Stacked T-Shaped Strips Compact Antenna for WLAN and WiMAX Applications

A compact triple band antenna with stacked T-shaped strips inside a rectangular ring monopole has been proposed. This novel structure with a slot in the defected ground achieves triple band opration i.e. 2.47–2.77 GHz, 3.3–3.7 GHz and 5.10–6.62 GHz. These bands find application in important wireless communication standards like WiMAX (3.3–3.8 GHz, and 5.25–5.85 GHz, WLAN (2.4 G-2.5 GHz, 5.1–5.3 GHz, and 5.72–5.85 GHz). The antenna is printed on a FR-4 substrate with an overall dimension of 33×17×1.6mm3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$33 \times 17 \times 1.6 \;{\text{mm}}^{3}$$\end{document}. An impedance bandwidth of 11% (2.47–2.77 GHz), 11% (3.3–3.7 GHz) and 25% (5.10–6.62 GHz) is obtained. A good conjunction between the simulated and measured results is inferred from the antenna design analysis.


Introduction
Compact printed monopole antennas with multiband operation is gaining attention as a result of the tremendous development of compact devices for wireless applications [1,2]. Antenna is the main component of wireless communication devices for adopting standards like WLAN, WiMAX etc. Now a days market demand for compact wireless portable 1 3 device is increasing rapidly. Hence, in regard to this demand, design of miniaturized multiband antenna has attracted many researchers [3][4][5][6]. Multiband antenna works for more than one standard is vital component for portable devices. Modified monopole antennas is considered as proficient among other antennas structures due to its simplest structures [7,8].
The literature details various dual and multiband antennas [9][10][11][12][13][14][15][16][17][18][19][20]. In [9], a pair of L-shaped strips are combined to obtain triple band operation. However, large antenna dimension (i.e., 33 × 28 mm 2 ) is a major drawback. Two self similar ring radiator was designed in [10] for achieving triple band operation with a dimension of 38 × 25 mm 2 . This antenna size is also large for compact devices. Li et al. [11] proposed a rectangular ring monopole antenna exhibiting triple band operation is designed using L-strips and inverted T-shaped stubs. In [12], L-Shaped strips on a 30 × 42 mm 2 substrate are used to obtain tri-band operation. Similarly, CPW fed slotted antenna with relatively larger dimension has been proposed in [13,14]. Li [20]. However, it exhibits less gain at operating frequencies. However, smaller antenna operational bandwidth and large size continues to remain the major loopholes in the aforementioned antenna designs.
In this research article, a compact tri-band antenna of 33 × 17 mm 2 dimensions with four T-shaped strips stacked one above another inside a rectangular ring monopole is proposed. The antenna operating bands are 2.47-2.77 GHz, 3.3-3.7 GHz and 5.10-6.62 GHz. The effect of adding strips on frequency and impedance matching are detailed. The parametric analysis are performed to justify the selection of antenna dimension.
The organization of the paper is briefed as follows: Section II details the methodology of the proposed design. The influence of various antenna parameters on performance has been explained in the third section. Section IV highlights the comparison between simulation and measured results. Section V concludes the paper.

Antenna Design
The development stages of the proposed antenna is depict in the Fig. 1. The first stage i.e., Antenna I consists of a rectangular ring ( w 1 × h 1 ), strip line ( w 8 × h 6 ), incremented strip line ( w 7 × h 5 ) and ground palne ( w × h 7 ). From [21], it is obvious that the tapered feed line gives better performance in terms of S 11 . It is observed from the Fig. 2 that, a dual band operation at 2.5 and 5 GHz is produced by the Antenna I. Then four T-shaped strips are stacked one over another inside the rectangular ring to produce Antenna II. This results in triple band operation as can be seen from the Fig. 2. Further to enhance the impedence bandwidth, a rectangular slot has been etched in the ground. Finally, the proposed antenna is evolved with triple oprating bands with ranges 2.47-2.77 GHz, 3.3-3.7 GHz and 5.10-6.62 GHz. This range corresponds to WLAN and WiMAX applications. The Fig. 3 depicts a detailed dimension layout of Antenna III. Figure 4 shows the fabricated antenna. The final dimensions of the proposed antenna are presented in Table 1.

Parametric Analysis
The influence of various geometric parameters on the frequency parameter characteristics are detailed in parametric analysis. Initially, the effect of various stubs is analyzed. The   gives better performance in comparison with other structures. Additionally, it can be easily observed that on adding each stub the performance of the antenna gradually increases. After finalizing the structure, the effect of dimensions of the stubs are studied through parametric analysis.
First, we analyze the effect of variation in length of the ground slot on the S 11 performance as described in Fig. 7. It can be inferred that w 9 = 7.2 mm produces only dual band operation, when w 9 is increased to 8.2 mm, the antenna operates at three bands. Hence, in the antenna design the ground slot width is chosen as 8.2 mm.     Fig. 9 shows the variation of S11 for the various values of w 5 . To get the operating band at 3.5 GHz 3.1 mm has been chosen for the fabricated antenna. It is observed Fig. 9 that, the other values of w 5 shifts the operating band towards 3 GHz.

Measurement Results
Agilent N5230A vector network analyzer (VNA) is used to perform the proposed antenna design measurements. The Fig. 10 shows that there is minimal difference between measured and simulated results. Additionally, it can be inferred from the  The surface current densities are given in Fig. 11. It is observed from figure that at 2.5 GHz, the current concentrates at the peripheral ring. The operating band at 3.5 GHz is due to the current at microstrip line. At 5.5 GHz, the current concentrates on ground slot as well as rectangular ring, thus validating the process of antenna design. Variation of S11 with respect to the variation in h4 Fig. 9 Variation of S 11 with respect to the variation in w 5 Figure 12 shows the measured and simulated radiation pattern. The far field radiation pattern exhibits bidirectional pattern at center frequency, i.e., 2.5 GHz, 3.5 GHz and 5.5 GHz in E-Plane. Omni directional pattern is observed in the H-Plane. The antenna gains and radiation efficiency is simulated over the operating ranges. It can be seen from the Fig. 13 that the gain varies from 2 to 3.8 dB and the efficiency varies from 60 to 80% over the operating bands. Comparative analysis of the proposed antenna with the existing similar antenna designs is highlighted in Table 2, thus implying the superiority of the proposed design in terms of size, gain and bandwidth over its counterpart.

Conclusion
A compact triple band antenna has been reported in this research article. The proposed design is compact in nature with dimension of 33 × 17 × 1.6 mm 3 . The printed monopole antenna exhibits triple band operation which supports important wireless communication standards like WLAN and WiMAX. The designed antenna achieves the omni-directional radiation pattern. It also achieves high gain and bandwidth. With these features and compact size, the proposed antenna could easily be adopted for real-time compact wireless communication devices.
Funding Open access funding provided by Manipal Academy of Higher Education, Manipal.  Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Sameena Pathan has completed PhD at MIT, Manipal. Her research interest includes microwave imaging, dermoscopic image analysis and pattern recognition. She has many publications in reputed national/ international journals and conferences. She is a reviewer of journals published by Springer.