Design and Realization of a Novel Compact Electromagnetic Band-Gap Structure

Li-Ye Fang, Jian-Hua Zhang, and Feng-Ge Hu

1.Introduction

The electromagnetic band-gap (EBG) structure is a kind of artificial material composed of periodical metallic or dielectric cells[1], which has the particular characteristic of bandgap in suppressing the propagation of surface-wave and in-phase reflection coefficient[2].This unique feature makes it an excellent candidate in microwave circuit and antenna community, such as increasing the patch antenna gain and reducing the back radiation, suppressing the mutual coupling between antenna elements, and eliminating scan blindness in phased array antenna system[3]–[6].However, due to the constraint of its periodical structure and electric property, the EBG structure has the disadvantage of relatively large size, which restrains practical applications in integration with compact patch antenna array.Therefore, how to minimize the unit size of the EBG structure has become a hot issue in the research area of EBG.

Many researchers make steps towards the miniaturization of EBG structure and the work of structure miniaturization has achieved practically by modifying the shape of the patch in recent years.For example, the interdigital structure[7]contributes to the miniaturization of EBG structure.The configuration of fork-like EBG structure has an extremely compact size, and its area is less than 40% of the conventional mushroom-like EBG(CML-EBG) structure[8].The period of the CSRR(complementary split ring resonator)-based EBG structure demonstrated in [9]presents a 28% size reduction.The period of the EBG lattice demonstrated in [10]is 3.86%and 7.3% of the two free space wavelengths, respectively.In addition, other methods like using edge-located vias are also effective[11],[12].

In this paper, a novel EBG structure with both of the RSRs (reverse split rings) and IE (inserting interleaving edge) configurations has been proposed, achieving a 13.6%size reduction in the center frequency of the bandgap,compared with the CML-EBG structure.The RSRs and IE were integrated, respectively, into the CML-EBG structure to investigate their contribution to compactness.A 5×5 sample was constructed and measured.The measured data show a proper agreement with the simulated results.

2.Structure Analysis and Design

The CML-EBG consisting of metal patch cells and vias is shown in Fig.1 (a).The stopband property of this structure can be described as an equivalent parallel LC resonator.The equivalent capacitance C is introduced by the gap electric field between the edges of adjacent cells,and the equivalent inductance L is engendered by the current flowing from upper patches to ground plane through vias.Therefore, the bandgap frequency and bandwidth (BW)can be calculated as

respectively, where the equivalent capacitance C is a gap structure, defined asthe periodic equivalent inductance L can be written as L=μ0h, w is the patch size, g is the distance between cells,εris the dielectric permittivity, h is the thickness of the substrate, d is the diameter of the via, and a is the size of the period.Thus, the bandgap frequency can be decreased by increasing the equivalent capacitance and inductance.

Fig.1.Geometry of the EBG structure: (a) CML EBG and (b)proposed RSRs-IE based EBG.

For the purpose of reducing the bandgap frequency without increasing the patch cell size, the RSRs and IE configuration are introduced in the patch of the CML-EBG to construct the RSRs-IE structure based EBG, as shown in Fig.1 (b).Comparing with the CML-EBG structure, the interleaving edge in the rectangle patch can be considered as interdigital capacitance, and the reverse split rings etched on the EBG structure are equivalent to additional inductance.Because of the extension of the current flowing path, which contributes to the increasing of equivalent inductance and capacitance, the RSRs-IE structure based EBG gains the feature of compactness.

3.Results and Discussions

The CML-EBG structure configuration has the chosen parameters as following: relative permittivity εr=2.65,substrate thickness h=1 mm, patch size w=6.2 mm, and distance between cells g=0.3 mm.In order to compare the RSRs-IE EBG structure with the CML-EBG structure, the width of the cell is w′=5.0mm, the length and the width of the IE cell are b=0.6 mm and c=0.4 mm, the parameters of the RSRs are w0=0.2 mm, w1=w2=0.2 mm, and r0=1.6 mm, other parameters are the same.The dispersion diagram method[13]is used to analyze the bandgap feature of the EBG structure.All the simulated results are calculated by Ansoft HFSS v10.

