International Journal of Antennas and Propagation
Volume 2015 (2015), Article ID 165270, 7 pages
http://dx.doi.org/10.1155/2015/165270

Research Article

A CPW-Fed Monopole Antenna with Double Rectangular Rings and Vertical Slots in the Ground Plane for WLAN/WiMAX Applications

Sangjin Jo, Hyunjin Choi, Jaehyuk Lim, Beomsoo Shin, Sangyeol Oh, and Jaehoon Lee

Department of Computer and Radio Communication Engineering, Korea University, 1 5Ka Anam-Dong, Seongbuk-gu, Seoul 136-701, Republic of Korea

Received 15 April 2015; Revised 15 June 2015; Accepted 15 June 2015

Academic Editor: Junping Geng

Copyright © 2015 Sangjin Jo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A compact triple-band monopole antenna consisting of double rectangular rings and vertical slots cut into the ground is proposed for WLAN and WiMAX operations. The antenna has a compact size of 27.1 × 38.8 × 1.6 mm³, with simulated and measured impedance bandwidths of 2.37~2.81, 3.21~3.82, and 4.61~6.34 GHz with a reflection coefficient of less than −10 dB. The antenna also exhibits an almost omnidirectional radiation pattern and stable gain levels in the triple bands. The characteristics of the proposed antenna have been investigated using the numerical simulations and experiments.

1. Introduction

Recently, there has been rapid progress in wireless communications technology employing various frequency bands. For short-range and long-range communication applications, many antenna designs have been studied that are suitable for wireless local area network (WLAN) and worldwide interoperability for microwave access (WiMAX) operation. There is also a growing demand in wireless communication technology for integration of the WLAN (2.412~2.482 GHz, 5.15~5.825 GHz) and WiMAX (2.500~2.690 GHz, 3.400~3.690 GHz, 5.250~5.850 GHz) frequency bands into a single device, thus necessitating the development of multiband antennas with simple structure and low profile. Numerous multiband antenna designs with simple structure have already been reported, including a printed planar wide-slot antenna [1], a CPW-fed planar monopole antenna [2], a printed inverted-L- (IL-) shaped monopole antenna [3], a slot-monopole antenna with embedded rectangular parasitic elements [4], and a monopole antenna with a split ring [5]. However, for the realization of multiband operation, additional structures must be employed on the bottom layer such as a T-shape, S-shape, vertical strip resonator, or parasitic elements.

In this paper, we propose a CPW double rectangular ring-shaped monopole antenna design that incorporates the frequency bands for WLAN and WiMAX applications (i.e., operates at frequencies in the ranges 2.4~2.690, 3.4~3.69, and 5.15~5.850 GHz). Triple band characteristics are achieved by an inner/outer ring structures and thin slots in the ground plane. The outer ring structure controls the resonance at the lower and upper frequency bands, whereas the inner ring structure controls the resonance at the middle frequency band. Two thin slots etched into the ground are used to reject an unwanted resonance at around 4 GHz. Details of the proposed antenna design and experimental results are presented and discussed.

2. Antenna Design

The geometry of the CPW-fed double rectangular ring-shaped antenna is shown in Figure 1, together with a photograph of the fabricated optimized structure. The antenna has overall dimensions of 27.1 × 38.8 × 1.6 mm³ and was fabricated on an FR4 substrate with relative permittivity . A 50 Ω CPW transmission line with a signal strip width of 3.5 mm and a gap distance of 0.3 mm is used to feed the antenna. Two ground planes of the CPW line with dimensions of 11.5 × 17 mm² are situated symmetrically on either side of the CPW line. The size of the outer rectangular ring is 17 × 21.8 mm², and the size of the inner ring is 7 × 8.8 mm². The antenna consists of a single-layer metallic structure for the simple fabrication, which is etched onto one side of the substrate. The optimal design parameters in Figure 1(a) are  mm,  mm,  mm,  mm,  mm,  mm,  mm,  mm,  mm,  mm,  mm,  mm,  mm,  mm, and  mm.

(a)

(b)

Figure 1: Geometry and photograph of the proposed antenna: (a) geometry, (b) photograph.

Figure 2 shows the effects on the return losses produced by the inner ring and the vertical slots in the ground plane via numerical simulations. Antenna 1 (in Figure 2), which consists solely of the outer rectangular ring, can easily achieve resonances at two frequency bands for dual-band WLAN operation [6]. In Antenna 2, which the inner rectangular ring is added, we found that it can excite a middle frequency of 3 GHz. However, the addition of the inner rectangular ring also produces wideband resonances in frequency bands around 4 GHz where they are not wanted. In Antenna 3, which vertical slots are inserted into the ground plane of Antenna 1, band-notched frequencies appear around 4 GHz, and the resonance in the notched band from 4.0 to 5.0 GHz is removed. The lengths of these slots are equal to the half-guided wavelength at the rejected frequency [7, 8]. Finally, the proposed double-rectangular-ring antenna with double rectangular rings and vertical slots in the ground plane (Antenna 4) can generate three distinct resonances in three wanted separate frequency bands.

Figure 2: Simulated return losses for the proposed antenna with and without the inner ring and vertical slots.

In the aforementioned, the lower and upper frequency bands are primarily determined by the outer ring and the middle frequency band is dominated by the inner ring. Figure 3 shows the influence of the geometries of the outer ring on the return loss at the lower and upper frequency bands in the proposed antenna. Figure 3(a) shows the effect of when the length, , of the outer ring was fixed at 19 mm and Figure 3(b) shows the effect of when  mm. Since, the parameter made a primary impact on not the lower and but the upper frequency band and the parameter made the frequency shift in the upper and lower bands at the same time, we adjusted the length of to finely tune the upper frequency band.

