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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 22, NO. 2, FEBRUARY 2012
Ridge Substrate Integrated Waveguide (RSIW) Dual-Band Hybrid Ring Coupler Tarek Djerafi, Hervé Aubert, Senior Member, IEEE, and Ke Wu, Fellow, IEEE
Abstract—A novel dual-band ring coupler is proposed in Ridge Substrate Integrated Waveguide (RSIW) technology. The structure is based on two concentric rings with the outer ring loaded by a low insertion loss de-multiplexing scheme allowing independent dual band operation. The RSIW ensures a wide-band fundamental mode operation covering both frequency bands while providing a compact design. A C/K band coupler is experimentally verified showing coupling bandwidths of 6.9% and 14.6% centered at 7.25 and 20.5 GHz, respectively. Index Terms—De-multiplexing, dual-band, ridge substrate integrated waveguide (RSIW), ring coupler.
I. INTRODUCTION
T
HE hybrid-ring directional coupler is one of the fundamental passive components used in microwave and millimetre wave circuits. The ring coupler offers equal power split at the coupled ports with both in-phase and anti-phase operations. An H-plane hybrid ring has been designed based on the Substrate Integrated Waveguide (SIW) technique [1] and on folded SIW in [2]. In current satellite and terrestrial communication systems, such as X/Ka-band dichroic mirror and dual-band couplers, are crucial components. A dual band ring coupler has been recently reported in [3] where the left handed propagation is explored together with the half-mode SIW structure giving a compact design; however, limited frequency ratios can be satisfied ( 1: 1.6 typically). A novel dual-band ring coupler is hereby proposed based on a double-layer RSIW topology. The coupler uses a pair of concentric rings and a novel low-loss de-multiplexing scheme. The ridge waveguide ensures fundamental mode operation over both frequency bands. The ridge waveguide parameters of the different sections are calculated based on the Transverse Resonant Method and the design methodology for the de-multiplexing scheme is outlined. The simulated and measured results of the fabricated coupler are presented and good agreement is found for a frequency ratio of 1:2.8. Manuscript received April 27, 2011; revised September 27, 2011; accepted November 06, 2011. Date of publication January 27, 2012; date of current version February 15, 2012. This work was supported in part by the Fonds Québécois de la Recherche sur la Nature et les Technologies (FQNRT) and Natural Sciences and Engineering Research Council of Canada (NSERC). T. Djerafi and K. Wu are with the Poly-Grames Research Center, École Polytechnique de Montréal, Montréal, QC H3C 3A7, Canada (e-mail:
[email protected]). H. Aubert is with the University of Toulouse and the Laboratory of Analysis and Architecture of Systems (LAAS-CNRS), Toulouse 31400, France. 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/LMWC.2011.2181158
II. STRUCTURE DESCRIPTION AND DESIGN CONSIDERATIONS A. Proposed Topology Fig. 1 shows the proposed dual-band coupler structure. The structure consists of two concentric ring couplers. The outer ring is responsible for the lower frequency band operation while the coupling at the higher frequency band is achieved by the inner one. This technique is used to ensure broadband performance of five ports [4]. The ridge waveguide is adopted to maintain wideband fundamental mode propagation, and thus providing a flexible separation of both frequency bands within a compact design. For the lower frequency band of operation (7–7.5 GHz), the electromagnetic wave propagation is confined in the outer ring by adjusting the inner ring ridge waveguide parameters to ensure a cutoff frequency higher than the lower band of operation. On the other hand, in order to forbid the propagation of the electromagnetic wave within the outer coupler in the higher frequency, a de-multiplexing scheme is applied. This scheme consists of a set of periodic defects that provides a stopband (bandgap effect) at the upper frequency band of operation (19–22 GHz) forbidding the electromagnetic signal from propagation in the outer ring in this band, directing it to the inner ring. B. Ridge Substrate Integrated Waveguide For proper operation, the fundamental mode must be ensured by these waveguides over the entire frequency band between the lowest operation frequency of the lower frequency band and the highest frequency in the upper band. Therefore, the use of ridge-waveguide appears as a very good candidate for such a structure to resolve the bandwidth problem. A planar ridged substrate-integrated waveguide (RSIW) with