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Novel Reconfigurable Left-handed Unit Cell for FilterApplications

Branka Jokanovic1 and Vesna Crnojevic-Bengin2

1Institute IMTEL, Belgrade, Serbia2Faculty 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 microstripline. Single unit cell is studied together with its modifications that could be realized with simplediode arrangements. By changing bias voltage, different resonant frequencies, insertion lossesand spurious responses can be obtained. Simulated characteristics of ForeS exhibit a smallerattenuation, better stop-band characteristic and considerable size reduction in comparison tozeroth-order resonator (ZOR).

DOI: 10.2529/PIERS061016120829

1. INTRODUCTION

Recently, revolutionary results were obtained in the field of metamaterials, artificial structuresthat exhibit electromagnetic properties generally not found in nature. Metamaterials are designedusing sub-wavelength particles, whose size is typically smaller then one tenth of the propagatingsignal wavelength. Due to this fact, quasi-static analysis can be performed and concept of arti-ficial effective media applied. Structures obtained by using this approach can be considered as acontinuous medium with effective parameters, namely effective dielectric permittivity and effectivemagnetic permeability. By properly choosing type and geometrical arrangement of constituentsub-wavelength particles, effective parameters of metamaterials can be made arbitrarily small orlarge, or even negative.

In the realization of metamaterials two main approaches are widely accepted in microwavecommunity: resonant-type left-handed metamaterials based on thin-wire structure and split-ringresonators (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 broadbandwidth structures.

Following the TL approach, a zeroth-order resonator (ZOR) [5] was proposed whose resonantfrequency depends only on the circuit elements of the unit cell and not on the physical length ofthe 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 end-coupled with microstrip line. Overall dimension of this novel particle is λg/13 by λg/13, where λg

is the wavelength in microstrip line. Generation of ForeS is shown in Fig. 1. This structure is also

L

LC

WC

gEC WEC

(a) (b)

WS gS

LS

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

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is possible to combine with diodes providing the structure reconfigurability with proper voltagebiasing.

2. SIMULATION RESULTS

In order to compare characteristics of the proposed structure with ZOR, ForeS is simulated onRogers 5880 substrate εr = 2.17, h = 1.575mm and tgδ = 0.0009. Performances of the structuresare determined using IE3D Power Pack Version 10.0 with conductor losses included through σ =58MS/m. Dimensions of the cell are optimized to provide an equal resonant frequency as ZOR.

20

Figure 2: Simulated s11 and s21 for three different ForSs occupying the same area.

Firstly, we analyzed the ForeS (Fig. 1(b)) having the following dimensions: Lc = 9.0mm, Wc =9.0mm, spiral line width Ws = 0.1mm, end-coupled gap gEC = 0.1mm, spiral gap gS = 0.1mmand via diameter d = 0.3mm. Keeping unit cell length (LC) and width (WC) constant as well asend-coupled gap (gEC), width of spiral lines (WS) and the gap (gS) are varied to investigate theflexibility 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

LC [mm] 9.0 9.0 9.0WC [mm] 9.0 9.0 9.0WS [mm] 0.4 0.5 0.6gS [mm] 0.1 0.1 0.2

gEC [mm] 0.1 0.1 0.1LS [mm] 36.7 30.7 22.5fR1 [GHz] 1.876 2.35 3.175s121 [dB] −1.86 −1.8 −2.05

s111 [dB] −14.17 −14.5 −13.79

B [MHz] 95.5 147.67 290QL 19.64 15.91 10.95Q0 33.26 27.32 17.85

fR2 [GHz] 7.75 7/8.15 7.9S2

21 [dB] −1.12 −2/−1.43 −1.3

By introducing small changes in dimensions of ForeS, significantly different resonant frequenciescan be obtained in range of 1.87GHz to 3.18GHz that is 59% in respect to the highest frequency. Inthe 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 (WS = 0.4mm).

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Generally, due to very small gap between cell and input microstrip line (gEC), simulated ForeSsexhibit low insertion loss at the resonance in range−1.8 dB to−2.05 dB and also very good reflectionof about −14.5 dB to −13.8 dB.

Simulated results for three different ForeSs in a wide frequency range are presented in Fig. 3. Itis shown that variation of spiral width and gap only influences the first resonance, while the secondone 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 greenpatches are considered as metal patches. Electronic reconfigurabilty of the ForeS is possible by

(a)

(b)

(d)

vias

voltage

voltage voltage

voltage

(c)

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 applyingvery simple biasing as indicated in Fig. 4. Both input ports are DC decoupled, while current returnpath is provided through the vias that simplifies the biasing circuit.

