A Millimeter Wave Center-SIW-Fed Antenna For 60 GHz Wireless Communication

Transcription

1 A Millimeter Wave Center-SIW-Fed Antenna For 60 GHz Wireless Communication M. Karami, M. Nofersti, M.S. Abrishamian, R.A. Sadeghzadeh Faculty of Electrical and Computer Engineering K. N. Toosi University of Technology Abstract An improved 2 2 microstrip patch antenna (MPA) array is proposed for impedance bandwidth enhancement, low back-lobe radiation, high co-to-cross polarization levels, and compact size at 60 GHz. Utilizing a novel center-fed coaxial-tosubstrate integrated waveguide (SIW) transition and a combshape choke structure, the antenna's electrical performance is improved. Compared to previous works, the antenna presents 10 db improvement in both the front-to-back ratio (FTBR) and the co-to-cross polarization levels in the radiation pattern. Also, the evaluated return loss of the antenna proves 100% enhancement in operating impedance bandwidth ( S db) around 60 GHz. Further, the overall size of the antenna is reduced by 22%. The simulation results are in good agreement and verify the design of the proposed antenna. Keywords-Microstrip patch antenna (MPA); substrate integrated waveguide (SIW); back-lobe; impedance bandwidth. I. INTRODUCTION In recent years, millimeter Wave (mmw) systems have been used in commercial communication systems, widely [1]. Since, these applications demand bandwidth enhancement and development in nano-metric Si-based integrated circuit technology. Microstrip patch antennas (MPAs) have been widely used in wireless communication systems due to advantages such as low profile, conformability to planar or curved surface, low cost, light weight, and compact size. In mmw frequencies, common feeding methods, such as microstrip line feed [2], coplanar waveguide feed [3], and rectangular waveguide excitation [4], have disadvantages such as high conduction losses, undesirable radiation production, bulky and fabrication cost. Utilizing substrate integrated waveguide (SIW) technology in the antenna feeding network has proposed to solve these problems. An aperture-coupled (AC) MPA array fed by SIW is proposed in [5] and [6]. In this paper, the electrical performance of the conventional 2 2 SIW-fed MPA array [7] is improved using a new centerfed coaxial-to-siw transition and a comb-shape choke structure. The proposed configuration is designed and simulated by the finite element method (FEM) and finite integration technique (FIT) respectively in two full-wave solvers, ANSYS HFSS and CST Microwave Studio (MWS). In order to attain the enhanced impedance bandwidth, the SIW discontinues are used in the transition of proposed antenna. Suppressing spillover surface current flowing through the MPA and SIW metal ground plate with adding the choke structure causes antenna back-lobe level significantly (10 db) improvement. The good agreement in simulation results of two solvers validates the proposed antenna design. II. CENTER-FED COAXIAL-TO-SIW TRANSITION AND COMB-SHAPE CHOKE STRUCTURE Major advantages of the coaxial-to-siw transition [8], [9] are external electromagnetic coupling noise cancellation, conductor loss reduction, and decreasing of connection leakage, and measurement errors by direct connecting to an electromagnetic signal source, or a measurement instrument up to mmw frequencies. The conventional SIW feeding network and the proposed center-fed coaxial-to-siw transition are shown in Fig. 1. The conventional feeding network is made of two sections: the 1 2 SIW power divider and the cavity back coaxial-to-siw transition [10]. To provide a better transition between the TEM coaxial mode and the TE 10 mode in SIW, the proposed transition is designed based on the cylindrical waveguide structure and the step matching. An equivalent lumped element circuit of the proposed SIW transition is shown in Fig. 2. Each coaxial-to-siw transition discontinuity is modeled with the shunt inductances, same as H-plane step in a rectangular waveguide. Therefore, adjusting the step discontinuities can be provides higher impedance bandwidth [11]. The inductances of the SIW discontinuities are proportional to the ratio of, where and are SIW aperture and transition center section widths, respectively. For better impedance matching, the length of and are assumed equal /4, same as Quarter-Wave Transformers g (QWT).

2 Fig. 1. Top view of the SIW feeding network, a) the conventional feeding network [10], b) the proposed center-fed coaxial-to-siw transition. Fig. 3. The S-parameters of the 60 GHz SIW feeding network, a) the conventional feeding network [10], b) the proposed SIW transition. Fig. 2. The equivalent circuit of the center-fed coaxial-to-siw transition. The conventional SIW feeding network and the proposed center-fed coaxial-to-siw transition operate at around 60 GHz. They are applied on the SIW substrate with a thickness h = 0.78 mm and a dielectric constant of According to design frequency, the coaxial cable has an inner conductor diameter ( ) of 0.1 mm, and a dielectric diameter ( ) of 0.35 mm. SIW has a center portion width ( ) of 2.5 mm, and QWT lengths (, ) of 1.25 mm. To validate the proposed SIW transition design, simulation is carried out with FEM and FIT in full-wave solvers ANSYS HFSS and CST MWS, respectively. Fig. 3 depicts the S- parameter simulation results of the conventional feeding network, which is fed by the cavity back coaxial-to-siw transition [10] and the proposed center-fed SIW transition, respectively. As can be seen, the conventional feeding network acts like a high-pass filter, while the developed Fig. 4. Comb-shape choke structure. transition s S-parameters are as same as a band-pass filter, resulting the higher order mode rejection and SNR improvement. In addition, according to the scattering property of the proposed SIW transition (see Fig. 3-b), it can be used as an equal-split three-port power divider and leads to a significant size reduction of the SIW feeding network. Since the incident power in port 1 has been equally coupled to the port 2 and 3. There are some methods proposed for antenna back-lobe reduction, such as thick side-walls waveguides [12], and EBG structures [13]. Major disadvantages of these methods are enormous size and high side-lobe levels (SLLs). As depicted in Fig. 4, in this paper, a comb-shape choke structure is proposed. The proposed structure suppresses the undesired surface-wave and back-lobe radiation. The proposed choke

