Microstrip Patch Antennas With Enhanced Gain by ... - IEEE Xplore

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 9, SEPTEMBER 2010

2811

Microstrip Patch Antennas With Enhanced Gain by Partial Substrate Removal Siew Bee Yeap, Member, IEEE, and Zhi Ning Chen, Fellow, IEEE

Abstract—A method to enhance gain of a microstrip patch antenna is investigated by partially removing the substrate surrounding the patch. The partial substrate removal reduces the losses due to surface waves and dielectric substrate. The effects of substrate removal in different configurations on the gain of the antenna are studied numerically and validated experimentally. Compared to a conventional patch antenna, the antennas with partial substrate removal can enhance gain, for example, up to 2.7 dB. Furthermore, it is observed that the enhancement of gain is more due to loss reduction of surface waves and dielectric substrate than increased patch size when the effective dielectric constant of substrate is lowered. Such a technique can be applied in designs operating at higher frequencies whereby surface wave and substrate losses are more significant though the 2.4-GHz design is exemplified here for ease of fabrication and measurement purposes. Index Terms—Microstrip antennas, surface waves.

I. INTRODUCTION

M

ICROSTRIP patch antennas is widely used due to being compact, conformal, and low cost. However, three types of losses i.e. conductor loss, dielectric loss and surface wave loss will lower gain of a patch antenna. The conductor loss and the dielectric loss depend on the quality of the materials being used such as copper or gold, and the substrate, respectively. The dielectric loss is dependent on the loss tangent of materials and substrate thickness while the surface wave loss depends on the permittivity of materials and the substrate thickness [1]. While better quality selection of conductor and substrate can reduce the conductor and dielectric losses therefore improving the gain of the antenna, the gain of the patch antenna can be further enhanced by suppressing surface waves. Jackson, et al. reported the designs that excite very little surface waves based on the principle that a ring of magnetic current in the substrate has a critical radius [2]. Another easy method is to replace the substrate of patch antennas with air of or with very low dielectric constant material, well known as suspended patch antennas [3]. The suspended patch requires an air gap and this is formed by a spacing material such as foam for fabrication purposes or the patch being supported by posts [4]. Both types can Manuscript received September 27, 2009; revised January 31, 2010; accepted March 19, 2010. Date of publication June 14, 2010; date of current version September 03, 2010. This work was supported by the Agency for Science, Technology and Research (A*STAR), Singapore through the Terahertz Science & Technology Inter-RI Program under Grant 082 141 0040. The authors are with Institute of Infocomm, Singapore 138632, Singapore (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2010.2052572

be fragile and not very durable at times making it unsuitable for mass production. Electromagnetic band gap structures are also used to improve antenna gain [5]–[7]. The periodic structures block the surface waves from propagating in a certain band gap. The alternative is to perforate the substrate i.e. drill holes in the substrate hence synthesizing a lower dielectric constant substrate [8], [9]. For ease of fabrication, the latter concept of synthesizing a lower dielectric constant by partially removing the substrate surrounding the patch is more practical. The method has also been applied by referring to it as trenches which are quite narrow but not studied with more detailed information [10]–[13]. In this paper, the effects of partial substrate removal on the performance of microstrip patch antenna are investigated to explore an effective yet simple way to enhance the gain of the antenna. The basic idea is to improve the gain of a microstrip patch antenna by suppressing surface waves and reducing dielectric loss through partial substrate removal surrounding the antenna. We refer to this method as open air cavity since the removed substrate can be a large portion. The designs with different substrate removals are studied numerically and verified experimentally. As an example, a microstrip patch antenna is designed to operate at 2.4 GHz to demonstrate the mechanism and effect of the method by studying the near-field distribution around the patch in Section II. Also in Section II, the gain of some open cavity designs are investigated. Then, the mutual coupling between two patch antennas lay opposite each other along the radiating edge direction is examined in Section III [14]. Section IV summarizes the findings with conclusions. II. MICROSTRIP PATCH WITH AND WITHOUT SUBSTRATE REMOVAL A. Aperture-Coupled Microstrip Patch Consider a conventional aperture-coupled microstrip patch antenna at 2.4 GHz on a dielectric substrate of Roger 6006 with and loss tangent of 0.0027. The geometry of the aperture-coupled microstrip patch is shown in Fig. 1(a). The patch and is positioned in the center of of square ground plane. The thicknesses of the a substrate are and . The thickness of is achieved by stacking multiple thin dielectric sheets of identical dielectric property. The width of the feeding strip . The coupled on the bottom of the substrate is slot positioned right underneath the center of the patch measures and . The strip stub has length of . Fig. 1(b) shows the simulated impedance bandwidth of 10% (2.33–2.57 GHz) for and gain against frequency.

