Research Article Triband Omnidirectional Circularly Polarized Dielectric Resonator Antenna with Top-Loaded Alford Loop

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1 Antennas and Propagation Volume 214, Article ID 79793, 7 pages Research Article Triband Omnidirectional Circularl Polaried Dielectric Resonator Antenna with Top-Loaded Alford Loop Chunia Cheng, Fushun Zhang, Yali Yao, and Fan Zhang National Laborator of Science and Technolog on Antennas and Microwaves, Xidian Universit, Xi an 7171, China Correspondence should be addressed to Chunia Cheng; cheng chunia@126.com Received 22 Ma 214; Revised 12 August 214; Accepted 1 September 214 Academic Editor: Bungje Lee Copright 214 Chunia Cheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in an medium, provided the original work is properl cited. A triband omnidirectional circularl polaried dielectric resonator antenna with a top-loaded modified Alford loop for GSM, WLAN, and WiMAX applications is proposed. Fed b an aial probe, the DRA (dielectric resonator antenna) radiates like a verticall polaried electric monopole. The top-loaded modified Alford loop provides an equivalent horiontall polaried magnetic dipole mode at triband. Omnidirectional CP (circular polaried) fields can be obtained when the two orthogonall polaried fields are equal in amplitude with phase quadrature. The antenna has been successfull simulated, fabricated, and measured. The eperimental and numerical results ehibit that the antenna can obtain usable CP bandwidths of GH, GH, and GH with return loss larger than 1 db and aial ratio less than 3 db. In addition, over the three bands, the antenna obtains ver good omnidirectional CP radiation patterns in the aimuth plane. Moreover, an average CP gain in the aimuth plane of 1.2, 1.6, and dbic for the lower, middle, and upper bands has been obtained. 1. Introduction In modern wireless communication sstems, the demand for antennas with multiband operations has increased since such antennas are vital for combining multiple different communication standards in a single compact wireless sstem. Also, circularl polaried antennas are widel used in modern wireless sstems to suppress multipath interferences. Moreover, omnidirectional antennas are ver attractive for applications in wireless communications because the can provide full coverage of signal to maintain good communicationlinksatallangles.thus,multibandomnidirectional circular polaried antennas are ideal for modern wireless communications. Several tpes of triband antennas have been reported, such as a CPW-fed monopole antenna with a parasitic circular patch and a pair of smmetrical inverted- L strips in [1], a monopole antenna with complementar split-ring resonators in [2], a single-loop resonator in [3], and a rectangle-loaded monopole antenna with inverted- L slot in [4]. However, those designs can onl provide triband linear-polaried omnidirectional patterns. As the growing interest in multiband CP antennas, various designs of multiband circularl polaried antennas have also been proposed, including the dual-band CPW-fed circularl polaried antenna with two orthogonal slots [5], a dual-band CP antenna with two square slots in the opposite corner of the ground plane and two truncated corners of the main slot [6], and a dual-band circularl polaried antenna with slots loaded in the two opposite corners and the halberd-shaped stripconnectedattheendofthesignalline[7]. Unfortunatel, although the above designs achieve multiband CP radiations, the all cannot achieve omnidirectional patterns. In addition, several tpes of omnidirectional CP antennas with singleband [8, 9] or dual-band [1] havebeenfoundinrecent literatures. However, those designs can onl provide one or dual operation band. In this paper, a triband omnidirectional CP clindrical dielectric resonator antenna (DRA) with a top-loaded modified Alford loop which satisfies 1.9 GH GSM, 2.4 GH WLAN, and 3.5 GH WiMAX applications is proposed. The DRA radiates like an electric monopole and the top-loaded modified Alford loop radiates as an equivalent magnetic dipole. B properl selecting shapes and dimensions of the DRAandtheAlfordloop,theorthogonalelectricfieldsofthe

2 2 Antennas and Propagation D d g d 1 L2 L1 W 2 L3 L 4 H d f W1 L h W D Figure 1: Configuration of the proposed antenna. Top view. Side view. Table 1: Dimensions of the proposed antenna (unit: mm). D H d g h d f d 1 L L 1 L 2 L 3 L 4 W W 1 W equivalent electric and magnetic dipoles which are equal in amplitude with phase quadrature within the three separate frequenc bands can be achieved. Details of the antenna design and eperimental results are presented and discussed. 2. Antenna Design The antenna design presented here is based on the omnidirectional circularl polaried dielectric resonator antenna with top-loaded Alford loop in [9]. In the present work, b modifing the top-loaded Alford loop the two orthogonall polaried fields are equal in amplitude with phase quadrature inthreebands.theconfigurationoftheproposedtriband omnidirectional CP clindrical DRA is shown in Figure 1. The DRA is designed to resonate in the middle band (about 2.4 GH) with a diameter of D = 5mm, a height of H = 22.5 mm, and a dielectric constant of ε r = 9.8. Theresonant frequenc of the DRA in TM 1δ -mode can be estimated using [11]: f= 2.933cε.468 r πd {1 [.75.5 ( D 4H )] [ε r 1 ]} 28 { ( D 2 D ).71( 4H 4H ) }. (1) The proposed DRA can be matched easil b simpl adjusting the length h of the probe. It is found that a good match can be obtained when h=1mm. As shown in Figure 1, the circular patch of the modified Alford loop that locates on the top of the DRA has a diameter of d 1.Fourconducting curved branches with the same width are etended from the patch and oriented in the counterclockwise direction. Four conducting curved branches with graduall narrow width are etended from the outer of the four inner branches. B adjusting the sies of the modified Alford loop and ground plane (d g ), the radiation fields of the electric and magnetic monopoles can be made equal in amplitude with phase quadrature, which are required for generating CP fields. The detailed dimensions of the proposed circularl polaried antenna are listed in Table 1.Thelengthsofthebranchesare approimatel determined b d 1 π 4 +L W L 2 +L 3 +L 2 1 =λ l, d 1 π 4 +L W L 2 +L 3 =λ 2 m, (2) d 1 π 4 +L 1 + W 1 2 +L 4 =λ u, where d 1 π/4 is a quarter of the circular patch circumference, L 1 is the length of the inner branch, (L 2 +L 3 )/2 is the average length of the outer branch, W 1 /2+L 4 is the distance between