(a)

(b)

Figure 3: Influence of the outer ring on the return loss of the proposed antenna: (a) various when  mm, (b) various when  mm.

For the adjustment of the middle frequency bands, we adjusted the geometries of the inner ring as shown in Figure 4. Figure 4 shows the effects of varying lengths of the rectangular inner ring, and , , and cannot make any variation at the lower and upper frequency bands when the parameters of the outer ring are fixed. As the parameters , , and increased, the middle resonant frequencies were decreased.

(a)

(b)

(c)

Figure 4: Influence of the inner ring on the return loss of proposed antenna: (a) various when  mm and  mm, (b) various when  mm and  mm, (c) and various when  mm and  mm.

3. Experiment Result

Figure 5 shows the simulated and measured return losses of the proposed antenna with the optimized parameters, and good agreement between simulation and measurement is observed. For a 10 dB return loss, the measured impedance bandwidths of the three individual operating bands are about 500 MHz (2.37~2.81 GHz), 800 MHz (3.21~3.82 GHz), and 1 GHz (4.61~6.34 GHz), simultaneously covering the 2.4/5.2/5.8 GHz WLAN and 2.5/3.5/5.5 GHz WiMAX operational bands.

Figure 5: Measured and simulated reflection coefficients of the proposed antenna.

Figure 6 shows the simulated current distributions under the optimized structure at the different frequencies. The current distributions varied significantly depending on the resonant frequency. It is evident from these current distributions that, at lower frequency band, the radiation mainly resulted from the outer ring, as shown in Figure 6(a). The inner ring mainly contributes the radiation at middle frequency band, as shown in Figure 6(b). Likewise, the horizontal strip line of the outer ring mainly contributes the radiation at upper frequencies band, as shown in Figure 6(c).

(a)

(b)

(c)

Figure 6: Simulated current density distribution of the proposed antenna at (a) 2.4 GHz, (b) 3.5 GHz, and (c) 5.5 GHz.

The measured XY-plane and ZX-plane radiation patterns are shown in Figure 7. The XY-plane patterns appear to be nearly omnidirectional, whereas the ZX-plane patterns are in the broadside direction at all operating frequencies. Figure 8 shows the measured peak gain of the proposed antenna in the 2.4 GHz, 3.5 GHz, and 5.5 GHz bands. The measured peak realized gain levels of the antenna are about 5.5, 4.0 and 3.5 dBi for 2.4, 3.5 and 5.5 GHz, respectively.

(a)

(b)

(c)

Figure 7: Simulated and measured radiation patterns of the proposed antenna at (a) 2.4 GHz, (b) 3.5 GHz, and (c) 5.5 GHz.

Figure 8: Measured peak gain of the proposed antenna at operational frequency bands.

4. Conclusion

We proposed a CPW-fed monopole antenna with double rectangular rings and vertical slots for WLAN/WiMAX applications. The antenna design is compact, with dimensions of only 27.1 × 38.8 × 1.6 mm³. A band notch for unwanted frequency bands is achieved by adding vertical slots to the ground plane. The antenna has a simple geometry and is relatively easy to fabricate because of its single-layer metallic structure. Because the bandwidths of the lower, middle, and upper bands of the proposed monopole antenna are sufficient to cover the 2.4, 3.5, and 5.5 GHz bands, the design is suitable for WLAN/WiMAX triple-band applications. Additionally, the proposed antenna provides good radiation patterns and stable gain in each of the operating bands.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology, Republic of Korea (NRF-2012R1A1B3002517 and NRF-2015R1A2A2A01005676).

References

Y.-F. Liu, K.-L. Lau, Q. Xue, and C.-H. Chan, “Experimental studies of printed wide-slot antenna for wide-band applications,” IEEE Antennas and Wireless Propagation Letters, vol. 3, no. 1, pp. 273–275, 2004.

Y. Xu, Y.-C. Jiao, and Y.-C. Luan, “Compact CPW-fed printed monopole antenna with triple-band characteristics for WLAN/WiMAX applications,” Electronics Letters, vol. 48, no. 24, pp. 1519–1520, 2012.

S. Verma and P. Kumar, “Compact triple-band antenna for WiMAX and WLAN applications,” Electronics Letters, vol. 50, no. 7, pp. 484–486, 2014.

C.-Y. Huang and E.-Z. Yu, “A slot-monopole antenna for dual-band WLAN applications,” IEEE Antennas and Wireless Propagation Letters, vol. 10, pp. 500–502, 2011.

S. C. Basaran, U. Olgun, and K. Sertel, “Multiband monopole antenna with complementary split-ring resonators for WLAN and WiMAX applications,” Electronics Letters, vol. 49, no. 10, pp. 636–638, 2013.

S. Jo, H. Choi, B. Shin, S. Oh, and J. Lee, “A CPW-fed rectangular ring monopole antenna for WLAN applications,” International Journal of Antennas and Propagation, vol. 2014, Article ID 951968, 6 pages, 2014.

H.-W. Liu, C.-H. Ku, and C.-F. Yang, “Novel CPW-fed planar monopole antenna for WiMAX/WLAN applications,” IEEE Antennas and Wireless Propagation Letters, vol. 9, pp. 240–243, 2010.

M. A. Antoniades and G. V. Eleftheriades, “A compact multiband monopole antenna with a defected ground plane,” IEEE Antennas and Wireless Propagation Letters, vol. 7, pp. 652–655, 2008.