A double layer EBG structure for slot-line printed devices 1

N. BOISBOUVIER 1,2,*, A. LOUZIR1, F. LE BOLZER1, A.-C. TAROT2, K. MAHDJOUBI2 Thomson, Corporate Research, 1 avenue Belle-Fontaine, CS17616, 35576 Cesson-Sévigné Cedex, France 2 IETR, UMR CNRS 6164, Université de Rennes1, Campus Beaulieu, 35042 Rennes Cedex, France {[email protected] or [email protected]}

Abstract This article proposes a double layer EBG structure for printed slot-line devices. Compared to the single layer EBG structure proposed in [1], the double layer EBG structure offers better rejection performances. Measured and simulated results are presented. I. Introduction EBG structures have already been proposed for filtering and phase shifting applications in Transmission Lines (TL). Such EBG structures are periodic structures whose unit cell is periodically repeated along the TL leading, on the one hand, to forbidden wave propagation over certain frequency bands and, on the other hand, to a modification of the propagation constant in its pass-band. The stop-band behaviour depends on the size of the unit-cell which should be around λg/2 where λg is the guided wavelength at the centre frequency of the expected stop-band. Like for filters, rejection improvement can be achieved by increasing the number of cells, leading to prohibitive TL length. Then compact EBG structures are highly desirable. In the case of slot-lines, a previous paper [1] has proposed a single layer EBG structure and its application to harmonic control on slot antennas. The single layer EBG structure consists in adding periodically spaced metallic discs on the opposite side of a slot-line. In order to reduce the size of EBG structure for printed slot-line devices, a resonator-like EBG structure for printed slot-line devices has also been proposed in [2]. This article proposes a double layer EBG structure for printed slot-line devices which allows to reduce the length of the EBG structure. II. Presentation of the structure Figure 1 sketches transversal and longitudinal sections of the single layer EBG structure for printed slot-line devices proposed in [1]. This EBG structure, the dual one of the Radisic structure for printed microstrip devices [3], consists in adding periodically spaced metallic discs on the opposite side of the substrate which contains the slot-line. This article proposes to reinforce the stop-band effect introduced by the single layer EBG structure. Unlike microstrip lines, field components in slot-lines are not confined to the substrate but extend in both sides of the substrate, as shown in Figure 2 [4]. The double layer EBG structure proposed in this article take advantage of the slot-line structure by adding periodically spaced metallic discs on a second layer leading to the structure shown in Figure 3. III. Simulation results The structure shown in Figure 3 has been simulated and compared to the single layer EBG structure. Simulations have been done with HFSS (ANSOFT). A 0.24mm wide slot-line is fed at both extremities by a Lumped Gap Source charged on the impedance of the slot-line alone. The substrate used is a 0.81mm thick dielectric substrate with a dielectric permittivity εr=3.38.

Figure 4 presents the simulated S-parameters of this slot-line without EBG structure compared to S-parameters of the same slot-line with a single layer EBG structure. This single layer EBG structure is made of 9 metallic discs with radius r=1.5mm and duplicated under the slot-line with a period a=18.4mm. As expected, a stop-band centred at 5.8GHz is obtained. Figure 5 shows the improvement obtained when a second layer is added on the slotline. A deeper rejection is achieved with the double layer EBG structure at the same central stop-band frequency. For this simulation, the second substrate parameters and the discs period are kept the same to that of the single layer EBG structure. Inversely, as shown in Figure 6, the same band-stop rejection can be achieved with only a 6-discs double layer EBG structure, leading to a more compact structure. IV. Experimental results In order to test the above structure, it is necessary to have a transition between the slot-line and measuring equipments. To that end, the slot-line is fed by electromagnetic coupling to microstrip lines following Knorr sizing rules [5]. Such structures have been simulated with I3eD (Zeland). For a slot-line without any EBG structures, good transmission performances are obtained over a bandwidth from 3 to 8 GHz, as shown in Figure 7. Then, as shown in Figure 8, a slot-line with a double layer EBG structure made of 5discs has been realized (a=18.4mm). Simulated and measured transmission coefficients of this slot-line with single and double layer EBG structures are respectively presented in Figure 9a and 9b. An improved rejection in the stop-band could be noticed. The measured rejection level improvement is closer to the simulated one obtained with HFSS (Figure5). The stopband frequency shift between single and dual layer EBG structure could be due to the reduction of the guided wavelength in the slot-line when the second layer is added. V. Conclusion & further work A double layer EBG structure for printed slot-line devices is proposed. This structure is more compact than the single layer EBG structure previously proposed. Simulated results fairly agreed with measurements. A new realization targeting the same stopband for single and double layer EBG structures, centred in the microstrip line to slotline double transitions frequency band is being designed. Obtained results will be presented at the conference. References [1] “Harmonic-less Annular Slot Antenna (ASA) using a novel PBG structure for slotline printed devices”, Boisbouvier N., Le Bolzer F., Louzir A., Tarot A.-C., Mahdjoubi K., IEEE AP-S 2003

[2] “Compact EBG structure for slot-line printed devices”, Boisbouvier N., Louzir A., Le Bolzer F., Tarot A.-C., Mahdjoubi K., 27th ESA Antenna Technology Workshop on Innovative Periodic Antennas, march 2004

[3] “Novel 2-D photonic bandgap structure for microstrip lines“, Radisic V., Qian Y., Coccioli R. & Itoh T., IEEE Microwave and Guided Wave Letters,vol.8, Feb 1998

[4] “Microstrip Lines and slotlines”, Gupta K.C., Garg R., Bahl I.J. Artech House, ISBN : 0-89006-074-6

[5] “Slot Line Transitions”, Knorr J.B., IEEE Transactions on Microwave Theory and Techniques, may 1974.

Figure 1 : single layer EBG structure for slot-line printed devices [1]

Figure 2 : Field distributions in slot-lines

Figure 3 : double layer EBG structure for slot-line printed devices

Figure 4 : Simulated S-parameters of a slot-line compared to the same slot-line with a 9-discs single layer EBG structure.

Figure 5 : Simulated S-parameters of a slot-line with a 9-discs single layer EBG structure compared to the same slot-line with a 9-discs double layer EBG structure.

Figure 6 : Simulated S-parameters of a slot-line with a 9-discs single layer EBG structure compared to the same slot-line with a 6-discs double layer EBG structure. 0 -5 S2 1

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Figure 7 : Simulated and measured S parameters of a slot-line fed by EM coupling to Microstrip lines

Figure 8 : Realized breadboard of a 5-discs double layer EBG structure (upper and bottom layers)

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b) Figure 9 : a) Simulated and b) measured transmission coefficients of a slot-line fed by EM coupling to microstrip lines with a 5-discs single and double layer EBG structures