3 Antennas and Propagation 3 (c) Figure 2: Three prototpes of the antenna. Ant. I, Ant. II, and (c) Ant. III Aial ratio (db) Aial ratio (db) Return loss of Antenna 1 Aial ratio of Antenna 1 Return loss of Antenna 2 Aial ratio of Antenna Aial ratio (db) Return loss of Antenna 3 Aial ratio of Antenna 3 (c). Figure 3: Simulated return loss and aial ratio (theta = 9 deg and phi = deg) of various antennas involved. Ant. I, Ant. II, and (c) Ant. III.

4 4 Antennas and Propagation E field (V per m) e e e e e e e e e e e e e e e GH 2.45 GH 3.51 GH E field (V per m) e e e e e e e e e e e e e e e GH 2.45 GH 3.51 GH E field (V per m) e e e e e e e e e e e e e e e GH 2.45 GH 3.51 GH (c) E field (V per m) e e e e e e e e e e e e e e e GH 2.45 GH 3.51 GH (d) Figure 4: Simulated E fields inside the DRA at the three resonant frequencies with four phase angles:,9,(c)18,and(d)27. Figure 5: Photograph of the proposed antenna.

5 Antennas and Propagation Aial ratio (db) Aial ratio (db) Return loss simulated Return loss measured Aial ratio simulated Aial ratio measured Return loss simulated Aial ratio simulated Return loss measured Aial ratio measured Aial ratio (db) Return loss simulated Aial ratio simulated (c) Return loss measured Aial ratio measured Figure 6: Simulated and measured return loss and AR of the proposed antenna. Lower band, middle band, and (c) upper band. the inner branch and the circular patch, and L W 2 /2 is the distance between the outer branch and the circular patch. λ l, λ m,andλ u are the lower, middle, and upper wave length, respectivel. The are given b λ l = λ m = λ u = C f l ε eff, C f m ε eff, C f u ε eff, where f l, f m,andf u are the lower, middle, and upper resonant frequenc and ε eff is the relative effective permittivit. For clarifing the improvement process, three prototpes of the antenna are defined as follows (Figure 2): Ant. I is the original antenna with four conducting curved branches of the same width etended from the top-loaded patch; Ant. II has four conducting curved branches with the same width which are etended from the outer of Ant. I; Ant. III is the final construction with the graduall narrow outer branches modified from those of Ant. II. These three prototpes are (3) simulated b a high frequenc structure simulator (HFSS 13.) and the detailed performances are presented in Figure 3: Ant. I has one usable circularl polaried band (overlap bands of return loss > 1 db and AR < 3 db), while Ant. II and Ant. III have three. Furthermore, the useable bandwidths of Ant. III are broader, especiall the middle band. It is observed that the graduall narrow outer branches have positive influence on circular polaried fields of the antenna. The resonant E fields of o-plane inside the DRA for the lower (1.937 GH), middle (2.45 GH), and upper (3.51 GH) frequencies of deg, 9 deg, 18 deg, and 27 deg are simulated using HFSS. The simulated fields are shown in Figure 4. In the three frequencies, the wave propagation direction is along the outer normal direction of the DRA. In order to determine the wave polariation, we observed electric field vector direction (inside the black dotted line) variet with phase angles. In the lower frequenc, the observed E field flows from the +-ais and -ais to -ais, and the wave propagation direction is +-ais, generating a RHCP radiation. In the middle frequenc, the observed E field flows from the +-ais and +-ais to -ais, resulting in a LHCP. And in the upper frequenc, the observed E field flows from the -ais and -ais to +-ais, also producing a LHCP.

6 6 Antennas and Propagation Simulated RHCP Simulated LHCP Measured RHCP Measured LHCP Simulated RHCP Simulated LHCP Measured RHCP Measured LHCP (c) Figure 7: Simulated and measured radiation patterns in the aimuth plane (left) and the elevation plane (right) GH GH. (c) 3.51 GH.