a center line of cylindrical posts is proposed in [5] constructed in two layers with 37% of bandwidth enhancement. In this work, ridge will be realized with continuously grooved slice with S of depth and of width as shown in Fig. 1(b). The bandwidth of the ridge waveguide can be controlled by a suitable selection of the geometry of the ridge. Based on the formulas presented in [6], the band. width increases significantly by reducing the ratio has been chosen equal to 1.524 mm which represents a relatively thick available substrate allowing therefore a good flexibility in the choice of with an of 0.2 the ridge waveguide covering the bandwidth from 6.25 to 28 GHz. C. Demultiplixing The bandgap effect is achieved by periodically loading the outer coupler by radial transverse slots in its un-ridged waveguide wall, centered above the quasi-uniform field in the ridge section. These slots are loaded by E-plane shunt stubs bent along
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DJERAFI et al.: RIDGE SUBSTRATE INTEGRATED WAVEGUIDE (RSIW) DUAL-BAND HYBRID RING COUPLER
Fig. 1. Proposed coupler structure: (a) 3D view, (b) exploded view, (c) a ridge cross section view and a microstrip to RSIW transition, (d) top view with a unit cell cross section of the loaded ridge waveguide. Ridge widths a are equal to 7.377, 7.087, 2.35 and 3.05 mm with W equal to 1.627, 1.428, 1.547, and 1.947 mm, for ridges 1 to 4 respectively (All dimensions are in mm).
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Fig. 3. Simulated and measured frequency responses of the dual band coupler (a) C-band operation. (b) K-band operation.
main EBG structure exhibits an evident band gap at 22.5 GHz with 30 dB of attenuation. The K band is suppressed and insertion loss of 0.1 dB is achieved in the passband. D. Fabrication
Fig. 2. Magnitudes of the scattering parameters for the ridge waveguide (ridge 2) loaded with the proposed defect of Fig. 1. Results are for 5 unit cells. Insets show the electric field distribution at 7.25 GHz (left side) and 20.5 GHz (right side).
the rings plane in an upper substrate layer. Short-ended stubs are used to avoid radiation and coupling problems. The position of the bandgap is controlled by selecting the period between the slots; the guided wavelength at bandgap center is twice the period [7], [8]. In order not to disturb the propagating mode within are under cutoff for the lower band, the waveguide stubs that band and allow propagation at a frequency that limits the defines the stopband width. lower bound of the bandgap. The parameters are optimized as a tradeoff between depth and bandwidth of the stopband and the ripple in the pass band. The final optimization is done with a full-wave simulator and the final dimensions are reported in Fig. 1. As shown in Fig. 2, the
The two rings couplers are designed and combined with the EBG structure to build the dual-band coupler. The ridges are fabricated by continuous drilling of height and widths while closely-spaced, arbitrary shaped perforations are used to define the lateral walls of the RSIW. All machining is carried out using a CNC milling machine. The structure is implemented on two layers of Rogers 6002 substrate with the bottom layer including both rings and the stacked upper one including the bent adhesive layer with relative permittivity stubs. A 5 is used to stick both layers. Wideband of 3.5 and microstrip-to-RSIW transitions with K-connectors are used for the measurements. Four transitions are realized separately then soldered to the access ridge waveguides as shown in the inset of Fig. 1(a). The optimization of the 75 transition parameters follows a similar procedure as in [9]. Electromagnetic simulations were carried out using Ansoft HFSS simulator. III. RESULTS AND DISCUSSION Based on the previously discussed design considerations, a C/K band coupler is designed. The optimized parameters are given in Fig. 1. The lower and upper frequency bands are chosen to be center at 7.25 and 20.5 GHz, respectively. Fig. 2 illustrates the simulated scattering parameters magnitudes of the loaded
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 22, NO. 2, FEBRUARY 2012
Fig. 5. Photograph of the fabricated coupler: (a) bottom and (b) top views.
at the lower frequency band besides the inherent losses of the bonding that are higher at the higher frequency band. Fig. 5 shows the photographs of the bottom (including the RSIW rings) and top (including the bent stubs) views of the fabricated coupler. IV. CONCLUSION
Fig. 4. Simulated and measured phase differences at the coupled ports for excitations at ports 1 and 2. (a) C-band operation. (b) K-band operation.
