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Design considerations for rectangular microstrip patch antenna on electromagnetic crystal substrate at terahertz frequency
Infrared Physics & Technology 53 (2010) 17–22
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Design considerations for rectangular microstrip patch antenna on electromagnetic crystal substrate at terahertz frequency G. Singh * Department of Electronics and Communication Engineering, Jaypee University of Information Technology, Solan 173 215, India
a r t i c l e
i n f o
Article history: Received 10 April 2009 Available online 20 August 2009 Keywords: Electromagnetic crystal Bandwidth Multi-frequency band Efficiency Gain Microstrip antenna
a b s t r a c t The effects of 2-D electromagnetic crystal substrate on the performance of a rectangular microstrip patch antennas at THz frequencies is simulated. Electromagnetic crystal substrate is used to obtain extremely broad-bandwidth with multi-frequency band operation of the proposed microstrip antennas. Multi-frequency band microstrip patch antennas are used in modern communication systems in order to enhance their capacity through frequency reuse. The simulated 10 dB impedance bandwidth of the rectangular patch microstrip antenna is 34.3% at THz frequency (0.6–0.95 THz). The radiation efficiency, gain and directivity of the proposed antenna are presented at different THz frequencies. The simulation has been performed using CST Microwave Studio, which is a commercially available electromagnetic simulator based on finite integral technique. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The growing demand of wireless applications has presented RF engineers with continuing call for low cost, power efficient, and small size system designs. Depending on the applications, required system characteristics, and system parameters such as operating frequency, transmitted power, and modulation scheme may vary widely. However, independent of the applications, compactness, wide bandwidth, high efficiency, ease of fabrication, integration and low cost are always sought in wireless systems. The demand for high-bit-rate, low-transmit-power, secured wireless communication capabilities continues unabated at the present time. THz electromagnetic radiation has recently found increasing prominence in communications, in which several order of magnitude of increase in the bandwidth, improved resolution and directivity as compared to current wireless technology is expected [1]. THz communication link is most likely secure communication for short-distance, point-to-point, and demanding high information data rate. Among the practical advantages of using THz regime for satellite communication system is the ability to employ smaller transmitting and receiving antennas. This allows the use of smaller satellite and a lighter launch vehicle. Microstrip patch antennas which are very important component of the communication systems, also known for their desirable physical characteristics such as, low profile, low cost, low weight, easy fabrication and conformability. Because of its resonance nature, the microstrip antenna has inherently a narrow bandwidth, * Tel.: +91 1792 239334; fax: +91 1792 245362. E-mail address: [email protected] 1350-4495/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2009.08.002
limiting their wide spread applications. However, there are some techniques that may be used to increase the bandwidth [2,3], but they all invariably increase the antenna volume by either increasing the patch size or the substrate thickness. Chang et al. [4] approach in principle can produce bandwidths as high as 20%, greatly reduces the antenna efficiency due to the increased surface wave losses. Brown et al. [5] proposed increasing the performance of planar antennas on a dielectric substrate, which suffer large radiation loss into the substrate with only 2–3% of the power radiated in the air. The increase in substrate thickness causes the antenna to more efficiently excite the substrate and surface modes, which in turn removes energy from the main radiation lobe. The surface waves may show up as spikes in the antenna pattern and can dominate the radiated power [6]. Thus, substrate of microstrip antennas plays a major role in achieving desirable electrical and physical parameters. The problems of surface wave losses in the microstrip antenna can be solved by using 2-D electromagnetic crystal as a substrate, from which the radiation will be fully reflected in all directions [7–12]. The electromagnetic crystal substrate would virtually eliminate any power loss into the substrate when the driving frequency is within the stop band, provided there are no evanescent surface modes. The aim of this paper is to demonstrate a gain and bandwidth enhancement method for the rectangular microstrip patch antennas at THz frequencies through the use of 2-D electromagnetic crystal substrate materials made of planar arrays of circular blocks (circular air implants) within the dielectric layers [13–17]. The high gain is due to the excitation of stronger leaky-wave fields. Thus, I have suggested one important possibility for the microstrip patch antennas at THz frequencies. The organization of the paper is
18
G. Singh / Infrared Physics & Technology 53 (2010) 17–22
as follows. The Section 2 is concerned with design configuration of the rectangular microstrip patch antenna at THz frequencies on electromagnetic crystal substrate. The Section 3 discusses the simulated results. Finally, Section 4 concludes the work.
