Compact EBG structure for slot-line printed devices N. BOISBOUVIER 1,2, A. LOUZIR1, F. LE BOLZER1, A.-C. TAROT2, K. MAHDJOUBI2 1
Thomson, Corporate Research, 1 avenue Belle-Fontaine, CS 17616, 35576 Cesson-Sévigné Cedex, France
[email protected] 2
IETR, UMR CNRS 6164, Université de Rennes1, Campus Beaulieu, 35042 Rennes Cedex, France
[email protected]
Abstract This article proposes a resonator-like EBG structure for printed slot-line devices which is more compact than Bragg-like EBG structures for printed slot-line devices. Validation of this EBG structure is presented by simulation and measure. 1. Introduction Among contributions on EBG structures applied to Transmission Lines (TL), one can make the distinction between Bragg-like EBG structures and resonator-like EBG structures [1]. A periodicity is involved in both structures but not in the same way Bragg-like EBG structures are periodic structures whose unit cell is periodically repeated along the slot-line. Such EBG structures have already been proposed in [2] with the uniplanar 1D EBG band-stop filter and in [3] with the conventional EBG structure. Analysis tools (Transmission Line Formulation [4], …) can be used in this context in order to predict their pass- and stopbands. Such an analysis leads to the appearance of a stop-band for a periodicity around λg/2 (λg is the guided wavelength in the TL). The major drawback of such EBG structures is their size. Several structures have then been proposed in order to reduce the size of EBG structures. One of this solution consists in strengthen the band-stop effect by adding metallic patterns under and above the slot-line [5]. Another solution is given by resonator-like EBG structures. Resonator-like EBG structures are periodic structures which are transverse to the TL with a periodicity which is much smaller than the wavelength. Such a structure has been proposed for microstrip lines as an anisotropic Uniplanar Compact PBG structure ([1], [6]). This structure can be seen as a transverse resonator and the first stop-band appears for a resonator length which is about λg/2 (λg is the guided wavelength in the transverse resonator). This paper proposes such a resonator-like EBG structure for printed slot-line devices. This structure, which is the dual one of the compact anisotropic UCPBG structure [1], is obtained by etching metallic line sections on the opposite side of the slot-line, as seen in Figure 1. Simulation results have been validated by measurement. This paper also proposes a performance comparison between Bragg-like and resonator-like EBG structures.
Figure 1 : EBG structures for slot-line printed devices
2. Compact EBG structure for printed slot-line devices 2.1. Structure design This compact EBG structure for printed slot-line devices is inspired by the anisotropic UC-PBG structure proposed in [1] for microstrip devices. As the slot-line is the dual structure of the microstrip line, the idea is to propose the dual structure of the anisotropic UC-PBG structure : instead of slots etched in the ground plane of a microstrip line, metallic patterns are etched on the opposite side of a slot-line. Figure 2 sketches the compact EBG structure and the way to design it. The unit cell is much smaller than the wavelength and is made of two connected metallic line sections with different width (Figure 2a). The unit cell is repeated Nx times along the X-axis and Ny times along the Y axis. A compact EBG structure made of 13x4 unit cells is plotted in Figure 2b, with the following dimensions px = py = p = 1.52 mm, W1 = 1.22 mm, W2 = L1 = L2 = 0.76 mm. This structure is then added on the opposite side of a slot-line in such a way that continuous line sections should be transverse to the slot-line, as plotted in Figure 2c.
Figure 2 : Compact EBG structure for printed slot-line devices
In such a configuration, the structure exhibits a stop-band behaviour at a frequency which is directly linked to the Lx dimension. This stop-band behaviour is then no more controlled by the period p of the periodic structure but by the transverse Lx dimension of the structure. The periodicity allows a reduction of the effective electrical length of the transverse resonator.
0 -5 -10
S parameters (dB)
2.2. Theoretical results In order to validate this stop-band behaviour, a slot-line with the previously described compact EBG structure has been simulated on a 0.81mm thick Rogers4003 dielectric substrate (εr=3.38) : a 0.24-mm wide slot-line is fed at both extremities by a Lumped Gap Source charged on the impedance of the slot-line alone. The Lx dimension of the compact EBG structure (Lx=19.76mm) leads to a stop-band which begins at 4.6GHz, as shown in Figure 3. This Lx dimension corresponds to half of the guided wavelength on the transverse resonator at this frequency.
-15 -20 -25 -30 -35 -40
S21_compact EBG structure
-45
S11_compact EBG structure
-50 3,5
4
4,5
5
5,5
frequency (Ghz)
Figure 3 : Simulated S-parameters of a slot-line with a compact EBG structure
6
2.3. Measurement results In order to validate by measure the stop-band behaviour of this compact EBG structure, the slotline has to be fed by EM coupling to microstrip lines following Knorr sizing rules [7]. Transmission and reflection coefficients of a single slot-line (Figure 4a) have been compared to S-parameters of the same slot-line in the presence of the compact EBG structure (Figure 4b).
Figure 4 : Slot-line fed by two mistrosrip lines (a) without and (b) with the PBG structure.