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Different types of diodes could be used to reconfigure the ForeS: PIN diodes, varactor or Schot-tky barrier diodes. Orientation of each diode in a pair should be an opposite: left and right inrespect to the central biasing line so, beam lead anti-parallel diode pairs or common cathode diodearrangement could be used. White patches represent the gaps which serve to isolate a centralbiasing line from the ground in Figs. 3. (b) and (c). By switching on/off bias voltage, differentresonant frequencies, insertion losses and spurious responses can be obtained.

Four modifications of Fores are simulated using metal patches instead of diodes and results aresummarized 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 (9.0× 9.0mm).

ForeS typeEnd-Opened

Cross

End-Connected

Cross

Center-Connected

Cross

Center-Disconnected

Cross

fR1 [GHz] 1.53 1.876 2.0 1.59

s121 [dB] −1.726 −1.86 −1.87 −1.58

s111 [dB] −14.85 −14.17 −14.3 −15.75

B [MHz] 58.5 95.5 108.3 69

QL 26.15 19.64 18.47 20.12

Q0 45.67 33.26 31.2 42.25

fR2 [GHz] 3.72 7.75 6.0 6

S221 [dB] −1.428 −1.12 1.22 −1.17

fR3 [GHz] 6.0 - 6.8 6.775

s321 [dB] −1.652 - −1.02 −1.0

It is seen that structure called “End-Opened Cross” offers the greatest decrease in resonantfrequency, but disadvantage is that the second resonance appears at 3.7 GHz. The structure called“Center-Disconnected Cross” demonstrated the best overall characteristics as: low resonant fre-quency, low insertion loss at the resonance, small reflection (−15.75 dB), a good Q-factor, while thesecond harmonic appears at 6.0 GHz. The best out of band spurious response is observed in ForeScalled “End-Connected Cross” (basic ForeS) because the second harmonic appears at 7.75GHz.It is noticed that simulated structures have different resonant frequencies in range 1.53 GHz to2.0GHz, which covers almost 24% of the tuning range.

Table 3: Comparison of end-coupled resonators: two types of ForeS and ZOR.

ResonatorEnd-Connected

Cross

Center-Disconnected

CrossZOR

LC [mm] 9.0 8.4 24.4gEC [mm] 0.5 0.6 0.2

WC/Lstub [mm] 9.0 8.4 9.9gS [mm] 0.1 0.1 -WS [mm] 0.4 0.4 -fR [GHz] 1.986 1.985 2.013s121 [dB] −4.75 −4.93 −7.3

B [MHz] 45 35.75 21QL 44 55.5 95.85Q0 54 67.2 106.26

4. COMPARISON WITH ZOR

Two structures are chosen for further optimization: End-Connected Cross and Center-DisconnectedCross. During optimization, end-coupled gap, cell length and width are changed to match charac-teristics 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 ob-served in resonator length (LC). Considerably lower attenuation at resonance is noticed (−4.75 dB

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and −4.93 dB in respect to −7.3 dB at ZOR). Also, ZOR s21-characteristic has spurious responsesat 3.695 GHz (−9.7 dB), and 4.325GH (−6.0 dB). The second resonance of End-Connected Crossappears at 8.0 GHz, which means spurious free region up to the 4th harmonic.

ZOR

S21

Center-Disconnected.Cross

End-Connected.Cross

S11

Figure 5: Simulated s11 and s21 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 thecell and microstrip line, resonant frequency can be changed from 1.87 GHZ to 3.18GHz only bychanging the spiral width and gap. Insertion loss at resonance remains almost unchanged (−1.8 dBto −2.0 dB), while 3-dB-bandwidth changes from 95MHz to 290MHz. Four different versions ofForeS are investigated to find the best ones to compare with zeroth-order resonator. Two chosenmodifications of ForeS demonstrated considerable low insertion loss, better out-of-band suppressionof 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 andsimple biasing which gives additional freedom in changing resonant frequency around 24% (from1.53GHz to 2.0 GHz), Q-factor and out-of-band response.

ACKNOWLEDGMENT

This work is supported by Eureka program (METATEC-METAmaterialCbased TEchnology forbroadband wireless Communications and RF identification E! 3853 project).

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