Electrically Small Triple-band SRR Antenna S.Zhu, K.L.Ford, A.Tennant and R.J.Langley1

1 Department of Electronic and Electrical Engineering,University of Sheffield,Mappin Street, Sheffield, UK S1 3JD e-mail: S.Zhu@sheffield.ac.uk, R.J.Langley@sheffield.ac.uk.

Abstract − A compact triple-band split ring antenna is presented. The antenna is electrically small and low profile. Its planar structure makes it easy to fabricate and integrate with other structures like AMC/EBG. The antenna operates at three frequency bands of 430MHz, 680MHz and 990MHz. It is 100mm long which corresponds to λ/7 at the lowest working frequency of 430MHz.

1 INTRODUCTION

The design of electrically small antennas (ESAs) is a topic of considerable interest within the antenna community. Of particular interest is the design of miniaturised antennas which operate at low frequencies, as traditional approaches to antenna design result in large and bulky structures. Various of approaches to antenna miniaturisation have been investigated over the years including the use of high permittivity materials and substrates [1]; electrically loading antennas with reactive components [2]; antennas which utilise the properties of metamaterial structures [3,4] and composite right/left handed transmission line based resonant antennas [5,6]. In this paper we present details of the design of a novel compact triple-band split ring resonator antenna. The antenna operates at three frequency bands of 430MHz, 680MHz and 990MHz. It is just 100mm long which corresponds to λ/7 at the lowest working frequency of 430MHz. The antenna was designed with the aid of the CST Microwave studio software suite. The results of numerical simulations of the antenna performance with regard to input match and radiation patterns are presented and compared to measured data.

2 ANTENNA DESIGN

2.1 Antenna geometry

The split ring dipole geometry is shown in Fig.1. It is composed of two split ring resonators (SRR) forming a dipole as the main radiating element. The two identical SRRs are printed symmetrically on opposite sides the substrate to form the dipole arms. The resonance of the structure is due to the resonance of the SRRs. Because of the negative permeability in a certain frequency range, the SRR can generate resonance at wavelengths that are much larger than its own size. The two driven elements are printed on each side of the substrate to

make for easy coaxial feed through the centre. The inner ring of each resonator is extended to a meandered ring to interact with the main resonator to generate extra resonances. The surface currents are mainly induced by the driven ring itself and the large capacitance existing in between the split rings. The ring width and the gap between the rings are tuned to generate resonances at specified frequency bands. At the feed of the antenna, a simple matching network which consists of a series capacitor and a parallel inductor connecting the bottom element is employed. The matching network is used to enhance the impedance matching of the resonances, and it also helps to balance the current at the coaxial feed to the dipole antenna itself.

(a)

(b)

Fig.1 SRR antenna geometry. (a) top view, (b) side view.

2.2 Antenna fabrication

The proposed SRR antenna is fabricated on FR4 substrate with a dielectric constant of 4.5 and thickness of 0.8mm. A coaxial feed from the front element to the bottom element and an LC matching network are used to feed the antenna. A series capacitance value of 12pF and shunt inductance value of 5.6nH are chosen according to C0 and L0 in Fig.1. The resonant frequencies are mainly defined by the radius of the rings, and the width of the split ring and the gap between the inner and outer rings can be tuned to achieve the best match at the resonance in this antenna design. Thus, the proposed antenna

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provides the features of easy fabrication and low profile configuration. The fabricated prototype of the SRR antenna is shown in Fig.2. The antenna's dimensions are (unit: millimetre): L=100, W=48, r1=22, r2=17, r3=12, r4=7, g1=1, g2=g3=2. The antenna has an overall size of 100mm×48mm×0.8mm, which at the free space operating wavelength of 430MHz measured

872147λλλ ×× .

Fig.2 Split Ring Antenna prototype Antenna dimensions 100mm x 48mm

3 RESULTS ANALYSIS

The performance of the SRR antenna was both simulated in CST Microwave Studio and measured in an anechoic chamber to make a comparison. The antenna is designed to have three useful resonances correspondingly at 435MHz, 680MHz and 990MHz in this study. The simulated and measured antenna reflection coefficients, radiation efficiency and radiation patterns will be discussed in the following sections. 3.1 Reflection Coefficient Fig. 3 shows the simulated and measured antenna reflection coefficients where the solid line represents the measurement and the dotted line is the simulated results. From the figure we can see that for the 430MHz and 990MHz band , the simulation and the measurement have a fairly good agreement, while at 680MHz, unlike the simulation, the measurement indicates quite poor matching. The -6dB frequency band coverage and the bandwidth for the three resonant bands are summarized in Table.1. It can be seen that a 1.63% bandwidth is achieved for the lower band of 430MHz, which is good for such a small antenna.

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Return loss-measurementReturn loss-simulation

Fig.3 Simulated and measured antenna return loss

Table 1: Resonant frequency summary

(-6dB return loss)

3.2 Current Distribution

The simulated current distribution of the proposed SRR antenna is plotted in Fig. 4. At 430MHz, it is observed that the primary current distribution is confined to the outer split ring and the capacitive coupling between the outer and the inner ring. This suggests that the size of the antenna could be reduced by increasing the capacitance coupling between the rings, for example by physically altering the ring shape or by simply loading lumped elements. This method has been proved by the authors in [7]. For the two upper frequencies, it is clear that the currents are mainly concentrated at the inner ring and the edges between the inner and outer ring, which indicates that the coupling between the rings is also important in determining these frequencies.

