ICICS-PCM 2003 15-18 December 2003 Singapore

2B7.6

A Triple Band Antenna for GSM and GPS Application Min Sze Yap, Lenna Ng, and Sheel Aditya Positioning and Wireless Technology Centre, School of Electrical and Electronic Engineering Nanyang Technological University, Singapore

Abstract

has reported an antenna combining GSM (linear polarization) and GPS (circular polarization) operation.

We report a dual-feed microstrip antenna for operation in the GSM 900/1800 MHz bands and GPS 1575 MHz band. The GPS antenna is right circularly polarized whereas the GSM antenna is linearly polarized. Simulation and measured results are presented for return loss and isolation. An isolation of better than –33dB between the two feed points has been achieved. The overall ground plane size is seen to be smaller than that for a triple-band PIFA design.

In the context of microstrip antennas, it is well known that for a given frequency, the triangular patch has a smaller size as compared to that of rectangular or circular patch. We report here a triple band antenna that consists of two triangular microstrip patch antennas. Each antenna has a separate feed. One of the patches works as a dual-frequency antenna covering the GSM bands. The other patch covers the GPS band. The two patch antennas can be put side-byside or back-to-back, sharing a common ground plane. Presented here are the simulation and measured results for return loss and isolation, for the back-to-back configuration.

1. Introduction In the recent years, there has been an increasing reliance on wireless communication to provide enhanced functionality for products and services. This places increased demands on the antennas used in wireless communication systems. In this context, three major areas of activity related to antennas are size reduction, wideband or multiband operation, and adaptive pattern control [1].

2. 900/1800 antenna

dual-frequency

The design is based on the single-feed dual-frequency equilateral-triangular microstrip antenna incorporating a pair of spur lines [9]. The antenna configuration and optimized dimensions are shown in Fig. 1. The substrate material is low cost FR-4 ( ξ r = 4.4, tanδ = 0.022, h = 1.6 mm). The triangular patch, without spur lines, excited in the TM10 mode, decides the higher resonant frequency. When a pair of uniform narrow width spur lines is embedded, with a length greater than about one-half the side length of the patch, as shown, a new resonant mode is also excited; this mode decides the lower resonant frequency. The spur lines, being parallel to the patch edges, have a very small effect on the higher resonant frequency. Furthermore, both resonant modes can be excited with good impedance match using a single probe feed located along the centre line. For the optimized dimensions, simulation results for return loss, obtained using Ansoft ENSEMBLE, are presented in Fig. 2. The resonant frequencies are 898 and 1804 MHz, matching well the target frequencies. At both frequencies, broad, linearly polarized radiation patterns are indicated (not shown).

With the introduction of 3G cellular communication systems in many parts of the world, the demand for a single compact antenna to support both 900 MHz and 1800 MHz operation is obvious. In addition, integration of GPS operation with the communication devices is also desirable to support Intelligent Transportation Systems, and to integrate location-finding service with mobile phones to enhance user safety. While the GSM operation requires linear polarization, GPS, operating at 1575 MHz (Coarse Acquisition), requires right hand circular polarization. This makes the development of an antenna that can cater to the three bands quite difficult. A number of approaches have been reported in the literature for achieving multi-frequency operation using printed antennas. Multi-layered aperture coupled microstrip antennas have been proposed in [2-3]; a single-feed multiband microstrip helical antenna is reported in [4]; triple-feed triple band planar inverted F antennas (PIFA) are proposed in [5]; a single-feed triple band multi-layer microstrip antenna for GSM/DCS/GPS operation, incorporating a feed network is reported in [6]; slot-ring antennas on thin low dielectric constant substrates are presented in [7]. Multi-frequency antennas based on fractal structures have also been studied by a number of authors, e.g. [8]. It can be noted that only one of these references [6]

0-7803-8185-8/03/$17.00 © 2003 IEEE

MHz

3. 1575 MHz antenna

circularly

polarized

The design consists of an equilateral triangular microstrip patch antenna with a narrow tuning stub loading the

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triangle tip [10]. The antenna configuration and optimized dimensions are shown in Fig. 3. The substrate material is the same as for the previous design. Due to the tuning-stub perturbation, the fundamental mode can be split into two orthogonal near-degenerate modes for circularly polarized operation. L1 and L2 represent, respectively, the 50-ohm feed-location loci of the two orthogonal modes. These loci can be determined experimentally. By selecting the probe feed at point A, right-hand circular polarization can be obtained. For the optimized dimensions, simulation results for return loss are presented in Fig. 4. The two orthogonal modes resonate at 1535.35 and 1575.75 MHz. At 1575 MHz, the upper resonant frequency, the two radiated orthogonal polarizations have magnitude difference of about 6 dB and a phase difference of 86°. The magnitude difference is expected to be smaller near the middle of these two frequencies.

