Novel Reconfigurable Left-handed Unit Cell for Filter Applications

Transcription

1 PIERS ONLINE, VOL. 3, NO. 3, Novel Reconfigurable Left-handed Unit Cell for Filter Applications Branka Jokanovic 1 and Vesna Crnojevic-Bengin 2 1 Institute IMTEL, Belgrade, Serbia 2 Faculty of Technical Sciences, Novi Sad, Serbia Abstract In this paper a novel, super compact, left-handed unit cell, called ForeS is proposed. ForeS consists of Four Spirals, which are both mutually coupled and end-coupled with microstrip line. Single unit cell is studied together with its modifications that could be realized with simple diode arrangements. By changing bias, different resonant frequencies, insertion losses and spurious responses can be obtained. Simulated characteristics of ForeS exhibit a smaller attenuation, better stop-band characteristic and considerable size reduction in comparison to zeroth-order resonator (ZOR). DOI: /PIERS INTRODUCTION Recently, revolutionary results were obtained in the field of metamaterials, artificial structures that exhibit electromagnetic properties generally not found in nature. Metamaterials are designed using sub-wavelength particles, whose size is typically smaller then one tenth of the propagating signal wavelength. Due to this fact, quasi-static analysis can be performed and concept of artificial effective media applied. Structures obtained by using this approach can be considered as a continuous medium with effective parameters, namely effective dielectric permittivity and effective magnetic permeability. By properly choosing type and geometrical arrangement of constituent sub-wavelength particles, effective parameters of metamaterials can be made arbitrarily small or large, or even negative. In the realization of metamaterials two main approaches are widely accepted in microwave community: resonant-type left-handed metamaterials based on thin-wire structure and split-ring resonators (SRRs) [1, 2] which are lossy and narrow-banded and non-resonant transmission line (TL) approach [3, 4] that provides useful tool for design of simultaneously low loss and broad bandwidth structures. Following the TL approach, a zeroth-order resonator (ZOR) [5] was proposed whose resonant frequency depends only on the circuit elements of the unit cell and not on the physical length of the resonator. This suggests that a zeroth-order resonator could be made arbitrarily small. In this paper we proposed a LH unit cell (ForeS) that consists of four grounded spirals endcoupled with microstrip line. Overall dimension of this novel particle is λ g /13 by λ g /13, where λ g is the wavelength in microstrip line. Generation of ForeS is shown in Fig. 1. This structure is also L g EC W EC W S g S W C L S L C (a) (b) Figure 1: (a) First step in ForeS generation. (b) ForeS view. very suitable as a constituent element for metasurfaces. Due to its unique construction, the ForeS

2 PIERS ONLINE, VOL. 3, NO. 3, is possible to combine with diodes providing the structure reconfigurability with proper biasing. 2. SIMULATION RESULTS In order to compare characteristics of the proposed structure with ZOR, ForeS is simulated on Rogers 5880 substrate ε r = 2.17, h = mm and tgδ = Performances of the structures are determined using IE3D Power Pack Version 10.0 with conductor losses included through σ = 58 MS/m. Dimensions of the cell are optimized to provide an equal resonant frequency as ZOR. 20 Figure 2: Simulated s 11 and s 21 for three different ForSs occupying the same area. Firstly, we analyzed the ForeS (Fig. 1(b)) having the following dimensions: L c = 9.0 mm, W c = 9.0 mm, spiral line width W s = 0.1 mm, end-coupled gap g EC = 0.1 mm, spiral gap g S = 0.1 mm and via diameter d = 0.3 mm. Keeping unit cell length (L C ) and width (W C ) constant as well as end-coupled gap (g EC ), width of spiral lines (W S ) and the gap (g S ) are varied to investigate the flexibility of the design. Results of simulations are shown in Table 1. and in Fig. 2. Table 1: Simulated results of the three different ForeSs occupying the same area. Resonator ForeS 1 ForeS 2 ForeS 3 L C [mm] W C [mm] W S [mm] g S [mm] g EC [mm] L S [mm] f R1 [GHz] s 1 21 [db] s 1 11 [db] B [MHz] Q L Q f R2 [GHz] / S21 2 [db] / By introducing small changes in dimensions of ForeS, significantly different resonant frequencies can be obtained in range of 1.87 GHz to 3.18 GHz that is 59% in respect to the highest frequency. In the same time, all configurations exhibit very small insertion loss, compared to other LH structures. It is seen that ForeS 1 has the best performances: the lowest resonant frequency and the highest Q- factor. Its insertion loss is somewhat higher ( 1.86 db), due to smaller spiral width (W S = 0.4 mm).

3 PIERS ONLINE, VOL. 3, NO. 3, Generally, due to very small gap between cell and input microstrip line (g EC ), simulated ForeSs exhibit low insertion loss at the resonance in range 1.8 db to 2.05 db and also very good reflection of about 14.5 db to 13.8 db. Simulated results for three different ForeSs in a wide frequency range are presented in Fig. 3. It is shown that variation of spiral width and gap only influences the first resonance, while the second one remains almost unchanged, around 8.0 GHz. Figure 3: Simulated results for three different ForeSs in wide frequency range. 3. RECONFIGURABILITY OF THE FORES Figure 4 shows some possible modifications of basic ForeS (Fig. 1). In the simulation, light green patches are considered as metal patches. Electronic reconfigurabilty of the ForeS is possible by vias (a) (b) (c) (d) Figure 4: Different modifications of ForeS realized with proper polarization of the diodes. diodes placed instead of light green patches. Various forms of ForeS could be generated applying very simple biasing as indicated in Fig. 4. Both input ports are DC decoupled, while current return path is provided through the vias that simplifies the biasing circuit.

