T-PROBE PROXIMITY-FED SHORTCIRCUITED PATCH ANTENNA Yong-Xin Guo,1 Kwai-Man Luk,2 and Kai-Fong Lee3 Institute for Communications Research National University of Singapore 20 Science Park Road, Singapore Science Park II Singapore 117674 2 Department of Electronic Engineering City University of Hong Kong 83 Tat Chee Avenue Kowloon, Hong Kong SAR, PRC 3 School of Engineering University of Mississippi University, MS 38677 1 Received 19 September 2002 Figure 2 SWR and gain versus frequency ABSTRACT: A broadband T-probe proximity-fed short-circuited quarter-wavelength rectangular patch antenna is proposed as one possible candidate for mobile communication applications. Using a foam layer of thickness ⬃0.09␭0 as a supported substrate, an impedance bandwidth of 59% and a gain of over 4.5 dBi have been obtained. The far-field patterns are stable across the passband. Experimental and computed results based on the finite-difference time-domain (FDTD) method are provided. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 37: 1–2, 2003; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mop.10805 Key words: microstrip antennas; small antennas; wideband antennas 1. INTRODUCTION Mobile communication systems have considerably improved mobility, but the antenna still remains a bulky element. Several techniques have been proposed to overcome the size problem of conventional patch antennas, which can be the preferred option for mobile communication applications because they have the attractive features of low profile and low weight, and can be made conformal to mounting structures. The simplest of these is to incorporate high-dielectric-constant substrates and cover layers [1]. However, this technique suffers from the need of expensive substrate, therefore, is unsuitable for most applications. The use of a short circuit [2] or a short pin [3] has also been proposed. Significant size reductions have been achieved by these methods, however, the bandwidth of the proposed antennas tend to be very narrow in nature, typically ⬍2%. Much attention has focused on the improvement of the bandwidth of short-circuited patch antennas [4 – 6]. Two stacked short-circuited patches were used to achieve a bandwidth of 30% [4]. With the use of a U-slot cut on the single-layer shorted patch, an impedance bandwidth of 28% was attained [5]. As for the use of the L-probe feeding a shorted rectangular patch, 39% bandwidth was achieved [6]. The L-probe fed patch is single-layer and single-patch. Thus, it is very simple in structure and more suitable for mobile communication systems. On the other hand, a regular patch with a T-probe feeding proved to have a wider bandwidth than that with an L-probe feeding [7]. Moreover, the T-probe feeding technique can be more suitable for the structure of some applications than the L-probe because the horizontal arm of the T-probe is placed parallel to the patch radiating edge, while the horizontal arm of the L-probe is placed vertical to the patch radiating edge [7]. In this paper, we report our experimental and computed results on a T-probe proximity-fed short-circuited rectangular patch antenna. The computation is based on the finite-difference timedomain (FDTD) method [8]. The proposed antenna achieves an impedance bandwidth of ⬃59% (SWR ⱕ 2), and a gain of over 4.5 dBi in the majority of the broadband. The far field patterns are stable across the passband. Compared with a ␭/2 regular patch resonating at the same frequency of the TM01 mode and with the same aspect ratio, the new antenna has a fourfold reduction in area. 2. GEOMETRY The antenna as shown in Figure 1 has a patch separated from the ground plane by a foam layer of dielectric constant close to unity with one side of the patch shorted. The patch is proximity fed by a T-shaped coaxial probe, and it is excited in the TM01 mode. It has the following parameters: L P ⫽ 12 mm (⬃0.18␭0, where ␭0 is the free-space wavelength corresponding to the center frequency, 4.46 GHz, of the patch), W P ⫽ 30 mm, T h ⫽ 9 mm (⬃0.134␭0), T v ⫽ 4.5 mm (⬃0.067␭0), D ⫽ 1 mm, R ⫽ 0.5 mm, and H ⫽ 6.0 mm (⬃0.09␭0). 3. RESULTS AND DISCUSSIONS Figure 1 Geometry of the T-probe proximity-fed short-circuited patch antenna The results for SWR and gain of the antenna are shown in Figure 2. The measured SWR is ⱕ2 in the frequency range 3.15–5.77 GHz, corresponding to an impedance bandwidth of 59% centered at 4.46 GHz. The broadband performance of the antenna is confirmed by the computation based on a FDTD code developed in-house [9]. It can be seen that good agreement between calculated and measured results is obtained. The T-probe incorporated MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 37, No. 1, April 5 2003 1 3. 4. 5. 6. 7. 8. 