3 includes a linear array of /4 short-end microstrip stubs located under the ground plane. The stubs generate ultra-high impedance at the edges of the substrate/ground plane for suppressing diffraction at these edges. The compact size is the main advantage of the proposed choke structure in practical application. III. ANTENNA DESIGN AND PERFORMANCE In [7], a 2D (2 2) SIW-fed MPA has been presented at 60 GHz to minimize the feeding loss shown in Fig. 5. It has two main parts, a SIW 1 2 power splitter and MPA elements. This antenna is fed ideally with a waveguide port. Narrow bandwidth (about 500 MHz) and the impossibility of directly connecting to the signal source are the main disadvantages of the conventional MPA array design [7]. To overcome these disadvantages, a 2 2 center-siw-fed MPA array is proposed. Fig. 6 depicts the proposed antenna configuration. In order to enhance the impedance bandwidth and the direct connection to the signal source, the proposed center-fed coaxial-to-siw transition is placed in the side portion of the antenna structure. The proposed transition removes the SIW 1 2 power divider section of [7] which leads to overall antenna size reduction. Another advantage of the proposed design is the back-lobe radiation reduction. The proposed comb-shape choke suppresses surface wave/current in the edges of antenna configuration and causes lower backlobe radiation and SLLs. Table I summarizes the dimensions of the proposed antenna and the reported [7]. The proposed antenna is designed using the electromagnetic simulators ANSYS HFSS and CST MWS at 60GHz. Fig. 7 illustrates the simulated return losses, S 11, which are in good agreement with together. The reflection coefficients of the MPA array [7] and the proposed antenna are under -10 db from to GHz and 59.6 to 60.6 GHz, respectively. It demonstrates that there is 100% enhancement compared to the MPA array [7]. TABLE I. DIMENSIONS OF THE CONVENTIONAL AND THE PROPOSED ANTENNAS (MILLIMETER) Fig. 5. The conventional SIW-fed MPA [7]. 3D view and side view. Slots Conventional MPA array [7] Coaxial-to- SIW Transition Proposed Antenna SIW Substrates L s = 2 D = 2.5 L 1 = 1.25 D = 2.5 ε r = 2.33 w s = 0.25 D s= 3.5 L 2 = 1.25 D s = 1.25 h SIWSUB = 0.78 W SIW = 2.4 W T = 3.6 W SIW = 2.4 D via = 0.3 L= 2.29 h ChokeSUB = S via = 0.6 W = 1.35 h MPASUB = CV-d=0.15 Ch-d=0.425 Fig. 6. The proposed SIW-fed MPA array, 3D view, and side view. Fig. 7. The simulated S 11 of proposed and conventional [7] antennas.

4 TABLE II. PERFORMANCE COMPARISON AT 60 GHZ MPA array Impedance bandwidth Gain FTBR (dbi) (db) Radiation efficiency co-to-cross polarization levels (db) Size Conventional [7] 500 MHz % Up to mm 2 Proposed 1000 MHz % Up to mm 2 Fig. 8. The antenna gain (dbi) and the radiation efficiencies of 2 2 mmw MPA antenna, the MPA array [7], the proposed antenna. Fig. 9. The simulated radiation patterns of 2 2 mmw SIW-fed MPA antennas at f = 60 GHz. E-plane, H-plane. Fig. 8 shows the calculated gain and radiation efficiency of [7] and the proposed antenna for different frequency points. In Fig. 8-a, the gain of the conventional MPA array [7] varies between dbi and dbi at operational bandwidth ( GHz). However, the changes of the gain of the proposed antenna are started from 11.1 dbi to 11.4 dbi between 59.6 GHz and 60.6 GHz. It is apparent that the conventional and the proposed antennas' gain are about 11.6 and 11.3 dbi in the operational frequency (60 GHz), respectively. The slight differences come from the comb-shape choke. The radiation efficiency of the proposed antennas is better than 90% from 59.6 to 60.6 GHz. The simulated E-plane and H-plane radiation patterns are shown in Fig. 9. The proposed antenna is fed by TEM coaxial mode, while [7] is ideally fed by TE 10 SIW mode. The symmetrical characterization of TEM mode field lines significantly improves the co-to-cross polarization levels. Thus, as depicts in Fig. 9, the cross-polarized fields (Exp and Hxp) of the proposed antenna are reduced more than 10 db compared to the conventional MPA array [7] in the broadside direction of the radiation patterns. The proposed chokes filter the undesired surface wave/current on the ground plane of the proposed antenna. Comparing with [7], the front-to-back ratios (FTBRs) of proposed antenna radiation pattern are increased more than 10 db in the both co-polarized fields (Ecop and Hcop). The simulated electrical performance and the total size of the proposed antenna and [7] are shown in Table II. As can be seen, the total antenna size is reduced by 22% because of combining the feeding network and the SIW power divider, and decreasing distance between the center of the final MPA element and terminated vias ( ). From the feeding network view point, the presented antenna achieves higher compact size than [7]. IV. CONCLUSION A 2D MPA array is proposed using center fed coax-to-siw transition and comb-shape choke configuration at 60 GHz. The proposed transition is designed based on the cylindrical waveguide structure and step matching. Thus, it provides a better transition between the TEM coaxial mode and the TE 10 mode in SIW. The comb-shape choke structure includes a linear array of /4 short-end microstrip stubs located under the