0018-926X/$26.00 © 2010 IEEE

2812


TABLE I DESIGN PARAMETERS OF THE APERTURE-COUPLED PATCH WITH/WITHOUT SUBSTRATE REMOVAL. UNITS ARE ALL IN mm EXCEPT V IN%

Fig. 3. Gain increment of Designs B-H compared to Design A at 2.4 GHz. Fig. 1. (a) Geometry, (b) simulated jS crostrip patch antenna.

j

and gain of an aperture-coupled mi-

Fig. 2. Aperture-coupled microstrip patch antenna with substrate removal.

Fig. 4. Aperture-coupled microstrip patch antennas with different substrate removal configurations.

B. Microstrip Patch Aperture-Fed With Open Air Cavity In this section, the substrate surrounding the patch is removed to synthesize a lower dielectric substrate with an open air cavity. All the cavities have the same depth . The cavity with a width is cut from the edge of the patch as shown in Fig. 2. The effect of increasing on the gain of the antenna is studied. The dimensions of the antenna are accordingly adjusted to maintain the resonance at 2.4 GHz when changes. Table I shows the design parameters, including the original patch without any cavity , that is Design A. Since the patch is not namely square, is not the same value in the radiating and non-radiating edge when all the substrate is removed. This is shown in Table I where has two values in the edges, respectively. The ratio of the substrate’s volume after partial removal over the total sub, in percentage is strate’s volume included as well. Fig. 3 shows the gain changes of the designs listed in Table I compared to the gain of Design A at 2.4 GHz. It is observed that

when , the gain increment of Designs A-D are only for Designs F and G. When 0.3 dB but 1.5 dB as all the side substrate is removed, namely Design H, the gain increases over 2 dB. The increase in gain is due to the increase in overall dimensions of the patch, the reduction of dielectric loss and suppression of surface waves. C. Comparison of Patch Antennas With Varying Open Air Cavity Configuration Besides varying the open air cavity size as mentioned above, the effects of removing substrate in different configurations on the antenna gain are numerically investigated and validated experimentally as shown in Fig. 4. For comparison purpose, Designs A and H were fabricated as Designs I and II, respectively. In Design III, the dielectric substrate was removed from its radiating edges whereas the dielectric substrate was removed from the non-radiating edges in Design IV. The dimensions of the

YEAP AND CHEN: MICROSTRIP PATCH ANTENNAS WITH ENHANCED GAIN BY PARTIAL SUBSTRATE REMOVAL

2813

Fig. 6. Normalized electric field distributions of Designs I–VI (in dB). Fig. 5. Simulated and measured jS j of aperture-coupled microstrip patch antennas with different substrate removal configurations.

patch antennas were changed accordingly to keep their resonance at around 2.4 GHz. To confirm that the improved gain is also from the reduction of surface waves and not only from the increase in patch size, Design V with the same patch size as Design II was designed by changing the dielectric constant of substrate to keep the antenna operating at 2.4 GHz. The achieved dielectric constant of can be considered as the effective permittivity for Design II. For fabrication purposes, Roger 4003 substrate with and the same loss tangent of 0.0027 was used to approximate . the substrate with the effective dielectric constant of Design VI has an embedded air cavity right beneath the patch. The cavity size is the same as the patch. All the cavities have the same depth, . for the Fig. 5 compares the simulated and measured designs, which agree quite well. The simulation was carried out using CST Microwave Studio. The slight discrepancy between the achieved resonances in simulation and measurement is mainly caused by fabrication tolerance and, especially, air gap between the thin dielectric sheets which were used to form the desired thick dielectric substrate. The bandwidth for Designs I–VI is as follow: 8.3% (Design I), 5% (Design II), 5.1% (Design III), 6% (Design IV), 11.2% (Design V) and 12% (Design VI). Due to higher surface wave loss, Design I and Design V have wider impedance bandwidths than Design II–IV. Design VI has the highest impedance bandwidth which shows that the cavity in Design VI improves the