9. sured bandwidth of quarter-wave microstrip radiators, IEEE Trans Antennas Propogat 36 (1988), 1615–1616. R.B. Waterhouse, Small microstrip patch antenna, Electron Lett 31 (1995), 604 – 605. L. Zaid, G. Kossiavas, J.Y. Dauvignac, J. Cazajous, and A. Papiernik, Dual-frequency and broad-band antennas with stacked quarter wavelength elements, IEEE Trans Antennas Propagat 47 (1999), 654 – 660. Y.X. Guo, A. Shackelford, K.F. Lee, and K.M. Luk, Broadband quarterwavelength patch antennas with a U-slot, Microwave Opt Technol Lett 28 (2001), 328 –330. Y.X. Guo, K.M. Luk, and K.F. Lee, L-probe proximity-fed shortcircuited patch antennas, Electron Lett 35 (1999), 2069 –2070. C.L. Mak, K.F. Lee, and K.M. Luk, Broadband patch antenna with a T-shaped probe, Proc IEE 147 (2000), 73–76. K.S. Yee, Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media, IEEE Trans Antennas Propagat 14 (1966), 302–307. Y.X. Guo, C.L. Mak, K.M. Luk, and K.F. Lee, Analysis and design of L-probe proximity-fed patch antennas, IEEE Trans Antennas Propagat 49 (2001), 145–149. © 2003 Wiley Periodicals, Inc. Figure 3 Radiation patterns at 4.46 GHz, 10 dB/div A LOW-COST SURFACE-MOUNT MONOPOLE ANTENNA FOR GSM/DCS OPERATION Gwo-Yun Lee,1 Hong-Twu Chen,2 and Kin-Lu Wong1 Department of Electrical Engineering National Sun Yat-Sen University Kaohsiung 80424, Taiwan 2 Department of Electrical Engineering Chinese Military Academy Feng-Shan 83056, Taiwan 1 with the patch introduces a capacitance suppressing some of the inductance introduced by the feed probe due to thick substrate, and another resonance near the resonance of the patch antenna can be created. Figure 3 shows the measured radiation patterns at 4.46 GHz. For the experiment, the short-circuited patch was mounted on a large ground plane (a circular disc with its diameter ⬃3␭0) to reduce diffraction off the edges. The radiation patterns have the same characteristics over the broad band. It should be noted that there is a beam squint of about 45° to 55° from broadside in the E-plane, which can be attributed to asymmetric current distribution of the patch due to the presence of the shorted wall and the T-probe. This was also observed in [4 – 6]. From Figure 2, it is interesting to see that, in the main beam direction, a gain of over 4.5 dBi in the majority of the broadband for the E-plane was measured. However, a maximum gain of ⬃2.5 dBi for the H-plane can be observed at broadside from Figures 2 and 3. This is from the fact that the H-plane cross-polarization level is relatively high, while the E-plane cross-polarization is at a low level in front, which can also be found in [4 – 6]. Although the H-plane crosspolarization is quite high, it may not be a disadvantage in some applications, for example, indoor mobile communications. 4. CONCLUSION We have described the combination of the T-probe bandwidth enhancement and the short-circuited size-reduction techniques, in the design of a broadband quarter-wavelength rectangular patch antenna. For a foam substrate thickness of ⬃0.09␭0, the resulting antenna appears to be 20% and 29% wider in bandwidth than the L-probe feeding and the stacked cases, respectively. REFERENCES 1. T.K. Lo, C.O. Ho, Y. Hwang, E.K.W. Lam, and B. Lee, Miniature aperture-coupled microstrip antenna of very high permittivity, Electron Lett 33 (1997), 9 –10. 2. S. Pinhas and S. Shtrikman, Comparison between computed and mea- 2 Received 8 October 2002 ABSTRACT: By folding a metallic strip onto a foam base, a low-cost surface-mount monopole antenna suitable for GSM/DCS dual-band operation is presented. The antenna has a compact size of 33 ⫻ 7.5 ⫻ 7.5 mm3 and requires no isolation distance to the system ground plane when surface-mounted to the system circuit board of a communication device. Experimental results of a constructed prototype covering the operating bandwidths of the GSM (890 –960 MHz) and DCS (1710 – 1880 MHz) systems are presented. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 37: 2– 4, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10806 Key words: surface-mount antennas; monopole antennas; mobile antennas 1. INTRODUCTION By folding a metallic strip onto a foam or plastic base, low-profile monopole antennas suitable for GSM/DCS dual-band operations have been recently demonstrated [1, 2]. In addition to their capability of dual-band operation, such antennas are also suitable to be surface-mounted on a circuit board of a communication device, similar to the conventional ceramic chip antennas [3– 6]. Moreover, in comparison to the ceramic chip antennas, which are fragile in nature, the foam or plastic chip antennas [1, 2] are inexpensive to construct and will not break. The foam or plastic chip antennas shown in [1, 2], however, require an isolation distance to the ground plane (a connection strip is required between the chip antenna and the 50⍀ microstrip feed line), when the antenna is surface-mounted to the system circuit board of a communication device. This isolation distance (4 mm in MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 37, No. 1, April 5 2003