na, since the fringing fields are tightly bound to the substrate. However, because of the relatively higher loss tangents, they are less efficient and have relatively smaller bandwidth. 2.1. Electromagnetic crystal substrate
2. Terahertz antenna design configuration A conventional microstrip patch antenna consists of a pair of parallel conducting layers separated by a dielectric medium, referred to as substrate as shown in Fig. 1. In this configuration, upper conducting layer is the source of radiation where electromagnetic energy fringes off the edges of the patch and into the substrate. The lower conducting layer acts as a perfectly reflecting ground plane, bouncing energy back through the substrate and into the free-space. The antenna size mostly depends on the frequency band of operation. There are several other factors that contribute to deciding the dimension of the antenna and its behavior such as the substrate material used and its thickness. The patch material affects the efficiency of the antenna, while the type of substrate plays a major role in the calculation of the antenna dimensions [18–20]. The excitation of the patch is accomplished via a microstrip feed line. This feed technique will supply the patch with electrical signal to be converted into an electromagnetic wave. When the patch is excited by the feed, the bottom of the patch at a certain point of time will have a positive charge distribution, and the ground plane will have a negative charge distribution. The attractive forces between these charges will hold most of them on the bottom and top surfaces of the patch and ground material, respectively. On the patch surface, repulsive charges within the same polarity tend to push some of the charges towards the edges. Commercial substrate materials are readily available for use at RF and microwave frequencies; selection is based on desired material characteristics for optimal performance over specific frequency ranges. The thickness of the substrate is of considerable importance when designing microstrip antennas. The most desirable substrates for antenna performance are the ones that are thick with low dielectric constant. This tends to result in an antenna with a large bandwidth and high efficiency due to loosely bound fringing fields that emanate from the patch and propagate into substrate. However, this comes at expense of a large volume antenna and reduced efficiency due to surface wave formation. On the other hand, thin substrates with high dielectric constants reduce the overall size of the anten-
A new technology has emerged which may be the key for developing ultra-wideband microstrip antennas. This technology manipulates the substrate in such a way that surface waves are completely forbidden from forming, resulting in improved antenna efficiency and bandwidth, while reducing side-lobes and electromagnetic interference levels. Although many applications have initially been proposed in the field of optics, the scalability of these structures opens up the possibility of using them in millimeter and THz wave regime. In this frequency range, electromagnetic crystal materials have attracted a lot of attention as a substrate for antennas. The basic idea is to match the operational bandwidth of the antenna with the band gap of the electromagnetic crystal. The utilization of the electromagnetic crystal substrate, instead of the original bulk substrate, has shown to reduce the excitation of the surface wave modes and as a consequence improve the antenna radiation efficiency, reduce the side-lobe and mitigate the problem related to coupling. Electromagnetic crystals are a new class of periodic metallic, dielectric or composite structures that exhibit transmission and reflection band in their frequency response. Electromagnetic wave propagation through such a medium is affected by the scattering and diffraction properties of the periodic elements. Planar conducting strip or patch on such materials has already had important applications in frequency selective surfaces. Since electromagnetic waves can be highly directional with the photonic band gap materials, it is conceivable that antennas with this material will have many unique characteristics [5]. Currently, the electromagnetic crystals have been used in many novel microwave and optical applications [21–26]. In the proposed antenna, planar arrays of circular blocks within the dielectric layer with radius 10 lm and 100 lm center-to-center distance drilled in a dielectric medium with a dielectric permittivity er = 9.1 as shown in Fig. 1 is used. This 2-D electromagnetic crystal structure can be fabricated using lithographically fabricated using custom designed lithographic mask [27,28]. The basic idea is that since an antenna placed on a dielectric substrate radiates more efficiently into the dielectric substrate
Fig. 1. Geometrical configuration of the rectangular microstrip patch antenna on 2-D electromagnetic crystal substrate at THz frequencies.
G. Singh / Infrared Physics & Technology 53 (2010) 17–22
than the air-side [29,30], one must replace the substrate with an electromagnetic crystal whose forbidden gap encompasses the antenna excitation frequency which provided no surface modes, power previously radiated into the substrate will be reflected towards the air-side, increasing the energy coupled to the radiated field. Yang et al. [31], originally proposed that high gain antenna structures could be obtained by printing on a 2-D electromagnetic crystal substrate. By reducing or eliminating the effects of these electromagnetic inhibitors with electromagnetic crystals, a broadband response can be obtained from inherently narrowband antennas. It is also useful to the reduction in pattern side-lobes resulting in improvements in the radiation pattern front-to-back ratio and overall antenna efficiency [32]. Agi and Malloy [33] have experimentally and computationally studied the integration of a microstrip patch antennas with a 2-D electromagnetic crystal substrate. 