impedance bandwidth the most. However, Design II–IV can achieve wider gain bandwidths than the conventional ones and better radiation performance as will be shown later on. Fig. 6 illustrates the electric field distributions along the plane of the patch, for all the designs respectively, which are normalized by the maximum electric filed amongst the designs. Design I is used as reference. Some important observations can be deduced from Fig. 6. First, compared with Design I, Designs II and III have much weaker radiation from the edges of the ground plane/substrate whereas Designs V and VI also illustrate strong radiation from their edges. Compared with Design I, Design V has weaker radiation from its substrate edges due to its lower dielectric constant. Such radiation is mainly caused by surface waves. It proves the fact that the dielectric causes the surface waves and the higher the dielectric constant, the stronger the surface waves. Secondly, Design IV has stronger radiation from the edges of dielectric slab which are positioned along the radiating edges of the patch, whereas Design III has much weaker radiation from the edges of the dielectric slab which is positioned along the non-radiation edges of the patch. It suggests that the majority of surface waves stems from the radiating edges of the patch. Table II compares the simulated and measured gain and halfpower beamwidths (HPBW) of Designs I–VI at their simulated and measured resonance frequency, respectively. The measured resonance frequency slightly shifts from the simulated due to fabrication tolerances as mentioned previously. From both simulated and measured gain against frequency, it is found that the achieved gain slightly changes around 2.4 GHz where the designs realized impedance matching. Designs II and III have the

2814


TABLE II APERTURE-COUPLED PATCH ANTENNAS WITH DIFFERENT SUBSTRATE REMOVAL CONFIGURATIONS

Fig. 7. (a) Measured gain of Design I–VI, (b) Gain increment of Designs II–VI at their respective resonance frequencies compared to Design I at 2.4 GHz.

narrowest HPBW due to less surface waves hence achieving the highest gain. Fig. 7(a) shows the measured gain for Design I–VI while Fig. 7(b) shows the gain increment of Designs II–VI with different substrate removal configurations at their respective resonance frequencies compared to Design I at 2.4 GHz. As mentioned earlier on, Design II–IV can achieve wider gain bandwidths than the conventional ones (gain above 4 dBi). The gain of 4 dBi is chosen as Design I has almost a flat gain response. Designs II–VI achieved higher gain at their respective resonance than Design I at 2.4 GHz. In particular, Design II realized the highest gain increment while Design III is not far off. By comparing Design II with partial substrate removal and Design V with the similar effective dielectric constant and loss tangent as Design II but without any substrate removal, the former is 2.7 dB higher gain than Design I whereas Design V has the 1.3 dB higher gain than Design I. Again, it suggests that the extra gain obtained is due to the reduction of surface waves and dielectric loss, not only from the increase in patch size. Gain of 6.7 dB for Design III and 5.2 dB for Design IV demonstrates that the surface wave loss caused from the radiating edges is much more than that from the non-radiating edges. Also, comparing 2.7 dB and 2.4 dB gain improvement for Design II and Design III suggests that the presence of substrate in the non-radiating edge is not as critical as the radiating edge for surface wave loss. Therefore, it will be reasonable to remove the dielectric substrate from the radiating edges instead of non-radiating edges if partial substrate removal is allowed in antenna fabrication. Finally, the gain enhancement for Design VI indicates that the introduction of the embedded cavity may reduce dielectric loss but not substantial surface wave loss compared to Designs II–IV.

It should be noted that the suspended design does not suffer any loss caused by surface waves and dielectric substrate. However, they have weakness in fabrication tolerance and installation robustness as well as ease of integration into other planar circuits, in particular at higher operating frequency, such as millimeter wave and sub-millimeter wave bands. Fig. 8 shows the measured radiation patterns for the designs at their respective resonant frequencies in the E- and H-planes. The measured results agree well with simulations as shown here. The cross-polarization levels are less than 20 dB for Designs II, III, and IV. Designs II and III have the narrowest beamwidths in both E- and H-planes as well as better front to back ratio. The reduced surface wave loss in Design II–IV does leads to improved quality factor. This in turn reduces the bandwidth as shown in Fig. 5. The design trade-offs would then be a compromise between bandwidth and the other antenna performance such as gain, beamwidth and front to back ratio for this type of design. III. MUTUAL COUPLING In this section, the inter-element mutual coupling between two adjacent patch antennas with/without substrate removal (Designs I and II as tabulated in Table II) along their E-planes is examined. The antennas are placed opposite to each other as shown in Fig. 9 at a distance of away from each other. The . separation is chosen at 0.1, 0.25, 0.5, 0.75, and Fig. 10 shows the isolation for a pair of Designs I and II for the varying at 2.4 GHz, respectively. Design II achieves better isoand a maximum for with lation for

YEAP AND CHEN: MICROSTRIP PATCH ANTENNAS WITH ENHANCED GAIN BY PARTIAL SUBSTRATE REMOVAL

2815

Fig. 9. Two aperture-coupled patch antennas placed opposite to each other with separation d.