Simulation Measurement

430MHz

Frequency range 429-433MHz 430-437MHz

Bandwidth 0.93% 1.63%

680MHz

Frequency range 679-687MHz N/A

Bandwidth 1.33% N/A

990MHz

Frequency range

951-1024MHz 964-1033MHz

Bandwidth 7.4% 7%

832

(a)

(b)

(c)

Fig.4 Simulated current distribution of the antenna at (a) 430MHz; (b) 680MHz and (c) 990MHz

3.3 Antenna Efficiency

The radiation efficiency of the antenna is measured by using the Wheeler Cap method [8]. The measured radiation efficiencies of the antenna from 390MHz-1050MHz is plotted in Fig.5. And the maximum achievable efficiencies at the three working frequency bands are listed in Table 2 together with the simulated results for comparison. It can be seen that the measured radiation efficiency at 430MHz is about 70%, which is considerably good for a electrically small antenna. And there is a fairly good agreement with our simulated radiation efficiency of 72% at the same frequency point. The measured efficiency at the other two bands are 72% for 680MHz and 83% for 990MHz band respectively. These are both lower than the simulated radiation efficiency of 78.5% and 96%. The reason that causes lower efficiency value in the measurement can be multiple. Factors such as the imperfect test environment and losses of the lumped elements which are not considered in simulation could be the main reasons.

300 400 500 600 700 800 900 1000 1100Frequency (MHz)

0

0.2

0.4

0.6

0.8

Rad

iatio

n Ef

ficie

ncy

Fig.5 Measured radiation efficiency

Table 2: Antenna radiation efficiency summary

430MHz 680MHz 990MHz Simulation 72% 78.5% 96%

Measurement 69.8% 72% 83%

3.4 Radiation Patterns The antenna radiation performance is very similar to a normal dipole which has a nearly omni-directional pattern at H plane and an eight shape at its E plane. The measured antenna E and H plane radiation patterns at three resonances are plotted in Fig.6. At 430MHz and 990MHz band, the received antenna power patterns are compared to those of two standard half wavelength dipoles to work out the antenna gain. The measured antenna gain at 434MHz is 0.8dBi, and 2.01dBi at 998MHz. As there is no standard reference antenna at 680MHz, the measured antenna gain is not available at the moment.

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Fig.6 Measured antenna radiation patterns

4 CONCLUSION

The performance of a compact triple-band split ring dipole which is electrically small and low profile has been presented. The split ring resonators provide the resonant frequency bands and by extending the inner ring extra resonance can be generated. The antenna has three resonances which are at 430MHz, 680MHz and 990MHz. It is just λ/7 long with a gain of 0.8dBi at the lowest resonant frequency 430MHz. The antenna size can be further reduced by simply loading lumped elements within the rings, and will be reported in future publications.

References

[1] D. Psychoudakis, J. L. Volakis, Z. N. Wing, S. K. Pillai and J. W. Halloran, "Enhancing UHF antenna functionality through dielectric inclusions and texturization," IEEE Transactions on Antennas and Propagation, Vol.54, No.2, pp.317-329, Feb 2006.

[2] M. Lee, B. A. Kramer, C. Chen and J. L. Volakis, "Distributed lumped loads and lossy transmission line model for wideband spiral antenna miniaturization and characterization," IEEE Transactions on Antennas and Propagation, Vol.55, No.10, pp.2671-2678, Oct 2007.

[3] F. Bilotti, A. Toscano and L. Vegni, "Design of spiral and multiple SRRs for the realization of miniaturized metamaterial samples," IEEE Transactions on Antennas and Propagation, Vol.55, No.8, pp.2258-2267, Aug 2007.

[4] P. Jin and R. Ziolkowski, "Broadband, efficient, electrically small metamaterial-inspired antennas facilitated by active near-field resonant parasitic elements," IEEE Transactions on Antennas and Propagation, Vol.58, No.2, pp.318-327, Feb 2010.

[5] F. Qureshi, M. Antoniades and G. V. Eleftheriades, "A compact and low-profile metamaterial ring antenna with vertical polarization," IEEE Antennas and Wireless Propagation Letters, Vol.4, pp.333-336, 2005.

[6] Q. Liu, P. S. Hall and A. L. Borja, "Efficiency of electrically small dipole antennas loaded with left-handed transmission lines," IEEE Transactions on Antennas and Propagation, Vol.57, No.10, pp.3009-3017, Oct 2009.

[7] S.Zhu, K. L. Ford, A. Tennant and R. J. Langley, "Loaded split ring antenna over AMC," IET Electronics Letters, Vol. 46, Issue 14, PP. 971-972, Jul 2010.

[8] R. H. Johnston and J. G. McRory, "An improved small antenna radiation-efficiency measurement method," IEEE Antenna and Propagation Magazine, Vol. 40, No. 5, pp.40-48. Oct 1998.

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