineering and Technology 2008 18 January 2008 Electronics Letters online no: 20080173 doi: 10.1049/el:20080173
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J. Jung and Y. Lim (Department of Electronics and Computer Engineering, Chonnam National University, Gwang-ju, Republic of Korea)
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H. Lee (Department of Electrical and Electronics Engineering, Dongkang College, Gwang-ju, Republic of Korea)
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References
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1 Lee, G.-Y., Chen, W.-S., and Wong, K.-L.: ‘A dual-frequency triangular chip antenna for WLAN operation’, Microw. Opt. Technol. Lett., 2003, 38, (3), pp. 244–247 2 Lee, D.-H., Lee, H.-J., Lee, Y.-W., and Shin, D.-H.: ‘Wireless broadband services and network management system in KT’, Int. J. Netw. Manage., 2006, 16, pp. 429– 442 3 Best, S.R., and Morrow, J.D.: ‘The effectiveness of space-filling fractal geometry in lowering resonant frequency’, Antennas IEEE Wirel. Propag. Lett., 2002, 1, pp. 112–115 4 Jung, J., Lee, H., and Lim, Y.: ‘Modified meander line monopole antenna for broadband operation’, Electron. Lett., 2007, 43, (22), pp. 1173– 1174
180 2.30 GHz 2.35 GHz 2.40 GHz 2.45 GHz 2.48 GHz
Fig. 4 Measured radiation patterns
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