Fig. 10. Comparison of isolation at 2.4 GHz for Designs I and II with varying separation d.

Fig. 11. Comparison of normalized electric field distributions at 2.4 GHz as d : for (a) Design I—top view, (b) Design II—top view, (c) Design I—side view, and (d) Design II—side view.

= 05

Fig. 8. Measured radiation patterns of the patch antennas with different substrate removal configurations (Designs I–VI).

difference in isolation of 9.4 dB. The isolation then becomes almost similar for when the surface waves strength starts

to deteriorate. Another point to note is that Designs I and II as space have the same isolation when spacing is around waves dominate the mutual coupling. Fig. 11 compares the normalized electric field distributions of the scenarios with a pair of Designs I and Design II as . It is found that the substrate removal in Design II reduces the surface waves from the radiating edges of the excited patch (left-hand side) so that the coupled field at the radiating edges

2816


of the patch (right-hand side) is weakened. Therefore, the substrate removal greatly reduces the mutual coupling by 6.9 dB compared to Design I.

IV. CONCLUSIONS The effects of partial substrate removal on the gain of microstrip patch antennas have been investigated numerically and validated experimentally. It has been seen that by suppressing surface waves and reducing dielectric loss, the gain of the microstrip patch antenna, especially with high permittivity has been improved up to 2.4–2.7 dB when the substrate surrounding the radiating edges of the patch antenna have been fully or partially removed. Meanwhile, the E-plane mutual coupling between the two identical patch antennas at a distance of a half operating wavelength has been reduced by 6.9 dB. Therefore, the partial substrate removal is capable of improving gain and maintaining mechanical robustness of patch antennas.

REFERENCES [1] R. B. Waterhouse, Microstrip Patch Antennas—A Designer’s Guide. Boston, MA: Kluwer Academic Publishers, 2003, ch. 4. [2] D. R. Jackson, J. T. Williams, A. K. Bhattacharyya, R. L. Smith, S. J. Buchheit, and S. A. Long, “Microstrip patch designs that do not excite surface waves,” IEEE Trans. Antennas Propag., vol. 41, no. 8, pp. 1026–1037, Aug. 1993. [3] Z. N. Chen, “Broadband probe-fed plate antenna,” in Proc. 30th Eur. Microwave Conf., Oct. 2000, pp. 1–5. [4] H. S. Lee, J. G. Kim, S. Hong, and J. B. Yoon, “Micromachined CPW-fed suspended patch antenna for 77 GHz automotive radar applications,” in Proc. Eur. Microwave Conf., Oct. 2005, vol. 3, pp. 4–6. [5] R. Gonzalo, P. de Maagt, and M. Sorolla, “Enhanced patch-antenna performance by suppressing surface waves using photonic-bandgap substrates,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 11, pp. 2131–2138, Nov. 1999. [6] H. Boutayeb and T. A. Denidni, “Gain enhancement of a microstrip patch antenna using a cylindrical electromagnetic crystal substrate,” IEEE Trans. Antennas Propag., vol. 55, no. 11, pp. 3140–3145, Nov. 2007. [7] N. Llombart, A. Neto, G. Gerini, and P. de Maagt, “Planar circularly symmetric EBG structures for reducing surface waves in printed antennas,” IEEE Trans. Antennas Propag., vol. 53, no. 10, pp. 3210–3218, Oct. 2005. [8] G. P. Gauthier, A. Courtay, and G. M. Rebeiz, “Microstrip antennas on synthesized low dielectric-constant substrates,” IEEE Trans. Antennas Propag., vol. 45, no. 8, pp. 1310–1314, Aug. 1997. [9] J. S. Colburn and Y. Rahmat-Samii, “Patch antennas on externally perforated high dielectric constant substrates,” IEEE Trans. Antennas Propag., vol. 47, no. 12, pp. 1785–1794, Dec. 1999. [10] R. A. R. Solis, A. Melina, and N. Lopez, “Microstrip patch encircled by a trench,” in Proc. IEEE Int. Symp. on Antennas Propag. Society, Jul. 2000, vol. 3, pp. 1620–1623. [11] Q. Chen, V. F. Fusco, M. Zheng, and P. S. Hall, “Micromachined silicon antennas,” in Proc. Int. Conf. on Microwave and Millimeter-Wave Tech., Aug. 1998, pp. 289–292. [12] Q. Chen, V. F. Fusco, M. Zhen, and P. S. Hall, “Trenched silicon microstrip antenna arrays with ground plane effects,” in Proc. 29th Eur. Microwave Conf., Oct. 1999, vol. 3, pp. 263–266.