2.2. Geometrical configuration Fig. 1 shows the geometrical configuration of the proposed rectangular microstrip patch antenna on 2-D electromagnetic crystal substrate. In this configuration, we have considered substrate material of dimensions 1000 1000 lm2 and 200 lm thickness as shown in Fig. 1. The radiating rectangular patch has dimensions 500 300 lm2. In this simulation model of rectangular microstrip patch antenna, we have used microstrip feed line technique. The dimension of the strip line is 350 30 lm2 with the thickness 50 lm as shown in Fig. 1. Microstrip line feed has been used in the proposed antenna is one of the most commonly used feeding techniques. Since feeding technique influences the input impedance, it is often exploited for matching purpose. In this feeding technique, a conducting strip is connected directly to the edge of the rectangular microstrip patch antenna. The advantage of this technique is that both the feed and patch lie on the surface of the substrate and therefore is plane in construction and provide the right impedance match between the patch and feed line. The antenna is placed at the central position of the electromagnetic crystal substrate. 3. Results and discussion General radiation characteristics of microstrip patch antennas can be obtained by investigating the far-zone fields from an elementary current source. The electromagnetic crystal layer considered is made of a dielectric slab with planar 2-D cylindrical grating (circular air implants) [34–36]. A complete field expression for the microstrip structure is in turn of a continuous plane wave spectrum. With planar material gratings, there exist three different propagating waves [37]. Space waves and surface waves are similar to those in conventional microstrip structures and can be found from the method of steepest-descent and pole-extraction, respectively. Leaky waves are due to the periodic nature of the planar grating structure [38]. The surface wave (bound wave) is a slow wave with normalized phase constant b/k0 (k0 = free-space wave number) that increases with frequency. When frequency increases to a point that the condition b=k0 k=a 1 holds, this bound wave becomes a leaky wave which is fast wave for 1 space harmonic with a complex propagation constant. The far-zone radiated fields are due to the combination of space waves and leaky waves. The energy carried by bounded surface waves that propagate laterally is considered a loss. Antenna directivity, gain and efficiencies are determined by the energy distribution among these three propagating waves. Antenna efficiency can be maximized by partial elimination of the bound surface waves within certain directions. For high efficiency antennas, directivity and gain are approximately the same and are determined by the energy distribution be-
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tween the space and leaky waves. Leaky wave radiation pattern is highly directive in contrast to the low directivity of the space waves. To achieve high gain antenna, it is necessary to excite a strong leaky wave [39–42]. The simulated input return loss of the rectangular microstrip patch antenna is shown in Fig. 2, which reveals that by using 2-D electromagnetic crystal substrate, we obtain extremely broadbandwidth with multi-frequency band in the frequency range 0.6–0.95 THz. The three different frequencies at which the antenna resonates are 693.45, 797.4 and 852 GHz. The 10 dB impedance bandwidth of this rectangular patch microstrip antenna using electromagnetic crystal substrate is 34.3%. The simulated far-zone radiation pattern for the gain and directivity at the frequencies 775, 693.45, 797.4 and 852 GHz are shown in Fig. 3. The radiation patterns within the operational frequency of the gap have been plotted for several frequencies. The radiation patterns in both E and H plane are rather stable using a electromagnetic crystal substrate, showing nearly the same radiation patterns. The radiation pattern of E-plane (the plane in which surface waves are more pronounced) is presented in Fig. 3. The electrical parameters such as gain, directivity and radiation efficiency of the rectangular patch microstrip antenna at the aforementioned frequencies have been shown in Table 1. From the Table 1, it is clear that the gain and directivity of the proposed antenna at frequency 797.4 GHz are very interesting. But the radiation efficiency point of view, the results of the proposed antenna at frequency 852 GHz is best among all. At the design frequency, the patch antenna on a electromagnetic crystal has more directivity, less side and back radiation and smoother pattern. The ability of the electromagnetic crystal substrate to reduce the surface wave mode propagation has been proven by better radiation efficiencies. This opens the door to design new devices with thicker substrates and higher dielectric constant without loosing performance by the undesired excitation of the surface wave mode. The effects of diameter of the air-gap in substrate material on the radiation efficiency and gain of the proposed microstrip patch antenna at frequency 775 GHz are shown in Fig. 4. The diameter of the air-gaps chosen in the simulation is 20 lm as marked in Fig. 4. The gain of antenna increases with the increase of diameter of the air-gaps and attain optimum value at 20 lm. At this point of interest even the efficiency is low as compare to others but the gain is maximum, which is the reason to consider this diameter in our simulation. The effects of dielectric permittivity of the electromagnetic crystal substrate on the radiation efficiency and gain of the rectan-
Fig. 2. The simulated frequency versus return loss of rectangular microstrip patch antenna at THz frequencies.
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G. Singh / Infrared Physics & Technology 53 (2010) 17–22
Fig. 3. Far-zone radiation pattern of the gain and directivity in E and H plane of the rectangular microstrip patch antenna at frequencies 775, 693.45, 797.4 and 852 GHz.
Table 1 Simulated electrical parameters of the microstrip antenna at different THz frequencies. Frequency (GHz)/electrical parameters Gain (dB) Directivity (dBi) Radiation efficiency (%)
775 8.451 9.934 71.06
693.45 4.501 6.239 67.02
797.4 9.19 10.36 76.45
852 8.248 8.789 88.30
gular patch microstrip antenna at 775 GHz has been shown in Fig. 5. In this simulation, we have considered the dielectric permittivity of electromagnetic crystal substrate as marked in Fig. 5. Sharma and Singh [19] have simulated a rectangular microstrip patch antenna by using conventional substrate material at terahertz frequency. In this simulation, the gain and radiation efficiency was 3.497 dB and 55.71%, respectively, at 775 GHz, but when it is sim-
G. Singh / Infrared Physics & Technology 53 (2010) 17–22
Fig. 4. The effects of air-gap diameter on the radiation efficiency and gain of the rectangular microstrip patch antenna.