7 Antennas and Propagation 7 Gain (dbic) combining equivalent electric and magnetic monopoles is adopted in the design; the former and the latter are obtained through the DRA and the modified Alford loop, respectivel. B tuning the dimensions of the Alford loop, the radiation fields of the equivalent electric and magnetic monopoles can be made equal in amplitude but different in phase b 9 at three resonant frequenc bands, thus generating triband omnidirectional CP fields. The usable bandwidths of the antenna are %, 5.%, and 1.%. In addition, the antenna showsgoodomnidirectionalcpradiationpatternsinthe aimuthplaneoverthethreebands.andtheaveragecpgains in o-plane are 1.2, 1.6, and dbic for the lower, middle, and upper bands, respectivel. Conflict of Interests Figure 8: Measured CP gains against frequenc for the proposed antenna. 3. Eperimental Results and Discussion According to the dimensions shown in Figure 1, a fabricated prototpe for the proposed antenna has been constructed and measured. The photograph of the prototpe is ehibited in Figure 5. Thesimulatedandmeasuredreturnlossand aial ratio (theta = 9 deg and phi = deg) versus frequenc are shown in Figure 6. The measured 1dB impedance bandwidths are 35 MH ( GH), 185 MH ( GH), and 135 MH ( GH), respectivel, and the 3 db aial ratio bandwidths are approimatel 7 MH ( GH), 12 MH ( GH), and 28 MH ( GH). It is observed that the usable bandwidths are 3 MH ( GH), 12 MH ( GH), and 28 MH ( GH). There has been a good agreement between the simulated and measured results. The RHCP and LHCP radiation patterns are measured in the aimuth (- plane) and elevation (- plane) planes at the frequenc of 1.937, 2.45, and 3.51 GH in Figure 7. Itisseen that the proposed antenna ehibits a good omnidirectional CPpatternintheaimuthplaneandabidirectionalpattern in the elevation plane. The measured CP gains in the o-plane of the proposed antenna are shown in Figure 8. From the right figure of Figure 7, it can be seen that the maimum CP gain for the upper band is not in the o-plane, but the maimum CP gains for the lower and middle bands are in the o-plane. The ground plane is larger relative to the upper frequenc wavelength; thus the maimum CP gain is above o-plane. So, the CP gain for upper bands is low. The average gains of 1.2, 1.6, and dbicforthelower,middle,andupperbands, respectivel, are obtained. 4. Conclusion In this paper, a triband omnidirectional CP DRA with a toploaded modified Alford loop is proposed. The DRA fed b an aial probe at the center of its bottom. The technique of The authors declare that there is no conflict of interests regarding the publication of this paper. References [1]L.Dong,Z.Zhang,W.Li,andG.Fu, AcompactCPW- FED monopole antenna with triple bands for WLAN/WiMAX applications, Progress in Electromagnetics Research Letters,vol. 39, pp , 213. [2] S.C.Basaran,U.Olgun,andK.Sertel, Multibandmonopole antenna with complementar split-ring resonators for WLAN and WiMAX applications, Electronics Letters,vol.49,no.1,pp , 213. [3] R. Wen, Compact planar triple-band monopole antennas based on a single-loop resonator, Electronics Letters,vol.49, no. 15, pp , 213. [4] H. Chen, X. Yang, Y.-Z. Yin, J.-J. Wu, and Y.-M. Cai, Tri-band rectangle-loaded monopole antenna with inverted-l slot for WLAN/WiMAX applications, Electronics Letters, vol.49,pp , 213. [5] M.-Z. Wang, F.-S. Zhang, Y. Zhu, and L.-T. Ma, Design of CPW-fed circularl polaried antenna with two orthogonal slots, Progress in Electromagnetics Research Letters, vol.33,pp , 212. [6] F.-X. Wu, W.-M. Li, and S.-M. Zhang, Dual-band CPW-fed circularl-polaried slot antenna for DMB/WiMAX application, Progress in Electromagnetics Research Letters, vol.3,pp , 212. [7] X.-Q. Zhang, Y.-C. Jiao, and W.-H. Wang, Compact dual-band dual-sense circularl-polaried CPW-fed slot antenna, Progress in Electromagnetics Research Letters, vol. 34, pp , 212. [8] A. Narbudowic, X. L. Bao, and M. J. Ammann, Omnidirectional circularl-polarised microstrip patch antenna, Electronics Letters, vol. 48, no. 11, pp , 212. [9] W. W. Li and K. W. Leung, Omnidirectional circularl polaried dielectric resonator antenna with top-loaded alford loop for pattern diversit design, IEEE Transactions on Antennas and Propagation,vol.61,no.8,pp ,213. [1] B.-C. Park and J.-H. Lee, Dual-band omnidirectional circularl polaried antenna using eroth- and first-order modes, IEEE Antennas and Wireless Propagation Letters, vol. 11, pp , 212. [11] A. Petosa, Dielectric Resonator Antenna Handbook, Artech House, Norwood, Mass, USA, 27.

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