The simulated dispersion diagram of the CML-EBG and RSRs-IE based EBG are shown in Fig.2 (a) and Fig.2(b), respectively.Only two modes are plotted to cut down the calculation time.As is shown, a complete bandgap between 6.5 GHz and 11.4 GHz is clearly observed for the CML-EBG, with a bandwidth of 27.4%, at the center frequency of 8.95 GHz.As to the RSRs-IE based EBG, the bandgap is from 3.9 GHz to 9.3 GHz, with a bandwidth of 41% at the center frequency of 6.6 GHz.Compared with the CML-EBG, the bandwidth of the RSRs-IE based EBG structure increases, however, the center bandgap occurs at a much lower frequency, thus, the RSRs-IE based EBG obtains compactness in size accompany with a broad bandwidth.

Fig.2.Dispersion diagram for the EBG structure: (a) CML EBG and (b) proposed RSRs-IE based EBG.

Table 1: Bandgap feature of different IE lengths

From the previous analysis, we could come to the conclusion that the RSRs and IE configurations play a significant role in the performance of the CML-EBG structure.To study the effect of structure and dimension of the RSRs and IE, two simulation cases have been carried out.

3.1 Case 1

The model of CML-EBG integrating with different interleaving edge is established and the dimensions of the IE and patch agree with those in the previous part.Fig.3 depicts the bandgap frequency of the IE-based EBG:6.2 GHz to 7.8 GHz (a bandwidth of 11.4% at 7 GHz).Note that the IE configuration inserting in the EBG makes a decline of 1.95 GHz at the center frequency with a drop of 16% in the bandwidth.Furthermore, the IE-based EBG with different IE lengths was simulated.It is observed in Table 1 that the center bandgap frequency tends to decrease with the increase of IE length b, moreover, the bandwidth gains an effective improvement when length b becomes smaller.

Fig.3.Dispersion diagram for the IE-based EBG structure.

Fig.4.Dispersion diagram for the RSR-based EBG structure.

Table 2: Bandgap feature of different RSRS radius

Fig.5.Figures of the constructed RSRs-IE EBG.

3.2 Case 2

Model of CML-EBG etched RSRs was built.The parameters of the RSRs and patch are the same as previous.Learning from Fig.4, RSRs-based EBG shows a bandgap of 3.5 GHz to 8.8 GHz (a bandwidth of 43% at 6.15 GHz).Note that the RSRs etched on the EBG bring about a decrease of 2.8 GHz at the center frequency, with an enlargement of 15.6% in the bandwidth.As shown in Table 2, when the radius of the inner ring increases, the middle bandgap frequency moves to the lower ends, while the bandwidth drops slowly.

Fig.6.Measured results: (a) mushroom-like EBG and (b)proposed EBG.

To further validate the bandgap performance of the RSRs-IE based EBG structure, a 5×5 lattice of the Mushroom-like EBG and the presented EBG structure were fabricated on printed circuit boards (PCBs) and measured by Agilent N5230 vector network analyzer, as shown in Fig.5.

The suspended microstrip method[14],[15]was applied to verify the bandgap property of the novel EBG in which a 50 Ω microstrip line was placed above upper substrate(εr=2.65, h=0.5 mm) and soldered with SMA connectors to measure the S-parameters.As a strong coupling measurement method, the bandgap is defined in the range with S21below –10 dB.The measured results are depicted in Fig.6:the bandgap is from 4.1 GHz to 6.7 GHz.Though it is narrower than the simulated result (Fig.2(b)), it is much lower than that of the CML-EBG (7.6 GHz to 11.8 GHz) as measured.

The measured results agree well with the simulated results, however, there is some error between them.The discrepancy is probably due to the following three reasons:1) ideal infinite periodic cells used in simulations, 2)manufacturing tolerances, and 3) interference from the surrounding environment.

4.Conclusions

A novel compact RSRs and IE embedded conventional mushroom-like EBG structure is studied and tested.The measure results confirm the location of the bandgap calculated by Ansoft HFSS.Compared with the conventional mushroom-like EBG structure, the combination of RSRs and IE results in decreasing the center frequency of the band-gap by 13.6%, therefore, the proposed RSRs-IE based EBG can be effectively applied in compact microstrip patch array.