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crystal substrate surfaces exhibited high efficiency and directivity as compare to conventional antenna on substrate. The gain of the printed circuit antenna can be greatly enhanced with the electromagnetic crystal material as substrate. From the simulation, we obtained multi-frequency band and 10 dB impedance bandwidth 34.3%, which is extremely broader bandwidth. At the frequency 852 GHz the antenna gain and directivity are 8.248 dB and 8.789 dBi, respectively, with radiation efficiency 88.30%. It was found that significant gain enhancement is achieved by exciting strong leaky waves through proper designs of planar periodic metallic structures. Multi-frequency band microstrip patch antennas are often used in modern communication systems in order to enhance their capacity through frequency reuse. The technological potential of electromagnetic crystal substrate for the development of novel microstrip patch antenna that could overcome the limitation of current technology. Consequently, this simulation study has shown the possibility of design of efficient, high gain broadband rectangular microstrip patch antennas on 2-D electromagnetic crystal substrate at THz frequencies. A very important need is to verify more of the simulation results with experimental measurements which will be reported in future communications. Acknowledgement Author is sincerely thankful to the reviewers for their critical comments and suggestions to improve the quality of the manuscript. References
Fig. 5. The effects of the dielectric permittivity of the substrate on radiation efficiency and gain of the rectangular microstrip patch antenna.
ulated over photonic crystal on same frequency the gain and radiation efficiency increases drastically as clear from Table 1. The radiation efficiency and gain of the proposed antenna is maximum at the dielectric permittivity of the substrate 9.1, above this value it is approximately constant. At this point, we have maximum gain and radiation efficiency as compare to all other points. The distance between air-gaps and arrays of air blocks is chosen for minimum return loss [42]. The size of the substrate is such that, in theory, the surface mode is added in counter phase for the conventional patch antenna, resulting in a very low value of gain in boresight direction. This effect of surface wave is approximately eliminated by the electromagnetic crystal substrate, leading to the smooth radiation pattern. A multi-frequency band and extremely broader bandwidth are the potential advantages of using electromagnetic crystal as substrate for the rectangular microstrip patch antenna at THz frequency.
4. Conclusion In this paper, I have simulated a rectangular microstrip patch antenna with electromagnetic crystal as a substrate material at THz frequencies. At millimeter and THz wave integrated circuits, the control of radiation from a microstrip patch antenna is of great interest. The microstrip antenna mounted on 2-D electromagnetic
[1] R. Piesiewicz, T.K.- Ostmann, N. Krumbholz, D. Mittleman, M. Koch, J. Schoebel, T. Kurner, Short-range ultra-broadband terahertz communications: concept and perspectives, IEEE Antennas Propag. Mag. 49 (2007) 24–38. [2] K.F. Lee, K.Y. Lto, J.S. Dahele, Circular-disk microstrip antenna with an air gap, IEEE Trans. Antennas Propag. 32 (1984) 880–884. [3] W.Y. Leung, R. Biswas, S.-D. Cheng, M.N. Sigalas, J.S. McCalmont, G. Tattk, K.-H. Ho, Slot antenna on photonic bandgap crystal, IEEE Trans. Antennas Propag. 45 (1997) 1569–1570. [4] E. Chang, S.A. Long, W.F. Richards, An experimental investigation of electrically thick rectangular microstrip antennas, IEEE Trans. Antennas Propag. 34 (1986) 767–772. [5] E.R. Brown, C.D. Parker, E. Yablonovitch, Radiation properties of a planar antenna on a photonic-crystal substrate, J. Opt. Soc. Am. B 10 (1993) 404–407. [6] R. Gonzalo, Enhanced patch-antenna performance by suppressing surface waves using photonic-band gap substrates, IEEE Trans. Microw. Theor. Tech. 47 (1999) 2131–2138. [7] L. Pajewaski, L. Rinaldi, G. Schettini, Enhancement of directivity using 2-D electromagnetic crystals near the bandgap edge: a full wave approach, Prog. Electromag. Res. 80 (2008) 179–196. [8] R. Gonzalo, G. Nagore, P. de Maagt, Simulated and measured performance of a patch antenna on a 2 dimensional photonic crystal substrate, Prog. Electromag. Res. 41 (2003) 257–269. [9] V. Radisic, Y. Qian, R. Coccioli, T. Itoh, Novel 2-D photonic bandgap structure for microstrip lines, IEEE Microw. Guided Wave Lett. 2 (1998) 69–71. [10] Y. Zhao, D.R. Grischkowsky, 2-D terahertz metallic photonic crystals in parallel plate waveguide, IEEE Trans. Microw. Theor. Tech. 55 (2007) 656–663. [11] H.Y. Yang, Finite difference analysis of 2-D photonic crystals, IEEE Trans. Microw. Theor. Tech. 44 (1996) 2688–2695. [12] R.N. Tiwari, P. Kumar, G. Singh, 2D-Photonic Crystal as Substrate for THz/ Millimeter Wave Microstrip Patch Antenna, in: Proceedings of Recent Advances in Microwave Theory and Applications (Microwave 2008), India, 21–24 November 2008, pp. 787–789. [13] H. Mosallei, K. Sarabandi, Antenna miniaturization and bandwidth enhancement using a reactive impedance substrate, IEEE Trans. Antenna Propag. 52 (2004) 2403–2414. [14] H.C.C. Fernandes, A.R.B. da Rocha, Analysis of antennas with PBG substrate, Int. J. Infrared Milli. Wave 24 (2003) 1171–1176. [15] K.M. Ho, C.T. Chan, C.M. Soukoulish, Existence of a photonic gap in periodic dielectric structures, Phys. Rev. Lett. 65 (1990) 3152–3155. [16] R.D. Meade, K.D. Brommer, A.M. Rappe, J.D. Joannopoulos, Photonic band states in periodic dielectric materials, Phys. Rev. B: Condens. Matter 44 (1991) 13772–13774. [17] E. Yablonovitch, Photonic band gap structures, J. Opt. Soc. Amer. B 10 (1993) 283–294. [18] Aditi Sharma, G. Singh, D.S. Chauhan, Design Considerations to Improve the Performance of a Rectangular Microstrip Patch Antenna at THz Frequency, in:
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[31] H.Y.D. Yang, N.G. Alexopoulos, E. Yablonovitch, Photonic band gap materials for high gain printed circuit antennas, IEEE Trans. Antennas Propag. 45 (1997) 185–187. [32] R. Coccioli, W.R. Deal, T. Itoh, Radiation characteristics of a patch antenna on a thin PBG substrate, IEEE Antennas Propag. Soc. Int. Symp. 2 (1998) 656–659. [33] K. Agi, J. Malloy, Integration of a microstrip patch antenna with a two dimensional photonic crystal substrate, Electromag. 19 (1999) 277–290. [34] I. Rumsey, M. Piket May, P.K. Kelly, Photonic band gap structure for microstrip line, IEEE Microw. Guided Wave Lett. 8 (1998) 336–338. [35] M.P. Kesler, J.G. Maloney, B.L. Shirley, G.S. Smith, Antenna design with the use of photonic band gap materials as all dielectric planer reflectors, Microw. Opt. Tech. Lett. 11 (1996) 169–174. [36] W. Kuang, W.J. Kim, A. Mock, J. O’Brien, Propagation loss of line defect photonic crystal slab waveguides, IEEE J. Sel. Top. Quant. Electron. 12 (2006) 1183– 1195. [37] Z. Zhang, S. Satpathy, Electromagnetic wave propagation in periodic structures: bloch wave solution of Maxwell equations, Phys. Rev. Lett. 65 (1990) 2650–2653. [38] D.R. Jackson, A.A. Oliner, A. Ip, Leaky-wave propagation and radiation for a narrow-beam multilayer dielectric substrate, IEEE Trans. Antennas Propag. 41 (1993) 344–348. [39] B. Temelkuran, M. Bayindir, E. Ozbay, R. Biswas, M.M. Sinjalas, G. Tyttle, K.M. Ho, Photonic crystal based resonant antenna with a vary high directivity, J. Appl. Phys. 87 (2000) 603–605. [40] M. Thevenot, C. Cheype, A. Reineix, B. Jecko, Directive photonic bandgap antennas, IEEE Trans. Microw. Theor. Tech. 47 (1999) 2115–2122. [41] J.D. Joannopoulos, R.D. Meade, J.N. Win, Photonic Crystals: Modeling the Flow of Light, Princeton Univ. Press, 1995. [42] Aditi Sharma, Design Consideration of Rectangular Microstrip Patch Antenna at THz Frequencies for Advanced Communication Systems, M. Tech. thesis, May 2008, Jaypee University of Information Technology, Solan, India.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Design considerations for rectangular microstrip patch antenna on electromagnetic crystal substrate at terahertz frequency G. Singh * Department of Electronics and Communication Engineering, Jaypee University of Information Technology, Solan 173 215, India
a r t i c l e
i n f o
Article history: Received 10 April 2009 Available online 20 August 2009 Keywords: Electromagnetic crystal Bandwidth Multi-frequency band Efficiency Gain Microstrip antenna
a b s t r a c t The effects of 2-D electromagnetic crystal substrate on the performance of a rectangular microstrip patch antennas at THz frequencies is simulated. Electromagnetic crystal substrate is used to obtain extremely broad-bandwidth with multi-frequency band operation of the proposed microstrip antennas. Multi-frequency band microstrip patch antennas are used in modern communication systems in order to enhance their capacity through frequency reuse. The simulated 10 dB impedance bandwidth of the rectangular patch microstrip antenna is 34.3% at THz frequency (0.6–0.95 THz). The radiation efficiency, gain and directivity of the proposed antenna are presented at different THz frequencies. The simulation has been performed using CST Microwave Studio, which is a commercially available electromagnetic simulator based on finite integral technique. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The growing demand of wireless applications has presented RF engineers with continuing call for low cost, power efficient, and small size system designs. Depending on the applications, required system characteristics, and system parameters such as operating frequency, transmitted power, and modulation scheme may vary widely. However, independent of the applications, compactness, wide bandwidth, high efficiency, ease of fabrication, integration and low cost are always sought in wireless systems. The demand for high-bit-rate, low-transmit-power, secured wireless communication capabilities continues unabated at the present time. THz electromagnetic radiation has recently found increasing prominence in communications, in which several order of magnitude of increase in the bandwidth, improved resolution and directivity as compared to current wireless technology is expected [1]. THz communication link is most likely secure communication for short-distance, point-to-point, and demanding high information data rate. Among the practical advantages of using THz regime for satellite communication system is the ability to employ smaller transmitting and receiving antennas. This allows the use of smaller satellite and a lighter launch vehicle. Microstrip patch antennas which are very important component of the communication systems, also known for their desirable physical characteristics such as, low profile, low cost, low weight, easy fabrication and conformability. Because of its resonance nature, the microstrip antenna has inherently a narrow bandwidth, * Tel.: +91 1792 239334; fax: +91 1792 245362. E-mail address: [email protected] 1350-4495/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2009.08.002
limiting their wide spread applications. However, there are some techniques that may be used to increase the bandwidth [2,3], but they all invariably increase the antenna volume by either increasing the patch size or the substrate thickness. Chang et al. [4] approach in principle can produce bandwidths as high as 20%, greatly reduces the antenna efficiency due to the increased surface wave losses. Brown et al. [5] proposed increasing the performance of planar antennas on a dielectric substrate, which suffer large radiation loss into the substrate with only 2–3% of the power radiated in the air. The increase in substrate thickness causes the antenna to more efficiently excite the substrate and surface modes, which in turn removes energy from the main radiation lobe. The surface waves may show up as spikes in the antenna pattern and can dominate the radiated power [6]. Thus, substrate of microstrip antennas plays a major role in achieving desirable electrical and physical parameters. The problems of surface wave losses in the microstrip antenna can be solved by using 2-D electromagnetic crystal as a substrate, from which the radiation will be fully reflected in all directions [7–12]. The electromagnetic crystal substrate would virtually eliminate any power loss into the substrate when the driving frequency is within the stop band, provided there are no evanescent surface modes. The aim of this paper is to demonstrate a gain and bandwidth enhancement method for the rectangular microstrip patch antennas at THz frequencies through the use of 2-D electromagnetic crystal substrate materials made of planar arrays of circular blocks (circular air implants) within the dielectric layers [13–17]. The high gain is due to the excitation of stronger leaky-wave fields. Thus, I have suggested one important possibility for the microstrip patch antennas at THz frequencies. The organization of the paper is