4 PIERS ONLINE, VOL. 3, NO. 3, Different types of diodes could be used to reconfigure the ForeS: PIN diodes, varactor or Schottky barrier diodes. Orientation of each diode in a pair should be an opposite: left and right in respect to the central biasing line so, beam lead anti-parallel diode pairs or common cathode diode arrangement could be used. White patches represent the gaps which serve to isolate a central biasing line from the ground in Figs. 3. (b) and (c). By switching on/off bias, different resonant frequencies, insertion losses and spurious responses can be obtained. Four modifications of Fores are simulated using metal patches instead of diodes and results are summarized in Table 2. Physical dimensions are kept the same as in Fores 1 (Table 1). Table 2: Summarized results of modified ForeSs occupying the same area ( mm). ForeS type End-Opened End-Connected Center-Connected Center-Disconnected f R1 [GHz] s 1 21 [db] s 1 11 [db] B [MHz] Q L Q f R2 [GHz] S21 2 [db] f R3 [GHz] s 3 21 [db] It is seen that structure called End-Opened offers the greatest decrease in resonant frequency, but disadvantage is that the second resonance appears at 3.7 GHz. The structure called Center-Disconnected demonstrated the best overall characteristics as: low resonant frequency, low insertion loss at the resonance, small reflection ( db), a good Q-factor, while the second harmonic appears at 6.0 GHz. The best out of band spurious response is observed in ForeS called End-Connected (basic ForeS) because the second harmonic appears at 7.75 GHz. It is noticed that simulated structures have different resonant frequencies in range 1.53 GHz to 2.0 GHz, which covers almost 24% of the tuning range. Table 3: Comparison of end-coupled resonators: two types of ForeS and ZOR. Resonator End-Connected Center-Disconnected ZOR L C [mm] g EC [mm] W C /L stub [mm] g S [mm] W S [mm] f R [GHz] s 1 21 [db] B [MHz] Q L Q COMPARISON WITH ZOR Two structures are chosen for further optimization: End-Connected and Center-Disconnected. During optimization, end-coupled gap, cell length and width are changed to match characteristics of ZOR as close as possible. The results are shown in Table 3. In comparison to ZOR, both optimized ForeSs demonstrate the considerable size reduction, especially great reduction is observed in resonator length (L C ). Considerably lower attenuation at resonance is noticed ( 4.75 db

5 PIERS ONLINE, VOL. 3, NO. 3, and 4.93 db in respect to 7.3 db at ZOR). Also, ZOR s 21 -characteristic has spurious responses at GHz ( 9.7 db), and GH ( 6.0 db). The second resonance of End-Connected appears at 8.0 GHz, which means spurious free region up to the 4th harmonic. ZOR S11 S21 Center-Disconnected. End-Connected. Figure 5: Simulated s 11 and s 21 for two types of ForeSs and for ZOR. 5. CONCLUSION In this paper a novel super compact LH resonator (ForeS) with small insertion loss is proposed. It consists of four spirals, which are both mutually coupled and end-coupled with microstrip line. It is shown that keeping overall dimensions of unit cell constant as well as the gap between the cell and microstrip line, resonant frequency can be changed from 1.87 GHZ to 3.18 GHz only by changing the spiral width and gap. Insertion loss at resonance remains almost unchanged ( 1.8 db to 2.0 db), while 3-dB-bandwidth changes from 95 MHz to 290 MHz. Four different versions of ForeS are investigated to find the best ones to compare with zeroth-order resonator. Two chosen modifications of ForeS demonstrated considerable low insertion loss, better out-of-band suppression of unwanted harmonics and tremendous size reduction in comparison to ZOR. It is also shown that the ForeS can be modified electronically by simple diode arrangement and simple biasing which gives additional freedom in changing resonant frequency around 24% (from 1.53 GHz to 2.0 GHz), Q-factor and out-of-band response. ACKNOWLEDGMENT This work is supported by Eureka program (METATEC-METAmaterialCbased TEchnology for broadband wireless Communications and RF identification E! 3853 project). REFERENCES 1. Smith, D. R., W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, Composite medium with simultaneously negative permeability and permittivity, Phys. Rev. Lett., Vol. 84, No. 18, , May Marqués, R., J. Martel, F. Mesa, and F. Medina, Left handed media simulation and transmission of EM waves in subwavelength SRR-loaded metallic waveguides, Phys. Rev. Lett., Vol. 89, , Eleftheriades, G. V., O. Siddiqui, and A. K. Iyer, Transmission line models for negative refractive index media and associated implementation without excess resonators, IEEE Microwave Wireless Compon. Lett., Vol. 13, 51 53, Feb Lai, A., C. Caloz, and T. Itoh, Composite right/left-handed transmission line metamaterials, IEEE Microwave Magazine, Vol. 5, No. 3, 34 50, September Sanada, A., C. Caloz, and T. Itoh, Zeroth order resonance in composite righ/left-handed transmission line resonators, Proc. Asia-Pacific Microwave Conference, Seoul, Korea, Vol. 3, , 2003.