ridge waveguide (ridge 2 with five cells) using the previous optimized dimensions. Allowed locations for the lower and upper frequency bands are defined by the passband (return loss 10 dB) and the bandgap, respectively. The electric field magnitudes at both frequency bands in the coupler are depicted in insets in Fig. 2 illustrating a good confinement of the field at each frequency band. The measured results including the effect of the transitions and K-connectors compared with the simulated ones are shown in Figs. 3 and 4. From the simulated results, the magnitude of the coupling coefficient exhibits an imbalance of 0.5 dB with an average value of 3.3 dB over the 7–7.5 GHz frequency range (lower band) with a reflection coefficient lower than 13 dB and isolation higher than 20 dB. In the K-band, 3.3 dB average coupling magnitude with imbalance of 0.7 dB is observed from 19 to 22 GHz (upper band). Both isolation and return losses are higher than 18 dB band. Measured results show, for the lower band, a coupling magnitude of 4.2 dB with isolation and return losses higher than 15 dB and 13 dB, respectively. For the upper band, the measured coupling is 5 dB with return and isolation losses higher than 11 and 18 dB, respectively. As shown in Fig. 4, the dispersion in the phase differences of and the in-phase and out-of-phase cases is less than in the lower and uppers bands, respectively. The four transitions are aligned and soldered to the four ports of the coupler. The higher losses at the upper band are probably related to slight misalignment in the soldering process for which the transmission at the upper band would be more sensitive than
A novel compact dual-band ring coupler has been presented. The coupler has an original structure based on two concentric rings in RSIW double-layer topology with a de-multiplexing scheme. A simple design methodology has been described and a C/K prototype with a 1:2.8 frequency ratio has been experimentally validated. The achievable frequency ratio between the lower and upper frequency bands is such that, the maximum separation is governed by the employed ridge waveguide fundamental mode bandwidth. The minimum separation is compromised by the occupied area of the coupler (over-dimensioning the outer rings provides larger degree of freedom for the inner coupler). Future work will focus on the implementation of dualband systems in multilayer hybrid substrate integrated circuit technology. ACKNOWLEDGMENT The authors would like to thank S. Dubé, Poly-Grames Research Center, for his help in the circuit fabrication and A. A. M. Ali, LAAS-CNRS, Toulouse, France, for help provided. REFERENCES [1] W. Che, K. Deng, K. N. Yung, and K. Wu, “H-plane 3 dB hybrid ring of high isolation in substrate integrated rectangular waveguide (SIRW),” Microw. Opt. Technol. Lett., vol. 48, no. 3, pp. 502–505, Mar. 2006. [2] Y. Ding and K. Wu, “Miniaturized hybrid ring circuits using T-type folded substrate integrated waveguide (TFSIW),” in IEEE MTT-S Int. Dig., Jun. 2009, pp. 705–708. [3] Y. Dong and T. Itoh, “Application of composite right/left-handed halfmode substrate integrated waveguide to the design of a dual-band raterace coupler,” in IEEE MTT-S Inc. Dig., May 2010, pp. 712–715. [4] D. I. Kim, K. Araki, and Y. Naito, “Properties of symmetrical five-port circuit and its broadband design,” IEEE Trans. Microw. Theory Tech., vol. MTT-32, no. 1, pp. 51–57, Jan. 1984. [5] W. Che, C. Li, P. Russer, and Y. L. Chow, “Propagation and band broadening effect of planar integrated ridged waveguide in multilayer dielectric substrates,” in IEEE MTT-S Int. Dig., Atlanta, GA, 2008, pp. 217–220. [6] S. Hopfer, “The design of ridged waveguides,” IRE Trans. Microw. Theory Tech., vol. 3, no. 5, pp. 20–29, Oct. 1955. [7] V. Radisic, Y. Qian, R. Coccioli, and T. Itoh, “Novel 2-D photonic bandgap structures for microstrip lines,” IEEE Microw. Guided Wave Lett., vol. 8, no. 2, pp. 69–71, Feb. 1998. [8] M. Rahman and M. A. Stuchly, “Transmission line periodic circuit representation of planar microwave photonic bandgap structures,” Microw. Opt. Technol. Lett., vol. 30, pp. 15–19, Jul. 2001. [9] Y. Rong, K. A. Zaki, M. Hageman, D. Stevens, and J. Gipprich, “Low-temperature cofired ceramic (LTCC) ridge waveguide bandpass chip filters,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 12, pp. 2317–2324, Dec. 1999.