0
0
-5
-5
-10
-10
-15
-15
S parameters (dB)
S parameters (dB)
On a 0.81mm thick Rogers4003 dielectric substrate (εr=3.38), the previous 13x4 structure (Lx=19.76mm & Ly=6.08mm) has been simulated with the EM simulator Ie3D (Zeland) and compared to measurement. Figure 5 shows simulated and measured S-parameters of the slot-line with and without the compact EBG structure.
-20 -25 -30 -35
-25 -30 -35
-40
-40
S11, measurement S21, measurement
-45
S11, simulation S21, simulation
S11, measurement S21, measurement
-45
-50
S11, simulation S21, simulation
-50 1
a)
-20
2
3
4
5
6
7
frequency (Ghz)
8
9
10
11
12
1
b)
2
3
4
5
6
7
8
9
10
11
frequency (Ghz)
Figure 5 : S parameters of a slot-line fed by two mistrosrip lines (a) without and (b) with the compact EBG structure.
For a slot-line without any EBG structures, the transmission is allowed over a large bandwidth approximately from 3 to 8 GHz. As expected, when a compact EBG structure is etched under the same slot-line configuration, a stop-band which begins at 4.6 GHz is created with sharp cut-off.
12
2.4. Parametric study Several simulations have been done in order to confirm the behaviour of this compact EBG structure. To that end, the two previous simulations plotted in Figure 5 will be used as references. A third simulation depicts the behaviour of a slot-line with a compact EBG structure with a smaller Lx dimension (Lx’Ly) and with a constant Lx dimension.
Figure 6 : variation of one parameter by keeping constant the other ones
Figure 6 sketches the transmission coefficient of those 4 compact EBG structures. Those simulations show that by keeping constant the Ly dimension, variations of the Lx dimension lead to move the centre frequency of the stop-band. Actually, the length of the transverse resonator is larger leading to a lower frequency stop-band. Those simulations also shows that by keeping constant the Lx dimension, variations of the Ly dimension lead to a deeper rejection and larger bandwidth of the stop-band at the same frequency. Then the sizing rules are the Lx transverse dimension controls the frequency of the stop-band. the Ly dimension controls the rejection and bandwidth of the stop-band. 3. Performances Comparison The compactness of the resonator-like EBG structure for printed slot-line devices proposed in this article is demonstrated. In order to compare their dimensions, we have sized two conventional Bragg-like EBG structures which exhibit similar stop-band characteristics than the previous resonator-like EBG structure. These Bragg-like EBG structures are the uniplanar 1-D PBG band-stop filter proposed in [2] and the 1-D EBG structure proposed in [3]. Both structures are plotted in Figure 7.
Figure 7 : Bragg-like and resonator-like EBG structure
For all three EBG structures, the stop-band is centred at 4.7GHz on a 0.81mm thick Rogers4003 dielectric substrate (εr=3.38). Their respective simulated transmission coefficients, plotted in Figure 8 show similar stop-band rejection, bandwidth and centre frequency with the following dimensions: For the uniplanar 1D EBG band-stop filter proposed in this article, the sizing is a=23.8 mm, W1=0.51mm, W2=3.51mm and n=8. For the EBG structure proposed in [2], the sizing is : a=23mm, r=3mm and n=9. Finally, for the compact EBG structure proposed in this article, the sizing is Ly=19.76mm , Ly=6.08mm and p=1.52mm. Then, the slot-line length needed to our proposed EBG structure is 9 times much smaller than the length needed for the two other classical Bragg-like EBG structures. 0
-5
-10
Transmission S21 (dB)
-15
[1] _ simulation [2] _ simulation
-20
[3] _ simulation
-25
-30
-35
-40
-45
-50 3,5
4
4,5
5 Frequency (GHz)
5,5
6
6,5
Figure 8 : structure EBG pour fentes
4. Conclusion In this article, a resonator-like EBG structure for printed slot-line devices and its sizing rules have been presented. Its stop-band behaviour has been validated by measure & simulation. Such an EBG structure is compared to conventional Bragg-like EBG structures for slot-line printed devices previously proposed. Finally, a performance comparison has been done : EBG structure sizes have been compared for similar band-stop characteristics.
5. References [1] “A Novel Anisotropic Uniplanar Compact Photonic Band-Gap (UC-PBG) Ground Plane”, ! " # # [2] “One-dimensional Photonic Bandgap Resonators and varactor tuned resonators”, $ $ % & ' ' ' [3] “Harmonic-less Annular Slot Antenna (ASA) using a novel PBG structure for slot-line printed devices”, ( ) * ! ( + ! , , " - ) % , . & # # [4] “An eigenvalue method to predict the parameters of photonic band-gap structure”, / $ 0 1 % / , . # # [5] “A double layer EBG structure for slot-line printed devices” ( ) * ! , ! ( + , " - ) % ) 2 " , . & # # [6] “A super-compact super-broadband tapered uniplanar PBG structure for microwave and millimeter-wave applications”, & # # [7] “Slot Line Transitions”, % 3 ( 4 " 5 2 4 ' 6
