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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
Broadband Substrate Integrated Waveguide Cavity-Backed Bow-Tie Slot Antenna Soumava Mukherjee, Student Member, IEEE, Animesh Biswas, Senior Member, IEEE, and Kumar Vaibhav Srivastava, Senior Member, IEEE
Abstract—A novel design technique for broadband substrate integrated waveguide cavity-backed slot antenna is demonstrated in this letter. Instead of using a conventional narrow rectangular slot, a bow-tie-shaped slot is implemented to get broader bandwidth performance. The modification of the slot shape helps to induce strong loading effect in the cavity and generates two closely spaced hybrid modes that help to get a broadband response. The slot an) in a tenna incorporates thin cavity backing ( single substrate and thus retains low-profile planar configuration while showing unidirectional radiation characteristics with moderate gain. A fabricated prototype is also presented that shows a bandwidth of 1.03 GHz (9.4%), a gain of 3.7 dBi over the bandwidth, 15 dB front-to-back ratio, and cross-polarization level below 18 dB. Index Terms—Bow-tie, broadband antenna, cavity-backed antenna, hybrid modes, slot antenna, substrate integrated waveguide (SIW).
I. INTRODUCTION
O
VER the years, slot antennas have been a popular choice among the researchers due to its various attractive features, e.g., low profile, conformability, good isolation from feeding network, easy integration to other planar circuits, etc. [1]. However, the slot antennas exhibit bidirectional radiation characteristics that limit their performance in some applications. The backside radiation can be removed by placing a metallic reflector or cavity behind the slot at an optimum distance of one quarter of a guided wavelength. Yet, the system becomes bulky due to integration of nonplanar metallic cavity behind the slot [2]–[4]. In recent times, a relatively new technology has emerged known as substrate integrated waveguide (SIW), which incorporates nonplanar waveguide structures in a planar substrate by the use of rows of metallic vias that implement the sidewall of the waveguide-based circuits in planar substrates [5]. Substrate integrated waveguide cavity was first proposed by Cassivi et al., where four rows of metallic vias were used to implement the cavity in a planar substrate [6]. The technology was incorporated in a cavity-backed antenna by Luo. et al., in which the nonplanar metallic cavity is replaced by an SIW cavity structure [7]. The proposed antenna presents unidirectional radiation Manuscript received March 28, 2014; revised May 12, 2014; accepted June 08, 2014. Date of publication June 12, 2014; date of current version June 24, 2014. (Corresponding author: Soumava Mukherjee.) The authors are with the Department of Electrical Engineering, Indian Institute of Technology Kanpur (IITK), Kanpur 208016, India (e-mail:
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2330743
characteristics with high gain while maintaining its low-profile planar configuration. However, due to the loading effect of the high- cavity, the antenna shows narrowband performance. Several techniques to improve the bandwidth of the SIW cavity-backed slot antenna are reported in recent years. In [8], a via-hole is placed above the slot to modify its electrical length and thus to add a second resonance to improve the bandwidth. Another method of enhancing bandwidth and efficiency is proposed in [9], where the dielectric substrate is removed under the slot to change the effective and to increase the bandwidth. However, all these techniques lead to a fractional bandwidth up to 2.16%, which is still narrowband. Finally in [10], a rectangular slot of much higher length is used in a rectangular-shaped cavity to generate hybrid modes, which helps to increase the operational bandwidth of the antenna up to 6.3%. The bow-tie-shaped slot antenna is investigated as a promising candidate for broadband application over the past few years [11]. Studies on this type of antenna reveal that it has potential to exhibit 20%–36% bandwidth with proper design of feeding network. In this letter, a study on a bow-tie-shaped slot backed by an SIW cavity is presented. The proposed antenna exhibits a broadband response of 9.4% bandwidth with a moderate gain and an unidirectional radiation pattern. The placement of the bow-tieshaped slot helps to resonate two hybrid modes in the cavity close enough to get a wider bandwidth response. The technique also replaces a complex feeding mechanism for bow-tie slot antenna with grounded coplanar waveguide (GCPW)-type feeding technique to simplify the design. The fabricated antenna shows uniform gain over the operating bandwidth while maintaining its planar form. II. PRINCIPLE OF OPERATION The geometry of the proposed antenna is shown in Fig. 1. The bow-tie-shaped slot is etched at the top metallic plate and placed at a distance of “ ” from one sidewall of the cavity. The SIW cavity is constructed in a single substrate by four rows of metallic vias implementing four sidewalls of the cavity. The diameter ( ) and pitch ( ) of the via-holes can be adjusted while maintaining the conditions and to ensure minimum leakage of energy [12]. The cavity dimensions have been optimized to get its dominant mode at 9.2 GHz. The modified bow-tie-shaped slot disturbs the current path of the higher-order mode, resulting in a strong loading effect to the cavity, and as a result, the higher-order mode shifts toward the lower-frequency end and interacts with dominant mode to generate two
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MUKHERJEE et al.: BROADBAND SUBSTRATE INTEGRATED WAVEGUIDE CAVITY-BACKED BOW-TIE SLOT ANTENNA
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Fig. 3. Surface current density vector at top metallic plate without slot (a) at mode), (b) at 14.7 GHz ( mode), and with slot loading 9.2 GHz ( (c) at 9.2 GHz and (d) at 10.52 GHz.
