Dual-Frequency Printed Dipole Loaded With Split Ring ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

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Dual-Frequency Printed Dipole Loaded With Split Ring Resonators Francisco Javier Herraiz-Martínez, Student Member, IEEE, L. Enrique García-Muñoz, David González-Ovejero, Vicente González-Posadas, and Daniel Segovia-Vargas, Member, IEEE

Abstract—A novel approach to design dual-frequency printed dipoles is presented. This approach is based on an antipodal printed dipole loaded with split ring resonators (SRRs). This technique allows the choice of any pair of working frequencies. Two prototypes, the first one working at 1.32 and 2.83 GHz and the second one working at 1.2 and 2.05 GHz, have been manufactured and measured. The experimental results show reasonable values for the efficiency at both working frequencies. Moreover, the obtained radiation pattern is dipolar at both frequencies with low cross polarization levels. Index Terms—Metamaterials, microstrip antennas, multifrequency antennas, split ring resonators (SRRs).

I. INTRODUCTION

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OWADAYS, the necessity for dual-frequency antennas is increasing in order to deal with the large demand for radiating elements for dual-band handheld devices. Dual-frequency printed dipoles could be a good solution due to their dual-band performance, reduced size, and low profile. The simplest approach to obtain a dual-frequency printed dipole consists of using two different dipoles fed through a single port [1]. Some approaches to obtain a single dipole with two resonances consist of making two U-slotted arms inside the dipole [2] or incorporating two compact resonant cells [3]. In both cases, the ratio between working frequencies is always larger than 2. It seems that smaller ratios cannot be achieved with these approaches. The main reason for that is that the additional resonance is fixed by the structure introduced inside the dipole (U-slots or compact resonant cells). Thus, a decrease in the frequency ratio involves an increase in the inner resonators’ size, causing these structures not to fit in the dipole. An interesting approach would be to develop dual-frequency printed dipoles with arbitrary ratios since frequency ratios smaller than 2 are needed in most wireless services, e.g., navigation systems such as GPS or Galileo.

Manuscript received September 16, 2008; revised November 04, 2008. First published January 09, 2009; current version published April 22, 2009. This work was supported in part by the Spanish MEC under Project TEC2006-13248C04-04/TCM, the FPU program from MEC, CCG06-UC3M/TIC-0803 from CAM, and COST IC0603, ASSIST. F. J. Herraiz-Martínez, L. E. García-Muñoz, and D. Segovia-Vargas are with the Department of Signal Theory and Communications, Carlos III University in Madrid, 28911 Madrid, Spain (e-mail: [email protected]). D. González-Ovejero is with the Communications Department, Université Catholique Louvain, 1348 Louvain-la-Neuve, Belgium. V. González-Posadas is with the Department of Ingeniería Audiovisual y Comunicaiones, Universidad Politécnica de Madrid, 28031 Madrid, Spain. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2009.2012402

Fig. 1. Sketch of the proposed antenna: (a) Top and side views of the SRRloaded dipole antenna with its design parameters. (b) SRR parameters. (c) Photograph of the manufactured prototypes (top: conventional antipodal dipole, bottom: proposed SRR-loaded antipodal dipole).

This letter presents an approach to develop dual-frequency printed dipoles by loading an antipodal dipole with split ring resonators (SRRs). The loaded dipole resonant frequencies are very close to the self-resonant frequencies of the dipole itself and the SRRs, respectively. This allows developing dual-frequency dipoles with arbitrary frequency ratios. It is important to note that the proposed antenna keeps the dipolar radiation pattern at both working frequencies. In addition, the cross-polar (XPOL) component is low. II. ANTENNA STRUCTURE AND OPERATION PRINCIPLE The geometry of the proposed dual-frequency printed dipole is shown in Fig. 1. This antenna is based on an antipodal dipole printed on both sides of a dielectric substrate with height . The parameters of the dipole are the length , and the width . Four SRRs are printed on the opposite side of each dipole branch. The SRR parameters, according to Fig. 1(b), are the external , and the gap between radius , the width of the strips strips . The SRRs are placed at a distance away from the dipole center, while the separation between the centers of the SRRs is . The antenna is fed through a paired strips transmission line [4] with a subminiature version A (SMA) connector

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Fig. 3. Measured reflection coefficient of the proposed SRR-loaded and conventional dipoles.