18
G. Singh / Infrared Physics & Technology 53 (2010) 17–22
as follows. The Section 2 is concerned with design configuration of the rectangular microstrip patch antenna at THz frequencies on electromagnetic crystal substrate. The Section 3 discusses the simulated results. Finally, Section 4 concludes the work.
na, since the fringing fields are tightly bound to the substrate. However, because of the relatively higher loss tangents, they are less efficient and have relatively smaller bandwidth. 2.1. Electromagnetic crystal substrate
2. Terahertz antenna design configuration A conventional microstrip patch antenna consists of a pair of parallel conducting layers separated by a dielectric medium, referred to as substrate as shown in Fig. 1. In this configuration, upper conducting layer is the source of radiation where electromagnetic energy fringes off the edges of the patch and into the substrate. The lower conducting layer acts as a perfectly reflecting ground plane, bouncing energy back through the substrate and into the free-space. The antenna size mostly depends on the frequency band of operation. There are several other factors that contribute to deciding the dimension of the antenna and its behavior such as the substrate material used and its thickness. The patch material affects the efficiency of the antenna, while the type of substrate plays a major role in the calculation of the antenna dimensions [18–20]. The excitation of the patch is accomplished via a microstrip feed line. This feed technique will supply the patch with electrical signal to be converted into an electromagnetic wave. When the patch is excited by the feed, the bottom of the patch at a certain point of time will have a positive charge distribution, and the ground plane will have a negative charge distribution. The attractive forces between these charges will hold most of them on the bottom and top surfaces of the patch and ground material, respectively. On the patch surface, repulsive charges within the same polarity tend to push some of the charges towards the edges. Commercial substrate materials are readily available for use at RF and microwave frequencies; selection is based on desired material characteristics for optimal performance over specific frequency ranges. The thickness of the substrate is of considerable importance when designing microstrip antennas. The most desirable substrates for antenna performance are the ones that are thick with low dielectric constant. This tends to result in an antenna with a large bandwidth and high efficiency due to loosely bound fringing fields that emanate from the patch and propagate into substrate. However, this comes at expense of a large volume antenna and reduced efficiency due to surface wave formation. On the other hand, thin substrates with high dielectric constants reduce the overall size of the anten-
A new technology has emerged which may be the key for developing ultra-wideband microstrip antennas. This technology manipulates the substrate in such a way that surface waves are completely forbidden from forming, resulting in improved antenna efficiency and bandwidth, while reducing side-lobes and electromagnetic interference levels. Although many applications have initially been proposed in the field of optics, the scalability of these structures opens up the possibility of using them in millimeter and THz wave regime. In this frequency range, electromagnetic crystal materials have attracted a lot of attention as a substrate for antennas. The basic idea is to match the operational bandwidth of the antenna with the band gap of the electromagnetic crystal. The utilization of the electromagnetic crystal substrate, instead of the original bulk substrate, has shown to reduce the excitation of the surface wave modes and as a consequence improve the antenna radiation efficiency, reduce the side-lobe and mitigate the problem related to coupling. Electromagnetic crystals are a new class of periodic metallic, dielectric or composite structures that exhibit transmission and reflection band in their frequency response. Electromagnetic wave propagation through such a medium is affected by the scattering and diffraction properties of the periodic elements. Planar conducting strip or patch on such materials has already had important applications in frequency selective surfaces. Since electromagnetic waves can be highly directional with the photonic band gap materials, it is conceivable that antennas with this material will have many unique characteristics [5]. Currently, the electromagnetic crystals have been used in many novel microwave and optical applications [21–26]. In the proposed antenna, planar arrays of circular blocks within the dielectric layer with radius 10 lm and 100 lm center-to-center distance drilled in a dielectric medium with a dielectric permittivity er = 9.1 as shown in Fig. 1 is used. This 2-D electromagnetic crystal structure can be fabricated using lithographically fabricated using custom designed lithographic mask [27,28]. The basic idea is that since an antenna placed on a dielectric substrate radiates more efficiently into the dielectric substrate
Fig. 1. Geometrical configuration of the rectangular microstrip patch antenna on 2-D electromagnetic crystal substrate at THz frequencies.