Fig. 1. Geometry of the proposed design. (a) 3-D view. (b) Bottom view ( mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm).
Fig. 4. Variation of reflection coefficient and radiation efficiency with change ) of the slot. in amount of flaring (
Fig. 2. Real and imaginary
plot of the cavity without and with slot loading.
hybrid modes resonating in a close frequency range. The loading effect of the bow-tie slot is evident from the plot as shown in Fig. 2, where the dominant mode (9.2 GHz) is not affected by the slot, whereas the higher-order mode shifts from 14.7 to 10.52 GHz. This can be better understood from the current distribution on the top metallic plate of the cavity as shown in Fig. 3. The surface currents corresponding to the two modes as shown in Fig. 3(a) and (b) interfere with each other
after placement of the slot and generate two hybrid mode distributions as shown in Fig. 3(c) and (d). The current distribution in the lower frequency is dominant in the right side of the cavity, whereas in case of higher frequency, the dominant field distribution exists in the left side of the cavity. In both cases, the magnitude and phase of the electric fields are different at opposite sides of the slot, which helps it to radiate. These two hybrid modes’ behaviors are very similar to that of the SIW cavity-backed slot antenna excited by hybrid modes as described by Luo et al. [10]. The input impedance and bandwidth of the bow-tie-shaped slot antenna strongly depends on the amount of flaring ( ). Variation of impedance matching and operating bandwidth with the change in is shown in Fig. 4. The second resonant frequency shifts to the higher end with increasing , and as a result, more bandwidth can be achieved by optimizing the flaring of the slot. The optimum flare value is chosen as mm, which is quite similar to the case of a simple CPW-fed bow-tie slot antenna where optimum bandwidth is achieved for a flare angle of 20 [11]. The variation of radiation efficiency with is also plotted in Fig. 4. The simulated results using Ansoft HFSS show that the radiation efficiency
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
Fig. 6. Fabricated prototype. (a) Top view. (b) Bottom view.
Fig. 5. Variation of reflection coefficient with change in length of slot (
).
above 92% can be obtained in the operating frequency band, making the antenna suitable for most practical applications. The length ( ) of the bow-tie slot is much higher than the half-wavelength resonant length of a conventional slot antenna. The effect on antenna performance with variation of is shown in Fig. 5. The optimum length to excite both hybrid modes and hence to get maximum bandwidth is dependent on the flaring of slot ( ). As it can be seen from the figure, the generation of lower-frequency hybrid mode is strongly dependent on the length of the slot. As the slot length increases, the lower-frequency hybrid mode becomes more and more prominent, which improves the impedance matching performance over the operating bandwidth. The length of the bow-tie slot in the current design is chosen to be 14.1 mm to get maximum bandwidth of 1.03 GHz. The feeding to a conventional bow-tie slot antenna requires a complex impedance-matching network due to its high input impedance [13], [14]. However, for the SIW cavity-backed bow-tie slot antenna, a simple feeding technique similar to a conventional SIW cavity-backed slot antenna can be used without much difficulty to get impedance matching. As shown in Fig. 1, a 50- GCPW line followed by a 50- microstrip line of same width is used to feed the antenna. The feedline excites corresponding modes in the cavity, which create the necessary variation in the field distribution at opposite sides of slot. As a result, the slot starts to radiate into free space. The inset of the feeding line is optimized to excite two necessary modes and of the cavity and hence to get a broadband impedance matching. The optimum dimensions of the design are given in Fig. 1. III. EXPERIMENTAL RESULTS The proposed design is fabricated on Rogers RT Duroid 5880 with the substrate thickness of 0.787 mm, which is less than . The slot is etched at the upper metallic plate, and vias are fabricated by standard printed circuit board (PCB) technology. The photograph of the fabricated prototype is shown in
Fig. 7. Comparative study between simulated and measured reflection coefficient and gain of the antenna.
Fig. 6. The antenna is fed by a coaxial SMA connector, and its performance is measured in Agilent VNA E5071C. The measured reflection coefficient response is shown in Fig. 7, which matches very well with the simulated result. The measured first and second resonances are at 9.98 and 10.6 GHz, respectively, which are very close to the simulated values of 9.96 and 10.56 GHz. The measured bandwidth of the proposed antenna is 1.03 GHz (9.43%), which is much higher than that achieved by a conventional SIW cavity-backed slot antenna [7]. The comparison between measured and simulated gain is also shown in Fig. 7. The variation of simulated gain of the antenna is almost uniform throughout the operating bandwidth within the range of 3.12–3.86 dBi. The measured gains of the antenna at 9.98 and 10.6 GHz are 3.53 and 5.4 dBi, respectively. However, there is some deviation in the measured gain plot with the simulated one, which may be due to imperfections in fabrication process. The measured radiation pattern of the antenna at two resonances matches very well with the simulated one as shown in Figs. 8 and 9. As we can see from Figs. 8 and 9, the patterns are much wider compared to that of the conventional narrow rectangular slot backed by SIW cavity. As a result, the directivity of the antenna decreases, and the gain of the antenna is limited to 3–4 dBi instead of 5.3 dBi in the case of a conventional slot antenna [7]. However, the radiation patterns at both resonances are almost similar, which is due to similar types of field distribution across the slot at both resonances. Therefore, we can conclude that the antenna maintains its radiation properties and gain throughout the operating bandwidth with very small deviation, which makes it suitable for broadband applications.