Fig. 2. (a) Simulated current distributions on the conventional dipole at the fundamental frequency; (b) simulated current distribution on the SRR-loaded dipole at f and (c) at f ; (d) E-field distribution in the SRRs.

soldered to the end of the line. The dimensions of the feeding and the width . line are the length This proposed configuration provides a dual-frequency performance. The first frequency ( ) is due to the dipole, while the second frequency ( ) occurs in the vicinity of the SRR self-resonant frequency. Fig. 2 (obtained with the software CST Microwave Studio–CST MWS® [5]) shows the current distributions on the conventional dipole at its fundamental working frequency [Fig. 2(a)] and on the SRR-loaded proposed dipole at the two working frequencies [Fig. 2(b) and (c)]. At the lower frequency, the current distributions on both dipoles [Fig. 2(a) and (b)] are very similar and dipolar-like. It can be appreciated that the effect of the SRRs at the lower frequency is negligible, and nearly no current passes along these SRRs. In fact, the only modification in the antenna performance is a very slight frequency shift in the resonant frequency toward lower frequencies due to the capacitive parasitic effect of the SRRs. Fig. 2(c) shows the performance of the SRR-loaded dipole at the second frequency. In this case, the working principle is somewhat different since the resonant frequency is imposed by the SRRs. At this frequency, the SRRs are resonating, causing a maximum current distribution on the SRRs, as can be seen in Fig. 2(c). It can be seen that the -field distribution is symmetric and with opposite phase in each half of the SRR. This can also be seen in [6], where the charge distribution is symmetric (with respect to the y-axis) with opposite signs in any of the two halves of the SRR. In addition, the radiation pattern at this second frequency is dipolar, which justifies the fact that the radiation mechanism is dominated by the dipole and not by the SRRs. This fact implies that the SRRs are not radiating but storing a large amount of reactive energy (this will be shown

in Section III by looking at the measured antenna radiation patterns). In this way, the radiating element is formed by the linear current distribution on the dipole between the SRRs’ arrangements. It should be noted that this new antenna is a dipole with a , sustaining a uniform current between length shorter than the edges where the SRRs are placed. Thus, we can conclude that the SRRs provide the second resonant frequency by imposing a hard boundary condition (an open circuit) where the SRRs are placed. In this way, the currents on the dipole (from the position where the SRRs are placed to the end of the overall structure) are nearly null. Thus, the overall structure (dipole plus SRRs) keeps a dipolar radiation pattern at . III. EXPERIMENTAL RESULTS Two prototypes of the proposed SRR-loaded dipole antenna have been implemented, one for a frequency ratio smaller than 2 and the other for a frequency ratio larger than 2. In both cases, mm, mm, the dimensions of the dipole are mm, and mm. The substrate is the low and ) with mm. The cost FR-4 ( resonant frequency of the unloaded dipole is 1.53 GHz. Both prototypes are loaded with two sets of four SRRs at a distance that is varied depending on the SRRs’ size. For the two designed dipoles, this distance has been set to 24.20 and 25.20 mm, respectively. The separation is 8.40 and 10.40 mm. The parammm for the first proeters of these SRRs are the radius ( totype, and mm for the second one), the width is 0.40 mm, and the gap between rings, , is 0.20 mm. According to [7], the resonant frequency of the first set of SRRs is 2.55 GHz, while the second one is 1.92 GHz. The proposed SRR-loaded dipoles and the conventional dipole (without SRR loading) have been manufactured [Fig. 1(c)]. The measured reflection coefficients of the manufactured antennas are shown in Fig. 3. The conventional dipole frequency is 1.48 GHz, while the proposed SRR-loaded dipoles present the desired dual-frequency performance. The GHz for the dipole loaded first resonance appears at GHz for the with 4-mm-radius SRRs, while at