Acknowledgment

The authors would like to thank Zhongyang Xingye Corporation and W.S.Zhan for fabrication of the prototypes.They also would like to thank Y.Huang for technical guidance in measurement.

[1]D.Sievenpiper, L.J.Zhang, R.F.J.Broas, N.G.Alexopolous, and E.Yabiomovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,”IEEE Trans.on Microw.Theory Tech., vol.47, no.11, pp.2059–2074, 1999.

[2]F.Yang and Y.Rahmat-Samii, Electromagnetic Band-Gap Structures in Antenna Engineering, Cambridge: Cambridge Univ.Press, 2008.

[3]F.Yang and Y.Rahmat-Samii, “Microstrip antennas integrated with electromagnetic band-gap (EBG) structures:A low mutual coupling design for array applications,” IEEE Trans.on Antenna Propag., vol.51, no.10, pp.2936–2946,2003.

[4]Z.Iluz, R.Shavit, and R.Bauer, “Microstrip antenna phased array with electromagnetic bandgap substrate,” IEEE Trans.on Antenna Propag., vol.52, no.6, pp.1446–1453, 2004.

[5]J.Liang and H.Y.D.Yang, “Radiation characteristics of a microstrip patch over an electromagnetic bandgap surface,”IEEE Trans.on Antenna Propag., vol.55, no.6, pp.1691–1697, 2007.

[6]M.Coulombe, S.F.Koodian, and C.Caloz, “Compact elongated mushroom (EM)-EBG structure for enhancement of patch antenna array performances,” IEEE Trans.on Antenna Propag., vol.58, no.4, pp.1076–1086, 2010.

[7]Y.-Q.Fu, N.-C.Yuan, and G.-H.Zhang, “Compact high-impedance surfaces incorporated with interdigital structure,” Electron.Lett., vol.40, no.5, pp.310–311, 2004.

[8]L.Yang, M.-Y.Fan, F.-L.Chen, J.-Z.She, and Z.-H.Feng,“A novel compact electromagnetic-bandgap (EBG) structure and its applications for microwave circuits,” IEEE Trans.on Microw.Theory Tech., vol.53, no.1, pp.183–190, 2005.

[9]L.Peng, C.-L.Ruan, and Z.-Q.Li, “A novel compact and polarization-dependent mushroom-type EBG structure using CSRR for dual/triple-band application,” IEEE Microw.Wireless Compon.Lett., vol.20, no.9, pp.489–491, 2010.

[10]Y.Yao, X.Wang, and Z-H.Feng, “A novel dual-band compact electromagnetic bandgap (EBG) structure and its application in multiantennas,” in Proc.of IEEE Antennas and Propagation Society International Symposium, Albuquerque,2006, pp.1943–1946.

[11]P.Jiang and K.Xie, “Design of novel compact electromagnetic bandgap structure with enhanced bandwidth,” J.Electron.Science Tech., vol.8, no.3, pp.262–266, Sep.2010.

[12]E.Rajo-Iglesias, L.Inclan-Sanchez, J.L.Vazquez-Roy, and E.Garcia-Muoz, “Size reduction of mushroom-type EBG surfaces by using edge located vias,” IEEE Microw.Wireless.Compon.Lett., vol.17, no.9, pp.670–672, Sep.2007.

[13]R.Remski, “Analysis of PBG surfaces using Ansoft HFSS,”Microwave Journal, vol.43, no.9, pp.190–198, Sep.2000.

[14]M.-Y.Fan, R.Hu, Z.-H.Feng, X.-X.Zhang, and Q.Hao,“New method for 2D-EBG structures’ research,” J.Infrared Millim.Waves, vol.22, no.2, pp.127–131, Oct.2003.

[15]Y.Ning, Z.-N.Chen, and Y.-Y.Wang, “A novel two-layer compact electromagnetic band-gap (EBG) structure and its applications in microwave circuits,” Science in China (Series E), vol.46, no.4, pp.439–447, 2003.