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bow-tie-shaped slot disturbs the field distribution in the slot, which results a higher cross polarization in E-plane pattern. However, the cross-polarization level is 26 and 40 dB below in the broadside direction at first and second resonance, respectively. The measured front-to-back ratio (FTBR) of the antenna is about 15 and 20 dB, respectively, at 9.98 and 10.6 GHz. IV. CONCLUSION
Fig. 8. Radiation pattern at 9.98 GHz. (a) E-plane. (b) H-plane.
A broadband substrate integrated cavity-backed bow-tie slot antenna is implemented in this letter. The proposed antenna replaces a conventional narrow rectangular slot with a bow-tieshaped slot and is excited by a simple GCPW feeding technique. The loading effect due to placement of the bow-tie-shaped slot on the top of the cavity can be adjusted by optimizing the dimensions of the slot to generate two closely spaced hybrid modes. These two resonances lead to a broadband response of 9.4%, which is much higher than that of the conventional SIW cavitybacked slot antenna (1.7%). The proposed antenna is fabricated in a single substrate using standard PCB technologies and thus maintains its low profile while retaining the advantages of unidirectional radiation pattern of conventional cavity backed antenna. The proposed antenna shows uniform gain versus frequency characteristics within the range of 3–4 dBi and a unidirectional radiation pattern over the operating bandwidth, which makes it suitable for many broadband practical applications. REFERENCES
Fig. 9. Radiation pattern at 10.6 GHz. (a) E-plane. (b) H-plane.
The cross polarization in H-plane is below 20 dB at both frequencies. The wider opening at both ends of the
[1] Y. Yoshimura, “A microstripline slot antenna,” IEEE Trans. Microw. Theory Tech., vol. MTT-20, no. 11, pp. 760–762, Nov. 1972. [2] A. T. Adams, “Flush mounted rectangular cavity slot antennas- Theory and design,” IEEE Trans. Antennas Propag., vol. AP-15, no. 3, pp. 342–351, May 1967. [3] C. Locker, T. Vaupel, and T. F. Eibert, “Radiation efficient unidirectional low profile slot antenna elements for X-band application,” IEEE Trans. Antennas Propag., vol. 53, no. 8, pp. 2765–2768, Aug. 2005. [4] A. Vallecchi and G. B. Gentili, “Microstrip fed slot antennas backed by a very thin cavity,” Microw. Opt. Technol. Lett., vol. 49, no. 1, pp. 247–250, Jan. 2007. [5] M. Bozzi, A. Georgiadis, and K. Wu, “Review of substrate integrated waveguide circuits and antennas,” Microw., Antennas Propag., vol. 5, no. 8, pp. 909–920, 2011. [6] Y. Cassivi, L. Perregrini, K. Wu, and G. Conciauro, “Low-cost and high- millimeter-wave resonator using substrate integrated waveguide technique,” in Proc. 32nd Eur. Microw. Conf., 2002, pp. 1–4. [7] G. Q. Luo, Z. F. Hu, L. X. Dong, and L. L. Sun, “Planar slot antenna backed by substrate integrated waveguide cavity,” IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 235–239, 2008. [8] S. Yun, D. Kim, and S. Nam, “Bandwidth enhancement of cavitybacked slot antenna using a via-hole above the slot,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 1092–1095, 2012. [9] S. Yun, D. Kim, and S. Nam, “Bandwidth and efficiency enhancement of cavity-backed slot antenna using a substrate removal,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 1458–1461, 2012. [10] G. Q. Luo et al., “Bandwidth-enhanced low-profile cavity-backed slot antenna by using hybrid SIW cavity modes,” IEEE Trans. Antennas Propag., vol. 60, no. 4, pp. 1698–1704, Apr. 2012. [11] J. Huang and C. Kuo, “CPW-fed bow-tie slot antenna,” Microw. Opt. Technol. Lett., vol. 19, no. 5, pp. 358–360, 1998. [12] F. Xu and K. Wu, “Guided-wave and leakage characteristics of substrate integrated waveguide,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 1, pp. 66–73, Jan., 2005. [13] J. Niu and S. Zhong, “A broadband CPW-fed bow-tie slot antenna,” in Proc. IEEE Antennas Propag. Int. Symp., 2004, vol. 4, pp. 4483–4486. [14] M. Z. Khan and M. Ali, “A CPW-fed inductively coupled modified bow-tie slot antenna,” in Proc. IEEE Antennas Propag. Int. Symp., 2005, vol. 3B, pp. 365–368.