5. Conclusion Design and measured results for a compact dual feed tripleband antenna assembly, for operation in the GSM and GPS bands, have been presented. The design features a small size and high isolation between the feed ports. Further optimization and more careful fabrication is expected to improve the performance with respect to operating frequencies and bandwidth.

References [1] C. B. Dietrich, Jr., R. M. Barts, W. L. Stutzman, and W. A. Davis, “Trends in antennas for wireless communications,” Microwave Journal, Jan. 2003, pp. 2244. [2] F. Croq and D. M. Pozar, “Multifrequency operation of microstrip antennas using aperture coupled parallel resonators,” IEEE Trans. Ant. and Propag., vol. 40, no. 11, Nov. 1992, pp. 1367-1374. [3] X. H. Yang and L. Shafai, “Multifrequency operation technique for aperture coupled microstrip antennas,” AP Symp., 1994, pp. 1198-1201. [4] Chien-Jen Wang, C. F. Jou, and S. T. Peng, “Small microstrip helical antenna,” AP Symp., 1999, pp. 367-370. [5] C. T. P. Song, P. S. Hall, H. Ghafouri-Shiraz, and D. Wake, “Triple band planar inverted F antennas for handheld devices,” El. Lett., 20 Jan. 200, vol. 36, no. 2, pp. 112-114. [6] Shyh-Tirng Fang and Jyh-Wen Sheen, “A planar tripleband antenna for GSM/DCS/GPS operations,” AP Symp., 2001, pp. 136-139. [7] H. Tehrani and Kai Chang, “Multifrequency operation of microstrip-fed slot-ring antennas on thin low-dielectric permittivity substrates,” IEEE Trans. Ant. and Propag., vol. 50, no. 9, Sept. 2002, pp. 1299-1308. [8] J. yeo and R. Mittra, “Modified Sierpinski gasket patch antenna for multiband applications,” AP Symp., 2001, pp. 134-137. [9] Jui-Han Lu and Kin-Lu Wong, “Single-feed dualfrequency equilateral-triangular microstrip antenna with pair of spur lines,” El. Lett., 11 June 1998, vol. 34, no. 12, pp. 1171-1173. [10] Jui-Han Lu and Kin-Lu Wong, “Single-feed circularly polarized equilateral-triangular microstrip antenna with a tuning stub,” ,” IEEE Trans. Ant. and Propag., vol. 48, no. 12, Dec. 2000, pp. 1869-1872.

4. Results and discussion for triple band antenna The antennas described above can be combined into a single assembly in two ways. These can be put side-by-side on the same substrate (Fig. 5), or these can be put back-toback and clamped together (Fig. 6 a and b). Measured isolation between the two feed ports is high in both configurations. The ground plane sizes in the two cases are 10x7 cm2 and 7.4x7.5 cm2, respectively. Since the latter configuration yields a smaller size, this is the one for which results are presented here. Simulation results for return loss and radiation patterns for the combined antenna are close to those mentioned in sections 2 and 3. The measured return loss is shown in Figs. 7a and b, for the GSM and the GPS antenna, respectively. For GSM bands, the resonant frequencies are 952 and 1812 MHz, with –10 dB return loss bandwidth of 1.5% and 1.2%, respectively. While the higher frequency is close to the simulation, the reason for deviation for the lower frequency appears to be error in the fabrication of the narrow spurs in the dual-frequency antenna. For the GPS band, the measured resonant frequencies for the two orthogonal resonant modes are 1555 and 1576 MHz. The measured isolation values between the two feed ports are better than –33dB. The ground plane dimensions are not mentioned in [6]. The ground plane size for the triple band PIFA [5], operating at 900, 1890, and 2440 MHz (all linearly polarized), is 11x5.8 cm2. The size of the design presented here, is smaller than that.

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Fig. 1: Design of the dual-frequency antenna for GSM bands; L = 53 mm, l = 42.5 mm, dp = 2.8 mm.

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Fig. 3: Design of the circularly polarized antenna for GPS band; L = 61 mm, ls = 5.6 mm, (xp, yp) = (-8.0, 4.5).

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Fig.2: Simulated return loss for the dual-frequency antenna shown in Fig.1

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Fig. 4: Simulated return loss for the circularly-polarised antenna shown in Fig. 3.

Fig. 5: Photograph of the combined antenna – side-by-side configuration with a S$ 1 coin.

Fig. 6(a): Plan view of the combined antenna – back-toback configuration.

Fig. 6(b): Side view of the combined antenna – back-toback configuration.

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Fig. 7(a): Simulated return loss for the dual-frequency antenna shown in Fig.1

Fig. 7(b): Measured return loss for the Circularly-polarised antenna shown in Fig. 3.

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