5 ground plane. So, it suppresses the undesired surface-wave and back-lobe radiation. The antenna has broadside gain up to 11.4 dbi and radiation efficiency better than 90% in the impedance bandwidth (1000 MHz). Compared to previous works, the proposed antenna reduces the Hxp and Exp (upper 10 db), and enhances the impedance bandwidth (100%) due to TEM symmetrical field lines, and SIW step discontinuities, respectively. Using comb-choke structure, the FTBRs are improved more than 10 db. Furthermore, the antenna achieves upper 22% size reduction. The results are in good agreement, which verify the design of the proposed antenna. [12] T. P. Wong and K. M. Luk, A wide bandwidth and wide beamwidth CDMA/GSM base station antenna array with low back-lobe radiation, IEEE Trans. Veh. Technol., vol. 54, pp , May [13] L. Li, X. J. Dang, B. Li, and C. H. Liang, Analysis and design of waveguide slot antenna array integratedwith electromagnetic band-gap structures, IEEE Antennas Wireless Propag. Lett., vol. 5, pp , December REFERENCES [1] W. M. A. Wahab, S. S. Naeini, and D. Busuioc, "Low cost microstrip patch antenna array using planar waveguide technology for emerging millimeter-wave wireless communication," in Proc. AANTEM- AMEREM. Int. Symp., pp. 1-4, July [2] W. L. Chen, G. M. Wang, and C. X. Zhang, "Bandwidth enhancement of a microstrip-line-fed printed wide-slot antenna with a fractal-shaped slot," IEEE Trans. Antennas Propag., vol. 57, pp , July [3] N. Mohammadian, M. N. Azarmanesh, and S. Soltani, "Compact ultrawideband slot antenna fed by coplanar waveguide and microstrip line with triple-band-notched frequency function," IET Microw. Antennas Propag., vol. 4, pp , November [4] K. W. Leung and K. K. So, "Rectangular waveguide excitation of dielectric resonator antenna," IEEE Trans. Antennas Propag., vol. 51, pp , September [5] T. Mikulasek and J. Lacik, Microstrip patch antenna fed by substrate integrated waveguide, in Proc. The 13th ICEAA, Torino, pp , September [6] T. Mikulasek, A. Georgiadis, A. Collado and J. Lacik, 2 2 Microstrip Patch Antenna Array Fed by Substrate Integrated Waveguide for Radar Applications, IEEE Antenn. Wireless Propag. Lett., vol. 12, pp , September [7] W. M. A. Wahab and S. S. Naeini, Wide-bandwidth 60-GHz aperturecoupled microstrip patch antennas (MPAs) fed by substrate integrated waveguide (SIW), IEEE Antenn. Wireless Propag. Lett., vol. 10, pp , September [8] T. Kai, Y. Katou, J. Hirokawa, M. Ando, H. Nakano, and Y. Hirachi, "A coaxial line to post-wall waveguide transition for a cost-effective transformer between a RF-device and a planar slot-array antenna in 60- GHz band," IEICE Trans. Commun., vol. E89-B, pp , May [9] D. Y. Kim, J. W. Lee, T. K. Lee, and C. S. Cho, "Design of SIW cavitybacked circular-polarized antennas using two different feeding transitions," IEEE Trans. Antennas Propag., vol. 59, pp , April [10] D. Y. Kim, W. S. Chung, C. H. Park, S. J. Lee, and S. Nam, "A series slot array antenna for 45 o -inclined linear polarization with SIW technology," IEEE Trans. Antennas Propag., vol. 60, pp , April [11] T. Kai, J. Hirokawa, and M. Ando, "A stepped post-wall waveguide with aperture interface to standard waveguide," in Proc. IEEE Antennas Propag. Int. Symp., pp , June 2004.