G. Singh / Infrared Physics & Technology 53 (2010) 17–22
than the air-side [29,30], one must replace the substrate with an electromagnetic crystal whose forbidden gap encompasses the antenna excitation frequency which provided no surface modes, power previously radiated into the substrate will be reflected towards the air-side, increasing the energy coupled to the radiated field. Yang et al. [31], originally proposed that high gain antenna structures could be obtained by printing on a 2-D electromagnetic crystal substrate. By reducing or eliminating the effects of these electromagnetic inhibitors with electromagnetic crystals, a broadband response can be obtained from inherently narrowband antennas. It is also useful to the reduction in pattern side-lobes resulting in improvements in the radiation pattern front-to-back ratio and overall antenna efficiency [32]. Agi and Malloy [33] have experimentally and computationally studied the integration of a microstrip patch antennas with a 2-D electromagnetic crystal substrate. 2.2. Geometrical configuration Fig. 1 shows the geometrical configuration of the proposed rectangular microstrip patch antenna on 2-D electromagnetic crystal substrate. In this configuration, we have considered substrate material of dimensions 1000 1000 lm2 and 200 lm thickness as shown in Fig. 1. The radiating rectangular patch has dimensions 500 300 lm2. In this simulation model of rectangular microstrip patch antenna, we have used microstrip feed line technique. The dimension of the strip line is 350 30 lm2 with the thickness 50 lm as shown in Fig. 1. Microstrip line feed has been used in the proposed antenna is one of the most commonly used feeding techniques. Since feeding technique influences the input impedance, it is often exploited for matching purpose. In this feeding technique, a conducting strip is connected directly to the edge of the rectangular microstrip patch antenna. The advantage of this technique is that both the feed and patch lie on the surface of the substrate and therefore is plane in construction and provide the right impedance match between the patch and feed line. The antenna is placed at the central position of the electromagnetic crystal substrate. 3. Results and discussion General radiation characteristics of microstrip patch antennas can be obtained by investigating the far-zone fields from an elementary current source. The electromagnetic crystal layer considered is made of a dielectric slab with planar 2-D cylindrical grating (circular air implants) [34–36]. A complete field expression for the microstrip structure is in turn of a continuous plane wave spectrum. With planar material gratings, there exist three different propagating waves [37]. Space waves and surface waves are similar to those in conventional microstrip structures and can be found from the method of steepest-descent and pole-extraction, respectively. Leaky waves are due to the periodic nature of the planar grating structure [38]. The surface wave (bound wave) is a slow wave with normalized phase constant b/k0 (k0 = free-space wave number) that increases with frequency. When frequency increases to a point that the condition b=k0 k=a 1 holds, this bound wave becomes a leaky wave which is fast wave for 1 space harmonic with a complex propagation constant. The far-zone radiated fields are due to the combination of space waves and leaky waves. The energy carried by bounded surface waves that propagate laterally is considered a loss. Antenna directivity, gain and efficiencies are determined by the energy distribution among these three propagating waves. Antenna efficiency can be maximized by partial elimination of the bound surface waves within certain directions. For high efficiency antennas, directivity and gain are approximately the same and are determined by the energy distribution be-
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tween the space and leaky waves. Leaky wave radiation pattern is highly directive in contrast to the low directivity of the space waves. To achieve high gain antenna, it is necessary to excite a strong leaky wave [39–42]. The simulated input return loss of the rectangular microstrip patch antenna is shown in Fig. 2, which reveals that by using 2-D electromagnetic crystal substrate, we obtain extremely broadbandwidth with multi-frequency band in the frequency range 0.6–0.95 THz. The three different frequencies at which the antenna resonates are 693.45, 797.4 and 852 GHz. The 10 dB impedance bandwidth of this rectangular patch microstrip antenna using electromagnetic crystal substrate is 34.3%. The simulated far-zone radiation pattern for the gain and directivity at the frequencies 775, 693.45, 797.4 and 852 GHz are shown in Fig. 3. The radiation patterns within the operational frequency of the gap have been plotted for several frequencies. The radiation patterns in both E and H plane are rather stable using a electromagnetic crystal substrate, showing nearly the same radiation patterns. The radiation pattern of E-plane (the plane in which surface waves are more pronounced) is presented in Fig. 3. The electrical parameters such as gain, directivity and radiation efficiency of the rectangular patch microstrip antenna at the aforementioned frequencies have been shown in Table 1. From the Table 1, it is clear that the gain and directivity of the proposed antenna at frequency 797.4 GHz are very interesting. But the radiation efficiency point of view, the results of the proposed antenna at frequency 852 GHz is best among all. At the design frequency, the patch antenna on a electromagnetic crystal has more directivity, less side and back radiation and smoother pattern. The ability of the electromagnetic crystal substrate to reduce the surface wave mode propagation has been proven by better radiation efficiencies. This opens the door to design new devices with thicker substrates and higher dielectric constant without loosing performance by the undesired excitation of the surface wave mode. The effects of diameter of the air-gap in substrate material on the radiation efficiency and gain of the proposed microstrip patch antenna at frequency 775 GHz are shown in Fig. 4. The diameter of the air-gaps chosen in the simulation is 20 lm as marked in Fig. 4. The gain of antenna increases with the increase of diameter of the air-gaps and attain optimum value at 20 lm. At this point of interest even the efficiency is low as compare to others but the gain is maximum, which is the reason to consider this diameter in our simulation. The effects of dielectric permittivity of the electromagnetic crystal substrate on the radiation efficiency and gain of the rectan-
Fig. 2. The simulated frequency versus return loss of rectangular microstrip patch antenna at THz frequencies.