HERRAIZ-MARTÍNEZ et al.: DUAL-FREQUENCY PRINTED DIPOLE LOADED WITH SRR

5-mm-radius SRRs. At these frequencies, the current along the SRRs is negligible, and the only effect of the SRRs is to slightly shift down the frequency of the proposed dipole. The second is 2.83 GHz for the first prototype and working frequency GHz for the second one, which are very close to the SRRs’ self-resonance frequencies. This frequency is shifted toward higher frequencies due to the overall effect of the SRRs coupled to the dipole and the tolerances of the substrate and the manufacturing process. The bandwidth at the lower frequency -dB level for the conventional dipole is around 15% at the and for the SRR-loaded dipoles. The bandwidth at the higher -dB level) for the proposed dipoles is much band (at the lower (1.27% and 3.5% for the first and second prototypes, respectively). This resonance is imposed by the SRR itself, factor that implies a much smaller which has a very large bandwidth. The measured radiation patterns of the dual-frequency dipole loaded with 4-mm-radius SRRs at both working frequencies (1.32 and 2.83 GHz) are shown in Fig. 4. The radiation patterns at both working frequencies are dipolar and similar to the ones presented by the conventional dipole. A ripple can be appreciated specially at the -plane ( -plane). This is due to the measurement procedure where the metallic plane of the positioner behind the antenna could not be avoided. At the first working frequency, the maximum cross-polarization level in the -plane ( -plane) (defined with respect to the normalized 0-dB copodB and occurs at with respect to larization level) is the 0-dB direction. For the -plane, the maximum cross-polardB and occurs at with respect to the ization level is 0-dB direction. At the second frequency (bottom part of Fig. 4) the effects are similar to the ones described before: A ripple appears due to the metallic plane of the positioner behind the dB for dipole. The maximum cross polarization level is -plane and occurs at 20 with respect to the 0-dB direction. dB and ocFor the -plane, the cross-polar component is curs at . The measured gain of the reference dipole is 1.99 dB. For GHz and the first dual-frequency dipole ( GHz), this magnitude is 1.81 dB at and 0.67 dB at . For the second dual-frequency dipole ( GHz and GHz), the gain is 1.89 dB at and 1.1 dB at . The decrease in the gain at the second frequency is due to the reactive energy stored in the SRRs. The efficiencies have been measured through the Wheeler cap method [8], where the efficiency is obtained as

(1) In this last expression, is the antenna free-space input resistance, and is the antenna input resistance inside the cap. The efficiency of the 4-mm-radius SRR-loaded dipole is 91% at the frequency associated with the dipole itself while decreasing to 62% at the frequency associated with the SRRs. For the other SRR-loaded prototype, the efficiency is 92% at the first working frequency and 70% at the second one. In both cases, this is due to the large amount of energy stored in SRRs that is not radiated. The effect of the high losses in the SRRs can also be appreciated

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Fig. 4. Measured radiation pattern of the proposed SRR-loaded dipole at 1.32 GHz (upper figure) and at 2.83 GHz (bottom figure).

in [7] for a filter application. Finally, all the measurements are summarized in Table I. From Table I, it can be seen that the efficiencies and gains for the 5-mm-radius SRR-loaded dipole are higher than those for the 4-mm-radius SRR-loaded one. In addition, the bandwidth at the SRRs’ frequency is larger in the second prototype. This implies that the loaded quality factor is lower in this second case. This can be due to two factors: an increase in the losses or a decrease of the energy stored in the SRRs. As the losses are, more or less, kept constant when the SRRs’ radius is increased, the amount of energy stored in the SRRs must have lessened. This implies that the overall antenna efficiency is increased since the radiation mechanism is mainly due to the dipole. These measurements demonstrate that the proposed approach allows the development of dual-frequency printed dipoles with similar characteristics to those of a conventional dipole at two arbitrary frequencies.

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TABLE I SUMMARY OF THE MEASURED ELECTRIC CHARACTERISTICS OF THE CONVENTIONAL DIPOLE AND SRR-LOADED DIPOLE ANTENNAS

is being undertaken in order to overcome this problem. Finally, several prototypes have been manufactured and measured, demonstrating the previous results. ACKNOWLEDGMENT The authors would like to thank Prof. C. Craeye from Université Catholique Louvain and Dr. T. Finn from IGN for his comments and help during this work and during the review process. They would also like to thank Antenas Moyano S.L. for its help during this work. REFERENCES

IV. CONCLUSION An approach to designing dual-frequency printed dipoles based on antipodal dipoles loaded with SRRs has been presented. The working frequencies are in the vicinity of the dipole and SRRs’ resonant frequencies. This fact has been used to design a dipole working at any pair of frequencies. The efficiency of the SRR-loaded dipole at the frequency associated with the dipole itself is similar to that of the conventional one. However, a loss in the efficiency at the second frequency has risen due to the reactive energy stored in the SRRs that is proportional to the SRR quality factor. The cross-polarization levels are low at both frequencies. Additionally, the bandwidth at the frequency associated with the dipole is maintained. Nonetheless, the bandwidth at the frequency associated with the SRRs is low due to the high of the SRRs. At this moment, further work

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