[13] Q. Chen, V. F. Fusco, M. Zheng, and P. S. Hall, “Silicon active slot loop antenna with micromachined trenches,” in Proc. Inst. Elect. Eng. National Conf. on Antennas and Propagation, April 1999, no. 461, pp. 253–255. [14] J. G. Yook and L. P. B. Katehi, “Micromachined microstrip patch antenna with controlled mutual coupling and surface waves,” IEEE Trans. Antennas Propag., vol. 49, no. 9, pp. 1282–1289, Sept. 2001. Siew Bee Yeap (M’07) received the B.Sc. degree (with honors) from the University of Malaya, Kuala Lumpur, in 1996, the M.Sc. from the National University of Singapore, in 1998, and Ph.D. degree from Queen Mary, University of London, London, U.K., in 2003. From 2003 to 2004, she was a Postdoctoral Fellow at Queen Mary, University of London, where she was involved in multiple input and multiple output antenna design. She later joined Laird Technologies, from 2006 to 2007, as a staff antenna Design Engineer from which she holds a U.S. patent. In 2007, she joined the Institute of Infocomm Research, Singapore, as a Senior Research Fellow. Her current research interests include design, modeling and measurements of millimeter-wave and terahertz antennas. She has authored more than 10 international journals and conferences.

Zhi Ning Chen (F’07) received the B.Eng., M.Eng., Ph.D. and Do.E. degrees all in electrical engineering from Institute of Communications Engineering (ICE), China and University of Tsukuba, Japan, respectively. From 1988 to 1995, he worked at ICE as a Teaching Assistant, Lecturer and Associate Professor as well as at Southeast University, China, as a Postdoctoral Fellow and later Associate Professor. From 1995 to 1997, he was with City University of Hong Kong, China, as a Research Assistant and later a Research Fellow. In 1997, he was awarded a JSPS Fellowship to conduct his research at the University of Tsukuba, Japan. In 2001 and 2004, he visited the University of Tsukuba under a JSPS Fellowship Program (at senior level). In 2004, he worked at IBM T. J. Watson Research Center, New York, as an Academic Visitor. Since 1999, he has worked at the Institute for Infocomm Research (formerly known as Center for Wireless Communications and Institute for Communication Research) as a Member of Technical Staff (MTS), Principal MTS, Senior Scientist, and Lead Scientist. He was currently appointed Principal Scientist and Department Head for RF & Optical. Concurrently he was/is an Adjunct/Guest Professor at Southeast University, Nanjing University, Shanghai Jiao Tong University, Tongji University, Zhejiang University, National University of Singapore, and Nanyang Technological University. He has published and presented 275 papers in journals and at conferences as well as authored and/or edited the books Broadband Planar Antennas, UWB Wireless Communication, Antennas for Portable Devices, and Antennas for Base Station in Wireless Communications. He also contributed chapters to the books UWB Antennas and Propagation for Communications, Radar, and Imaging, and the Antenna Engineering Handbook. He is holding 25 granted and filed patents with 15 licensed deals with industry. His current research interests are in applied electromagnetics as well as antennas for microwave, mmW, submmW, and THz communication and imaging systems. Dr. Chen was nominated a Fellow of the IEEE for his “contribution to small and broadband antennas for wireless applications” and is also an IEEE AP-S Distinguished Lecturer. He has organized many international technical events as General Chair, Technical Program Committee Chair, and has been a key member of organizing committees. He is the founder of the International Workshop on Antenna Technology (iWAT). He is the recipient of the CST University Publication Award 2008, IEEE AP-S Honorable Mention Student Paper Contest 2008, IES Prestigious Engineering Achievement Award 2006, I2R Quarterly Best Paper Award 2004, and IEEE iWAT 2005 Best Poster Award.