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G. Singh / Infrared Physics & Technology 53 (2010) 17–22
Fig. 3. Far-zone radiation pattern of the gain and directivity in E and H plane of the rectangular microstrip patch antenna at frequencies 775, 693.45, 797.4 and 852 GHz.
Table 1 Simulated electrical parameters of the microstrip antenna at different THz frequencies. Frequency (GHz)/electrical parameters Gain (dB) Directivity (dBi) Radiation efficiency (%)
775 8.451 9.934 71.06
693.45 4.501 6.239 67.02
797.4 9.19 10.36 76.45
852 8.248 8.789 88.30
gular patch microstrip antenna at 775 GHz has been shown in Fig. 5. In this simulation, we have considered the dielectric permittivity of electromagnetic crystal substrate as marked in Fig. 5. Sharma and Singh [19] have simulated a rectangular microstrip patch antenna by using conventional substrate material at terahertz frequency. In this simulation, the gain and radiation efficiency was 3.497 dB and 55.71%, respectively, at 775 GHz, but when it is sim-
G. Singh / Infrared Physics & Technology 53 (2010) 17–22
Fig. 4. The effects of air-gap diameter on the radiation efficiency and gain of the rectangular microstrip patch antenna.
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crystal substrate surfaces exhibited high efficiency and directivity as compare to conventional antenna on substrate. The gain of the printed circuit antenna can be greatly enhanced with the electromagnetic crystal material as substrate. From the simulation, we obtained multi-frequency band and 10 dB impedance bandwidth 34.3%, which is extremely broader bandwidth. At the frequency 852 GHz the antenna gain and directivity are 8.248 dB and 8.789 dBi, respectively, with radiation efficiency 88.30%. It was found that significant gain enhancement is achieved by exciting strong leaky waves through proper designs of planar periodic metallic structures. Multi-frequency band microstrip patch antennas are often used in modern communication systems in order to enhance their capacity through frequency reuse. The technological potential of electromagnetic crystal substrate for the development of novel microstrip patch antenna that could overcome the limitation of current technology. Consequently, this simulation study has shown the possibility of design of efficient, high gain broadband rectangular microstrip patch antennas on 2-D electromagnetic crystal substrate at THz frequencies. A very important need is to verify more of the simulation results with experimental measurements which will be reported in future communications. Acknowledgement Author is sincerely thankful to the reviewers for their critical comments and suggestions to improve the quality of the manuscript. References
Fig. 5. The effects of the dielectric permittivity of the substrate on radiation efficiency and gain of the rectangular microstrip patch antenna.
ulated over photonic crystal on same frequency the gain and radiation efficiency increases drastically as clear from Table 1. The radiation efficiency and gain of the proposed antenna is maximum at the dielectric permittivity of the substrate 9.1, above this value it is approximately constant. At this point, we have maximum gain and radiation efficiency as compare to all other points. The distance between air-gaps and arrays of air blocks is chosen for minimum return loss [42]. The size of the substrate is such that, in theory, the surface mode is added in counter phase for the conventional patch antenna, resulting in a very low value of gain in boresight direction. This effect of surface wave is approximately eliminated by the electromagnetic crystal substrate, leading to the smooth radiation pattern. A multi-frequency band and extremely broader bandwidth are the potential advantages of using electromagnetic crystal as substrate for the rectangular microstrip patch antenna at THz frequency.
4. Conclusion In this paper, I have simulated a rectangular microstrip patch antenna with electromagnetic crystal as a substrate material at THz frequencies. At millimeter and THz wave integrated circuits, the control of radiation from a microstrip patch antenna is of great interest. The microstrip antenna mounted on 2-D electromagnetic
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